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Safe joint module for safe robot arm based on passive and active compliance method
Mechatronics 22 (2012) 1023–1030
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Safe joint module for safe robot arm based on passive and active compliance method Hwi-Su Kim, In-Moon Kim, Chang-Nho Cho, Jae-Bok Song ⇑ School of Mechanical Engineering, Korea University, Seoul, Republic of Korea
a r t i c l e
i n f o
Article history: Received 8 September 2011 Accepted 26 August 2012 Available online 15 September 2012 Keywords: Joint module Manipulator design Collision safety Collision absorption
a b s t r a c t In recent years, collision safety between humans and robots has drawn much attention as service robots are being increasingly used in human environments. Since various types of collisions can occur during a robot’s operation, multiple safety methods should be implemented to ensure human safety. The most common and practical solutions are the active and passive compliance methods and each has its own merits and disadvantages. To combine the active and passive compliance methods to achieve higher reliability, we propose a novel safe joint module composed of a speed reducer, a torque sensor and a safety mechanism. The torque sensor embedded in the safe joint module can be used to detect a collision; then the actuator appropriately reacts to minimize the impact. However, if the collision detection by the embedded joint torque sensor and the subsequent reaction fail due to the limited bandwidth of the sensor, the safety mechanism composed of purely mechanical elements such as springs and a cam follower absorbs the collision force. With the proposed safety joint module, the combined active and passive compliance method can ensure collision safety. Moreover, the embedded torque sensor can be used for force control. The embedded harmonic drive can achieve high gear reduction and low backlash. Several experiments on static and dynamic collisions showed that the proposed module can guarantee safety while maintaining good performance. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In recent year, humanoid robots and service robots have drawn a great deal of attention. Since these robots often operate in human environments to help and co-operate with humans, the safety issues related to physical human–robot interaction are becoming increasingly important [1–3]. There are two major steps to achieve collision safety between humans and robots. Collisions can be predicted before the contact occurs so that the robot using non-contact sensors (e.g., a vision camera) can regenerate its path to avoid the collision [3–6]. On the other hand, after the collision, the collision force can be detected by force/torque sensors and the robot arm can either react against the collision or absorb the impact to protect the human. In this research, we focus on the post-collision safety methods. As shown in Fig. 1, there are two strategies to improve collision safety, the active compliance method and the passive compliance method. In an actively compliant arm, a collision is detected by sensors, and the robot arm is controlled to reduce the collision force. The robot arm efficiently handles a collision by recognizing different collision situations and choosing the most appropriate reaction ⇑ Corresponding author. E-mail address: [email protected] (J.-B. Song). 0957-4158/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mechatronics.2012.08.007
strategy. The collision safety of the robot arm has been improved by using active compliance methods: covering the manipulator with skin sensors [7], attaching a six axis force/torque sensor at the end-effector of a manipulator [8], embedding joint torque sensors at each robot joint to estimate the collision force [9], and adjusting the stiffness of a joint by serial-type dual actuator unit [10]. On the other hand, the passive compliance method consists of purely mechanical elements. Since this approach does not use any sensors or actuators, it can provide fast and reliable responses even for a dynamic collision. Several safety mechanisms have been developed using the passive compliance method: the programmable passive compliance shoulder mechanism [11], the mechanical impedance adjuster with a leaf spring and an electromagnetic brake [12], and a passive compliance joint with rotary springs and a MR damper [13]. However, most previous approaches adopted only one of these compliance methods, which alone often could not guarantee collision safety. The active compliance method has limited bandwidth since it involves sensing and actuation in response to a dynamic collision. Moreover, the scheme of sensing and actuation inevitably results in high cost, sensor noise, and possible malfunctions. On the other hand, the passive compliance method activates the safety mechanism too frequently even against a low-speed collision which would not seriously harm a human. Also, the passive compliance
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Active compliance
Passive compliance
Collision detection Force/torque sensor
Collision detection Mechanical device
Safe ?
threshold ?
Fail
No Collision reaction Reflex motion / E-stop
Yes Collision absorption Low stiffness
Collision safety Fig. 1. Safety strategies after collision.
method provides a smaller safe region compared to the active compliance method due to the singularity problem of the robot arm. Since various types of collisions can occur during robot operations, multiple safety methods should be implemented to prevent possible human injury. As an example, collision prediction and path regeneration using a vision camera was combined with Variable Stiffness Transmission (VST) actuator which could observe a collision force based on the passive compliance method [14]. However, prediction of a fast collision requires an expensive high-speed vision system because a normal camera has limited bandwidth. Moreover, the VST decreases the stiffness of a robot arm by actuator control, which results in the reduced bandwidth of the robot arm. This paper proposes a novel design of a safe joint module composed of a speed reducer, a joint torque sensor and a safety mechanism. The safe joint module implements both active and passive compliance methods to provide collision safety with higher reliability. The torque sensor can detect a collision; then the actuator, controlled properly, reacts against the collision. If the torque sensor fails to detect a collision because of its low bandwidth, the safety mechanism absorbs the collision force. Moreover, using the embedded joint torque sensors, the proposed module can measure the external force applied to the robot arm for force control without an expensive force/torque sensor. Furthermore, the modular structure allows the easy construction of various safe robot arms [15–17]. The rest of the paper is organized as follows. The active compliance method using an embedded torque sensor is presented in Section 2. The operation principle and the structure of the safety mechanism are discussed in Section 3. Section 4 shows the design of the safety joint module combining the active and passive compliance methods. Various experimental results on collision safety and force estimation are discussed in Section 5. Finally, the conclusion is presented in Section 6.
(a)
2. Active compliance method with embedded torque sensor As mentioned in Section 1, both force control and collision safety based on the active compliance approach require sensors to detect the contact force. Therefore, a 6-DOF force/torque sensor is often mounted at the wrist of a robot arm, but this sensor is too expensive for practical applications. Moreover, it can detect only the collision at the end-effector. However, a joint torque sensor inside the proposed safe joint module can monitor the external torque and thus, detect collisions regardless of where they occur. When the torque exceeds a certain threshold, the actuator can be properly controlled to react against the collision. In this section, the active compliance method using the embedded torque sensor will be presented. Also, the possibility of using the torque sensor to establish a low-cost force control system will be discussed. 2.1. Design of joint torque sensor The joint torque sensor should be designed for sufficient deformation in response to an external torque for precise measurement. However, a large deformation has an adverse effect on the positioning accuracy of a robot, so there is a tradeoff. In this research, the joint torque sensor has a hub-spoke type structure that provides appropriate stiffness and high sensitivity. As shown in Fig. 2a, the structure of the torque sensor is divided into outer rim, hub and spoke [18]. The several strain gauges attached to the torque sensor measure the external torque. FEM analysis is used to find the best positions to place the strain gauges, which are the positions of the maximum deformation at the spoke, where deformation is multiplied several times. The torque coefficient of the designed torque sensor can be found through experiments. The strain gauge wires are connected to an amplifier though the hollow shaft of the safe joint module as shown in Fig. 2b. 2.2. Force estimation and collision detection using torque sensor When a robot collides with an object, an external torque is induced at each joint of the robot. Thus, by monitoring the external torque, collisions can be successfully detected. The equation of motion of a robot can be described by
_ h_ þ gðhÞ sreq ¼ MðhÞ€h þ Cðh; hÞ
ð1Þ
where h is the joint angle vector, M is the inertia matrix, C is the matrix involving the Coriolis and centrifugal terms, g is the gravity vector, and sreq is the torque required for the actuators to perform the desired motion. In case of a collision, the torque due to the dynamics of the robot, sreq, and the torque due to the collision, sext are applied to each torque sensor. To extract sext from the sensor data, sreq must be subtracted from the torque sensor reading, so that
(b)
Fig. 2. (a) Structure of embedded torque sensor, and (b) installation of torque sensor.
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Fig. 5. Procedure for the active compliance method.
(a)
(b)
Fig. 3. Multi-DOF robot arm with several numbers of safety joint modules used to estimate external force: (a) 3-DOF, and (b) 6-DOF arm.
Fig. 4. Block diagram of natural admittance controller.
sext ¼ sm sreq
ð2Þ
where sm is the measured torque vector by the joint torque sensors. When sext exceeds the pre-defined active threshold torque, a collision is detected. From sext, the location, magnitude, and orientation of the collision can be found. With this information, the robot analyzes the collision and selects the most suitable reaction strategy, either an emergency stop or a move-back motion. The threshold must be set appropriately to avoid possible malfunctions. Furthermore, during a heavy-payload operation or a force control operation, increasing the threshold would allow the robot to detect collisions as it performs the desired task. Also, using the joint torque sensors, a low-cost force control system can be established. Using Eq. (2), the force due to the external torque at the end effector can be computed by
F ¼ ðJ T Þ1 sext
ð3Þ
As shown in Fig. 3, the forces Fx, Fy and Fz can be obtained by using three safety joint modules. Six joint modules would provide all three forces and three moments. Thus, depending on the application, an appropriate number of the safety joint modules would allow the robot to perform force control and collision detection at the same time. In this paper, an admittance control scheme, which is another form of impedance control, is used for the 3-DOF robot arm equipped with three safe joint modules. Fig. 4 shows the controller, where xd and Fd are the desired position and force, xc and xt are the compensated position and the target position represented in the task space, respectively. The variables ht and st are the target angle and torque represented in the joint space, respectively, and the robot position can be obtained by solving the inverse kinematics of the robot arm. The admittance filter Y(s), which represents the ratio of the velocity to the force in the s-domain, can be described by
YðsÞ ¼
V c ðsÞ 1 ¼ F d ðsÞ FðsÞ B þ k=s
ð4Þ
where B is the damping coefficient and k is the stiffness [19]. Therefore, the proper admittance filter can be selected experimentally by adjusting these values. If the external torque estimated from
Eqs. (1) and (2) is not identified as a collision, the actual contact force in the task space can be computed from the external torque by the Jacobian relation. In this admittance control scheme, the external force estimated from joint torque data is converted to the robot position information through the force controller. This position information is used to provide the desired impedance effect (i.e., spring and damper effects), while the robot arm moves to the target position in joint space through the position controller. The inner-position/outer-force control scheme is advantageous for impedance control of the robot arm that is based on a position controller [20]. On the other hand, if the external torque is identified as a collision by the collision detector shown in Fig. 5, the active compliance method is applied to control the robot arm. Fig. 5 shows the procedure for the active compliance method, where a is the collision index. During a normal operation, the index is set to 0, and the position controller controls the robot to follow a pre-defined path. However, when a collision is detected, the index is switched to 1, which switches the controller from the task planner to the collision reactor. Then, the robot generates the proper reaction motion and delivers it to the position controller which directs the robot to perform the safety motion. However, the robot arm usually cannot react quickly against a collision due to the limited bandwidth of the control system and torque sensor. Each control scheme requires a certain computation time, and thus a time delay is inevitable [21]. Furthermore, this time delay directly depends on the performance of the controller, and thus an expensive controller is required to improve collision detection and reaction time. 3. Passive compliance method with safety mechanism based on nonlinear stiffness To deal with the limitations of the active compliance method, a passive compliance device, which consists of purely mechanical elements such as springs and cams, was proposed. A spring is the most popular mechanical element for shock absorption. However, a spring used at a robot arm joint leads to positioning inaccuracy because it significantly lowers the stiffness of the joint. Although a hard spring can provide high positioning accuracy for a robot arm, it has much lower shock absorbability than a soft spring, increasing the probability of human injury upon collision. To achieve both collision safety and positioning accuracy, the safety mechanism based on nonlinear stiffness was proposed and installed inside of the safe joint module. In this section, the passive compliance method for collision absorption using the safety mechanism is presented. 3.1. Design of safe mechanism to realize nonlinear stiffness Nonlinear stiffness for both positioning accuracy and collision safety of a robot arm is difficult to realize using only linear springs. Therefore, in our previous research, nonlinear stiffness was realized by a safety mechanism composed of a double-slider mechanism and linear springs [22,23]. This safety mechanism maintains
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However, the increased spring force, due to compression, is not large enough to sustain this balance. This unbalanced state causes the slider to rapidly slide, and thus the collision force is absorbed by the spring via its compression. Thus, the stiffness of the safety mechanism abruptly drops. From Eq. (5), the passive threshold torque can be described by
T th ¼
(a)
(b)
Fig. 6. Safety mechanism based on cam and cam-follower mechanism: (a) structure, and (b) operation (after collision).
¼
(b)
Fig. 7. Cam and cam-follower mechanism for nonlinear stiffness: (a) zero configuration, and (b) general configuration.
very high stiffness for positioning accuracy up to a preset threshold torque, but for collision safety, the stiffness drops abruptly when the torque exceeds the threshold. Note that upon a collision, an external torque less than this threshold does not cause serious human injury. Moreover, after the collision force is removed, the safety mechanism and robot arm return to their initial states because of the stored spring energy. This paper presents a new safety mechanism shown in Fig. 6, which can be installed much easily than the previous safety mechanism at each joint of a robot. The proposed safety mechanism consists of a cam, a cam follower, and linear springs pre-compressed to achieve the nonlinear spring characteristics. To explain the principle of nonlinear stiffness, a close-up view of area A in Fig. 6 is shown is Fig. 7. The input torque Tin exerted on the cam rotates the cam and applies a corresponding force F0 to the output slider. However, due to the spring force by spring compression, the output slider does not move. If the input torque Tin from the external load is smaller than the predetermined passive threshold torque, the safety mechanism would provide enough stiffness to maintain static equilibrium, which prevents the rotation of the link. In other words, the resisting force Fr due to spring compression, and the x-component of F0 by Tin are in equilibrium. From the equation of closure of the mechanism, the relationship between Tin and Fr is given by
cosð270 hÞ T in Fr ¼ c
ð5Þ
where the parameters c and h are defined in Fig. 7, and c and h are related by c = 270° h. However, if the input torque exceeds the passive threshold torque, then the cam plate turns around point P as shown in Fig. 6b and the spring block (output slider in Fig. 7) is forced to move along the spring guide to compress the spring. This movement of the output slider reduces the transmission angle, so sustaining the static balance requires a greater resisting force for the same input force.
kso
ð6Þ
where the subscript o represents the zero configuration, so is the spring compression at the zero configuration, k is the spring constant, and l is the friction coefficient. From Eq. (5), the equivalent stiffness, keq seen from the input shaft can be obtained as a function of c as follows:
keq ¼
(a)
co
l cosð270 ho Þ
T in ks ¼ Dh h h o
c
l cos c
k l so þ bo ða tan cÞ ðh ho Þl cos c cos c a l tan c cos c
ð7Þ
where s is the spring compression, a, b and l are defined in Fig. 7. Fig. 8 shows an equivalent stiffness curve as a function of angular displacement of the input shaft when k = 5.2 kN/m, a = 14.5 mm, bo = 19.4 mm, co = 22 mm, lo = 10.2 mm, so = 12 mm, ho = 192° and l = 0.9. The equivalent stiffness keq of the nonlinear system remains very high for a small angular displacement of the input slider, but quickly drops as the displacement increases. Hence, a nonlinear stiffness system is realized by the cam and cam-follower mechanism combined with a pre-compressed linear spring. 3.2. Limitation of passive compliance method Although the passive compliance method can provide fast response against a high-speed collision since it only consists of purely mechanical elements, this method also has limitations in reacting against various types of collisions. First of all, the passive threshold of the safety mechanism cannot be changed once the mechanism is built, although it is desirable to have the ability to change the threshold during a task such as a human–robot co-operative task, lowering it to prevent any human injury, but increasing it for a high-payload task. Furthermore, frequent activation of the safety mechanism would significantly decrease the performance of the robot. Moreover, collision safety near the singularity of the robot arm can be a serious problem with the passive compliance method having a fixed threshold torque. The threshold torque should be lowered to ensure collision safety when the robot arm collides with a human near its singular position since high collision force yields only a small torque at each joint near singular points. Second, the operational angle of the safety mechanism is limited due to the kinematic constraints, such as the spring compression
100 80
Nonlinear spring
60 40
Linear spring
20 0
0
2
4
6
8
10
Δ (°) Fig. 8. Nonlinear stiffness using cam and cam-follower mechanism.
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Operational angle
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Front view
(a)
(b)
Fig. 9. (a) Operational angle of safety mechanism, and (b) available range of compression spring.
length as shown in Fig. 9. Excessive compression of the spring may permanently deform the spring and thus changes the threshold of the safety mechanism. If the mechanism fails to stop or withdraw the robot within its operational range, a follow-up collision may occur, which would significantly harm a human. To effectively protect human from collisions, therefore, the robot must be able to detect the collision when the safety mechanism is activated so that it can perform appropriate reactions. 4. Safe joint module to combine active and passive compliance methods 4.1. Design of safe joint module The safe joint module, which consists of a gear reducer, a safety mechanism, a torque sensor, cross-roller bearings, and a hollow shaft, was constructed as shown in Fig. 10. A component type harmonic drive was used for speed reduction, low backlash and lightweight design. The safety mechanism and torque sensors were designed such that they could all fit into the safe joint module. A cross-roller bearing can support the moment load applied to the torque sensor so that only the rotational torque is applied to the torque sensor. A hollow shaft was used to get the wires through the safety joint module. Every casing and plate were made of duralumin to minimize the weight of the safety joint module. Prototypes of the safe joint mechanism were constructed and their performances were evaluated, as shown in Fig. 11. Two types of safety modules were constructed depending on the passive threshold torque and size (Types A and B). The specifications of the safe joint modules are listed in Table 1. 4.2. Combination of active and passive compliance method
Fig. 11. Prototype of safe joint module.
Table 1 Specifications of safe joint module. Type
A
B
Weight Dimension Passive threshold torque Operation range Torque sensing
1 kg /86 63 mm 7.2 N m 25° 10 N m
1.5 g /98 77 mm 15 N m 20° 20 N m
and the passive compliance method to provide collision safety with high reliability. The robot arm constructed of the safe joint modules can detect collisions and react to reduce the collision force. The active threshold torque for the collision detection needs to be set low when the robot arm is operating around humans, but set high for tasks that require heavy payloads. Therefore, both collision safety and high task efficiency can be achieved at the same time. Moreover, the robot arm with the proposed safe joint modules can absorb the collision force when the robot collides with the environment at a high speed, a situation too fast for only the active method to perform a proper reaction. When the safety mechanism is not applied, due to the low bandwidth of the controller and the inertial effect of the robot arm, there will be a time delay before the controller stops or reacts against the collision. During this delay, due to the position controller of the robot, the desired position would be placed inside the environment, causing the robot to continuously push the environment as shown in Fig. 12a. This would induce a contact force between the robot arm and the environment, damaging both the robot and the environment (e.g., human). However, the contact force during the time delay between collision detection and reaction can be absorbed by the safe joint mechanism. Then, after the time delay, the active compliance method would perform an appropriate reaction against the collision. Fig. 12b shows the operation of both safety systems when a collision occurs. Upon a collision, the stiffness of the robot arm decreases abruptly by the passive safety mechanism and it stops completely after the delay by the active compliance method. Thus,
Motor Speed
Stiffness
This paper proposes a novel design of a safe joint module to combine the advantages of both the active compliance method
Rear view
(a) Fig. 10. Structure of proposed safety joint module.
(b)
Fig. 12. Collision of robot arm: (a) without passive compliance method, and (b) with passive and active compliance methods.
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II
Fig. 13. Safety region with respect to the configuration of robot arm.
through the combination of the active and passive compliance methods, the proposed joint module can provide reliable collision safety against various types of collisions. Also, the proposed safe joint module can provide a larger safety region against a collision than when the passive compliance method alone is used. As mentioned, the passive compliance method may fail near singular points. As shown in Fig. 13, inside region I, both the active and passive compliance methods can be applied. On the other hand, inside region II, the passive compliance method alone can no longer ensure human safety since the robot is near its singular points. However, with the proposed safe joint module composed of the active and passive compliance methods, this region is covered by the active compliance method.
Angular position of robot arm Torque (w/o safe joint module) 30 Torque (w/ safe joint module) Tthp (7.1Nm)
8.0 6.0
Commanded position
4.0
Collision
2.0
Motor stop
Robot arm
Safe joint module I
10.0
0 0
10
20
20
10
Actual position 30
Angular Position (deg)
Reachable distance Operational limitation
External torque (Nm)
1028
0
40
Time (s) Fig. 15. Experimental results with static collision to verify nonlinear stiffness characteristic of proposed mechanism.
As shown in Fig. 15, the external torque of the robot arm without the safe joint module increased more than 10 N m due to the high stiffness of the harmonic drive, which corresponds to the contact force between the robot arm and the wall. However, the joint torque of up to only 7.1 N m was generated by the robot arm with the safe joint module, which was similar to the pre-determined passive threshold torque Tthp of 7.2 N m because the excess torque was absorbed by the safe joint module.
5. Experiments To verify collision safety and force control performance of the safe joint module, various experimental results are presented in this section. Section 5.1 verifies the nonlinear stiffness of the safe joint module. Section 5.2 shows the result of the force estimation with the safe joint module in comparison with that with a commercial force sensor. Finally, Section 5.3 discusses the collision safety of a 3-DOF robot arm with and without the safe joint module. 5.1. Performance test of safety mechanism with static collision The experimental setup shown in Fig. 14 was constructed to evaluate the nonlinear stiffness of the safety mechanism. The 1-DOF robot arm consists of the safe joint module, a motor, a link and a belt-pulley connection was prepared. The motor was connected to the input shaft of the safety joint module using a timing belt. Therefore, the motor torque can be transmitted to the robot link via the safety mechanism, which is included in the safe joint module. To press the wall continuously after a collision, the desired position of the robot was placed 20° inside the wall. In the experiments without the safe joint module, only a harmonic drive was used for speed reduction. During this experiment, collision detection was turned off.
Fig. 14. Experimental setup for 1-DOF robot arm with and without the safety joint module.
5.2. Force control with 3-DOF robot arm constructed of three safe joint modules To show that the proposed module can be used for force control, a wall-following experiment was conducted using the 3-DOF robot arm constructed using three safe joint modules. The experimental setup shown in Fig. 16a was constructed to evaluate the force control performance of the robot. A ball caster was attached at the end-effector of the 3-DOF robot arm so that the arm could follow the wall smoothly. The wall was mounted on a linear guide, and springs were installed to provide the stiffness. A commercial F/T sensor was attached on the wall to measure the contact force between the ball caster and the wall. As shown in Fig. 16b, the robot arm followed the wall in two directions as it maintained a certain contact force with the wall. Fig. 17 shows the experimental results for the case when the robot arm was controlled to move in the vertical direction and the horizontal direction at constant contact force of 22 N. As shown in Fig. 17a, the contact force at the wall was kept at 22 N during the wall following in both horizontal and vertical directions. It should be noted that the measured force from the F/T sensor agrees with the estimated force using the three embedded torque sensors
Fig. 16. Experimental setup for force control (wall following): (a) 3-DOF robot arm, and (b) direction of wall following.
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30
Estimated force Measured force
25
Force in y direction (N)
Force in y direction (N)
30
20 15
Force in y direction
10
Wall following (x direction) Contact
5 0 -5
Force in -y direction 0
5
10
15
Estimated force Measured force
25 20 15
Force in y direction
10
Wall following (z direction)
5
Contact
0
Force in -y direction
-5 20
5
0
15
10
Time (s)
20
Time (s)
(a)
(b)
Fig. 17. Experimental results with force control applied to wall following: (a) horizontal direction, and (b) vertical direction.
The experimental setup shown in Fig. 18 was constructed to evaluate the safety achieved by the 3-DOF robot arm. The experiments on a dynamic collision were conducted to investigate the collision safety of the 3-DOF robot arm with and without the safety joint modules. In this experiment, the impactor of the robot arm collided at a constant velocity of 2 m/s. To detect collisions, the active threshold torque Ttha was set to 6 N m for joints 1 and 3, and 14 N m for joint 2, as joint 2 had a longer moment arm and heavier weight. For the reaction strategy, the robot was programmed to stop when the external torque reached the active threshold torque (i.e., when collision was detected). Fig. 19 shows the resulting external torques at joints 2 and 3 of the 3-DOF robot arm when the robot arm collided with the block at a constant velocity of 2 m/s. The external torques exceeded 20 N m and 10 N m, respectively, without the safe joint module. With the safe joint modules, however, the robot arm stopped when the external torque reached 14 N m/6 N m due to the active compliance method. However, a time delay occurred because of the limited bandwidth. Due to this delay, the external torque increased continuously
Impactor
3-DOF safe robot arm
Joint 3 (Type A) Motor Silicon cover Block
Joint 2 (Type B) Joint 1 (Type A) Motor
Fig. 18. Experimental setup for 1-DOF robot arm with and without safety joint module.
Angular pos. (°) External torque (Nm)
5.3. Collision safety with 3-DOF robot arm constructed of three safe joint modules
until the motor stopped. However, using the safe joint module, the safety mechanism inside the module absorbed the external torque during this delay by abruptly dropping its stiffness. The areas B/C shown in Fig. 19 represent the safe zone, which clearly shows that the safety mechanism was able to absorb the impact from the
20 Tthp (15.5 Nm)
15
Ttha (14 Nm)
Collision detection
10 5 0
Collision
External torque
-5 -10 55
w/o safe joint module w/ safe joint module
Motor stop s. ed po mand Com
50 45 0
20
40
g
B Collision Delay 60 80 100
l
120
Time (ms)
(a) Angular pos. (°) External torque (Nm)
installed inside the safe joint modules shown in Fig. 17b, which indicates that force control was performed at high precision with the embedded joint torque sensors.
10 Tthp (7.6 Nm) Ttha (6 Nm)
8 6
Collision 4 detection 2
Collision
External torque
0 -2
w/o safe joint module w/ safe joint module
-4 75
Motor stop pos. ded 70 omman C 65 0
20
40
g
C Collision Delay 60
80
100
l
120
Time (ms)
(b) Fig. 19. Experimental results on dynamic collision with and without safe joint module in 3-DOF robot arm: (a) joint 2, and (b) joint 3.
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collision during the delay of the actuator. Angles hg and hl shown in Fig. 19 are the rotation angles of the input and output parts of the safety mechanism, respectively, which have the same values before the collision due to the high stiffness of the safety mechanism. However, after the collision and the activation of the safety mechanism, the stiffness of the safety mechanism drops abruptly, and hg and hl are no longer the same. This allows the springs to absorb the external torque. Therefore, the maximum torque was limited to 15.5 N m/7.6 N m whereas the pre-determined passive threshold torques Tthp of the safety mechanism were 15 N m/7.2 N m. 6. Conclusion This paper proposed a safe joint module composed of a speed reducer, a torque sensor and a safety mechanism. With the proposed safety joint module, collision safety was guaranteed by the active and passive compliance methods. Therefore, reliable collision safety against various types of collisions was achieved using the safe join module. Based on our analysis and experiments, the following conclusions are drawn: 1. The limited bandwidth of the active compliance method is compensated for by the passive compliance method which can provide a fast response since it consists of purely mechanical elements. 2. Using the active compliance method, a robot can effectively react to collisions. Moreover, the lack of flexibility of the passive compliance method can be compensated by merging it with the active compliance method. 3. The proposed safe joint module enables low-cost force control and easy construction of a safe robot arm.
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Acknowledgments This work was supported by the Center for Autonomous Intelligent Manipulation under Human Resources Development Program for Convergence Robot Specialists (Ministry of Knowledge Economy) (NIPA-2011-C7000-1001-0003) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2012-0000792). References
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Mechatronics journal homepage: www.elsevier.com/locate/mechatronics
Safe joint module for safe robot arm based on passive and active compliance method Hwi-Su Kim, In-Moon Kim, Chang-Nho Cho, Jae-Bok Song ⇑ School of Mechanical Engineering, Korea University, Seoul, Republic of Korea
a r t i c l e
i n f o
Article history: Received 8 September 2011 Accepted 26 August 2012 Available online 15 September 2012 Keywords: Joint module Manipulator design Collision safety Collision absorption
a b s t r a c t In recent years, collision safety between humans and robots has drawn much attention as service robots are being increasingly used in human environments. Since various types of collisions can occur during a robot’s operation, multiple safety methods should be implemented to ensure human safety. The most common and practical solutions are the active and passive compliance methods and each has its own merits and disadvantages. To combine the active and passive compliance methods to achieve higher reliability, we propose a novel safe joint module composed of a speed reducer, a torque sensor and a safety mechanism. The torque sensor embedded in the safe joint module can be used to detect a collision; then the actuator appropriately reacts to minimize the impact. However, if the collision detection by the embedded joint torque sensor and the subsequent reaction fail due to the limited bandwidth of the sensor, the safety mechanism composed of purely mechanical elements such as springs and a cam follower absorbs the collision force. With the proposed safety joint module, the combined active and passive compliance method can ensure collision safety. Moreover, the embedded torque sensor can be used for force control. The embedded harmonic drive can achieve high gear reduction and low backlash. Several experiments on static and dynamic collisions showed that the proposed module can guarantee safety while maintaining good performance. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In recent year, humanoid robots and service robots have drawn a great deal of attention. Since these robots often operate in human environments to help and co-operate with humans, the safety issues related to physical human–robot interaction are becoming increasingly important [1–3]. There are two major steps to achieve collision safety between humans and robots. Collisions can be predicted before the contact occurs so that the robot using non-contact sensors (e.g., a vision camera) can regenerate its path to avoid the collision [3–6]. On the other hand, after the collision, the collision force can be detected by force/torque sensors and the robot arm can either react against the collision or absorb the impact to protect the human. In this research, we focus on the post-collision safety methods. As shown in Fig. 1, there are two strategies to improve collision safety, the active compliance method and the passive compliance method. In an actively compliant arm, a collision is detected by sensors, and the robot arm is controlled to reduce the collision force. The robot arm efficiently handles a collision by recognizing different collision situations and choosing the most appropriate reaction ⇑ Corresponding author. E-mail address: [email protected] (J.-B. Song). 0957-4158/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mechatronics.2012.08.007
strategy. The collision safety of the robot arm has been improved by using active compliance methods: covering the manipulator with skin sensors [7], attaching a six axis force/torque sensor at the end-effector of a manipulator [8], embedding joint torque sensors at each robot joint to estimate the collision force [9], and adjusting the stiffness of a joint by serial-type dual actuator unit [10]. On the other hand, the passive compliance method consists of purely mechanical elements. Since this approach does not use any sensors or actuators, it can provide fast and reliable responses even for a dynamic collision. Several safety mechanisms have been developed using the passive compliance method: the programmable passive compliance shoulder mechanism [11], the mechanical impedance adjuster with a leaf spring and an electromagnetic brake [12], and a passive compliance joint with rotary springs and a MR damper [13]. However, most previous approaches adopted only one of these compliance methods, which alone often could not guarantee collision safety. The active compliance method has limited bandwidth since it involves sensing and actuation in response to a dynamic collision. Moreover, the scheme of sensing and actuation inevitably results in high cost, sensor noise, and possible malfunctions. On the other hand, the passive compliance method activates the safety mechanism too frequently even against a low-speed collision which would not seriously harm a human. Also, the passive compliance
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Active compliance
Passive compliance
Collision detection Force/torque sensor
Collision detection Mechanical device
Safe ?
threshold ?
Fail
No Collision reaction Reflex motion / E-stop
Yes Collision absorption Low stiffness
Collision safety Fig. 1. Safety strategies after collision.
method provides a smaller safe region compared to the active compliance method due to the singularity problem of the robot arm. Since various types of collisions can occur during robot operations, multiple safety methods should be implemented to prevent possible human injury. As an example, collision prediction and path regeneration using a vision camera was combined with Variable Stiffness Transmission (VST) actuator which could observe a collision force based on the passive compliance method [14]. However, prediction of a fast collision requires an expensive high-speed vision system because a normal camera has limited bandwidth. Moreover, the VST decreases the stiffness of a robot arm by actuator control, which results in the reduced bandwidth of the robot arm. This paper proposes a novel design of a safe joint module composed of a speed reducer, a joint torque sensor and a safety mechanism. The safe joint module implements both active and passive compliance methods to provide collision safety with higher reliability. The torque sensor can detect a collision; then the actuator, controlled properly, reacts against the collision. If the torque sensor fails to detect a collision because of its low bandwidth, the safety mechanism absorbs the collision force. Moreover, using the embedded joint torque sensors, the proposed module can measure the external force applied to the robot arm for force control without an expensive force/torque sensor. Furthermore, the modular structure allows the easy construction of various safe robot arms [15–17]. The rest of the paper is organized as follows. The active compliance method using an embedded torque sensor is presented in Section 2. The operation principle and the structure of the safety mechanism are discussed in Section 3. Section 4 shows the design of the safety joint module combining the active and passive compliance methods. Various experimental results on collision safety and force estimation are discussed in Section 5. Finally, the conclusion is presented in Section 6.
(a)
2. Active compliance method with embedded torque sensor As mentioned in Section 1, both force control and collision safety based on the active compliance approach require sensors to detect the contact force. Therefore, a 6-DOF force/torque sensor is often mounted at the wrist of a robot arm, but this sensor is too expensive for practical applications. Moreover, it can detect only the collision at the end-effector. However, a joint torque sensor inside the proposed safe joint module can monitor the external torque and thus, detect collisions regardless of where they occur. When the torque exceeds a certain threshold, the actuator can be properly controlled to react against the collision. In this section, the active compliance method using the embedded torque sensor will be presented. Also, the possibility of using the torque sensor to establish a low-cost force control system will be discussed. 2.1. Design of joint torque sensor The joint torque sensor should be designed for sufficient deformation in response to an external torque for precise measurement. However, a large deformation has an adverse effect on the positioning accuracy of a robot, so there is a tradeoff. In this research, the joint torque sensor has a hub-spoke type structure that provides appropriate stiffness and high sensitivity. As shown in Fig. 2a, the structure of the torque sensor is divided into outer rim, hub and spoke [18]. The several strain gauges attached to the torque sensor measure the external torque. FEM analysis is used to find the best positions to place the strain gauges, which are the positions of the maximum deformation at the spoke, where deformation is multiplied several times. The torque coefficient of the designed torque sensor can be found through experiments. The strain gauge wires are connected to an amplifier though the hollow shaft of the safe joint module as shown in Fig. 2b. 2.2. Force estimation and collision detection using torque sensor When a robot collides with an object, an external torque is induced at each joint of the robot. Thus, by monitoring the external torque, collisions can be successfully detected. The equation of motion of a robot can be described by
_ h_ þ gðhÞ sreq ¼ MðhÞ€h þ Cðh; hÞ
ð1Þ
where h is the joint angle vector, M is the inertia matrix, C is the matrix involving the Coriolis and centrifugal terms, g is the gravity vector, and sreq is the torque required for the actuators to perform the desired motion. In case of a collision, the torque due to the dynamics of the robot, sreq, and the torque due to the collision, sext are applied to each torque sensor. To extract sext from the sensor data, sreq must be subtracted from the torque sensor reading, so that
(b)
Fig. 2. (a) Structure of embedded torque sensor, and (b) installation of torque sensor.
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Fig. 5. Procedure for the active compliance method.
(a)
(b)
Fig. 3. Multi-DOF robot arm with several numbers of safety joint modules used to estimate external force: (a) 3-DOF, and (b) 6-DOF arm.
Fig. 4. Block diagram of natural admittance controller.
sext ¼ sm sreq
ð2Þ
where sm is the measured torque vector by the joint torque sensors. When sext exceeds the pre-defined active threshold torque, a collision is detected. From sext, the location, magnitude, and orientation of the collision can be found. With this information, the robot analyzes the collision and selects the most suitable reaction strategy, either an emergency stop or a move-back motion. The threshold must be set appropriately to avoid possible malfunctions. Furthermore, during a heavy-payload operation or a force control operation, increasing the threshold would allow the robot to detect collisions as it performs the desired task. Also, using the joint torque sensors, a low-cost force control system can be established. Using Eq. (2), the force due to the external torque at the end effector can be computed by
F ¼ ðJ T Þ1 sext
ð3Þ
As shown in Fig. 3, the forces Fx, Fy and Fz can be obtained by using three safety joint modules. Six joint modules would provide all three forces and three moments. Thus, depending on the application, an appropriate number of the safety joint modules would allow the robot to perform force control and collision detection at the same time. In this paper, an admittance control scheme, which is another form of impedance control, is used for the 3-DOF robot arm equipped with three safe joint modules. Fig. 4 shows the controller, where xd and Fd are the desired position and force, xc and xt are the compensated position and the target position represented in the task space, respectively. The variables ht and st are the target angle and torque represented in the joint space, respectively, and the robot position can be obtained by solving the inverse kinematics of the robot arm. The admittance filter Y(s), which represents the ratio of the velocity to the force in the s-domain, can be described by
YðsÞ ¼
V c ðsÞ 1 ¼ F d ðsÞ FðsÞ B þ k=s
ð4Þ
where B is the damping coefficient and k is the stiffness [19]. Therefore, the proper admittance filter can be selected experimentally by adjusting these values. If the external torque estimated from
Eqs. (1) and (2) is not identified as a collision, the actual contact force in the task space can be computed from the external torque by the Jacobian relation. In this admittance control scheme, the external force estimated from joint torque data is converted to the robot position information through the force controller. This position information is used to provide the desired impedance effect (i.e., spring and damper effects), while the robot arm moves to the target position in joint space through the position controller. The inner-position/outer-force control scheme is advantageous for impedance control of the robot arm that is based on a position controller [20]. On the other hand, if the external torque is identified as a collision by the collision detector shown in Fig. 5, the active compliance method is applied to control the robot arm. Fig. 5 shows the procedure for the active compliance method, where a is the collision index. During a normal operation, the index is set to 0, and the position controller controls the robot to follow a pre-defined path. However, when a collision is detected, the index is switched to 1, which switches the controller from the task planner to the collision reactor. Then, the robot generates the proper reaction motion and delivers it to the position controller which directs the robot to perform the safety motion. However, the robot arm usually cannot react quickly against a collision due to the limited bandwidth of the control system and torque sensor. Each control scheme requires a certain computation time, and thus a time delay is inevitable [21]. Furthermore, this time delay directly depends on the performance of the controller, and thus an expensive controller is required to improve collision detection and reaction time. 3. Passive compliance method with safety mechanism based on nonlinear stiffness To deal with the limitations of the active compliance method, a passive compliance device, which consists of purely mechanical elements such as springs and cams, was proposed. A spring is the most popular mechanical element for shock absorption. However, a spring used at a robot arm joint leads to positioning inaccuracy because it significantly lowers the stiffness of the joint. Although a hard spring can provide high positioning accuracy for a robot arm, it has much lower shock absorbability than a soft spring, increasing the probability of human injury upon collision. To achieve both collision safety and positioning accuracy, the safety mechanism based on nonlinear stiffness was proposed and installed inside of the safe joint module. In this section, the passive compliance method for collision absorption using the safety mechanism is presented. 3.1. Design of safe mechanism to realize nonlinear stiffness Nonlinear stiffness for both positioning accuracy and collision safety of a robot arm is difficult to realize using only linear springs. Therefore, in our previous research, nonlinear stiffness was realized by a safety mechanism composed of a double-slider mechanism and linear springs [22,23]. This safety mechanism maintains
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However, the increased spring force, due to compression, is not large enough to sustain this balance. This unbalanced state causes the slider to rapidly slide, and thus the collision force is absorbed by the spring via its compression. Thus, the stiffness of the safety mechanism abruptly drops. From Eq. (5), the passive threshold torque can be described by
T th ¼
(a)
(b)
Fig. 6. Safety mechanism based on cam and cam-follower mechanism: (a) structure, and (b) operation (after collision).
¼
(b)
Fig. 7. Cam and cam-follower mechanism for nonlinear stiffness: (a) zero configuration, and (b) general configuration.
very high stiffness for positioning accuracy up to a preset threshold torque, but for collision safety, the stiffness drops abruptly when the torque exceeds the threshold. Note that upon a collision, an external torque less than this threshold does not cause serious human injury. Moreover, after the collision force is removed, the safety mechanism and robot arm return to their initial states because of the stored spring energy. This paper presents a new safety mechanism shown in Fig. 6, which can be installed much easily than the previous safety mechanism at each joint of a robot. The proposed safety mechanism consists of a cam, a cam follower, and linear springs pre-compressed to achieve the nonlinear spring characteristics. To explain the principle of nonlinear stiffness, a close-up view of area A in Fig. 6 is shown is Fig. 7. The input torque Tin exerted on the cam rotates the cam and applies a corresponding force F0 to the output slider. However, due to the spring force by spring compression, the output slider does not move. If the input torque Tin from the external load is smaller than the predetermined passive threshold torque, the safety mechanism would provide enough stiffness to maintain static equilibrium, which prevents the rotation of the link. In other words, the resisting force Fr due to spring compression, and the x-component of F0 by Tin are in equilibrium. From the equation of closure of the mechanism, the relationship between Tin and Fr is given by
cosð270 hÞ T in Fr ¼ c
ð5Þ
where the parameters c and h are defined in Fig. 7, and c and h are related by c = 270° h. However, if the input torque exceeds the passive threshold torque, then the cam plate turns around point P as shown in Fig. 6b and the spring block (output slider in Fig. 7) is forced to move along the spring guide to compress the spring. This movement of the output slider reduces the transmission angle, so sustaining the static balance requires a greater resisting force for the same input force.
kso
ð6Þ
where the subscript o represents the zero configuration, so is the spring compression at the zero configuration, k is the spring constant, and l is the friction coefficient. From Eq. (5), the equivalent stiffness, keq seen from the input shaft can be obtained as a function of c as follows:
keq ¼
(a)
co
l cosð270 ho Þ
T in ks ¼ Dh h h o
c
l cos c
k l so þ bo ða tan cÞ ðh ho Þl cos c cos c a l tan c cos c
ð7Þ
where s is the spring compression, a, b and l are defined in Fig. 7. Fig. 8 shows an equivalent stiffness curve as a function of angular displacement of the input shaft when k = 5.2 kN/m, a = 14.5 mm, bo = 19.4 mm, co = 22 mm, lo = 10.2 mm, so = 12 mm, ho = 192° and l = 0.9. The equivalent stiffness keq of the nonlinear system remains very high for a small angular displacement of the input slider, but quickly drops as the displacement increases. Hence, a nonlinear stiffness system is realized by the cam and cam-follower mechanism combined with a pre-compressed linear spring. 3.2. Limitation of passive compliance method Although the passive compliance method can provide fast response against a high-speed collision since it only consists of purely mechanical elements, this method also has limitations in reacting against various types of collisions. First of all, the passive threshold of the safety mechanism cannot be changed once the mechanism is built, although it is desirable to have the ability to change the threshold during a task such as a human–robot co-operative task, lowering it to prevent any human injury, but increasing it for a high-payload task. Furthermore, frequent activation of the safety mechanism would significantly decrease the performance of the robot. Moreover, collision safety near the singularity of the robot arm can be a serious problem with the passive compliance method having a fixed threshold torque. The threshold torque should be lowered to ensure collision safety when the robot arm collides with a human near its singular position since high collision force yields only a small torque at each joint near singular points. Second, the operational angle of the safety mechanism is limited due to the kinematic constraints, such as the spring compression
100 80
Nonlinear spring
60 40
Linear spring
20 0
0
2
4
6
8
10
Δ (°) Fig. 8. Nonlinear stiffness using cam and cam-follower mechanism.
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Operational angle
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Front view
(a)
(b)
Fig. 9. (a) Operational angle of safety mechanism, and (b) available range of compression spring.
length as shown in Fig. 9. Excessive compression of the spring may permanently deform the spring and thus changes the threshold of the safety mechanism. If the mechanism fails to stop or withdraw the robot within its operational range, a follow-up collision may occur, which would significantly harm a human. To effectively protect human from collisions, therefore, the robot must be able to detect the collision when the safety mechanism is activated so that it can perform appropriate reactions. 4. Safe joint module to combine active and passive compliance methods 4.1. Design of safe joint module The safe joint module, which consists of a gear reducer, a safety mechanism, a torque sensor, cross-roller bearings, and a hollow shaft, was constructed as shown in Fig. 10. A component type harmonic drive was used for speed reduction, low backlash and lightweight design. The safety mechanism and torque sensors were designed such that they could all fit into the safe joint module. A cross-roller bearing can support the moment load applied to the torque sensor so that only the rotational torque is applied to the torque sensor. A hollow shaft was used to get the wires through the safety joint module. Every casing and plate were made of duralumin to minimize the weight of the safety joint module. Prototypes of the safe joint mechanism were constructed and their performances were evaluated, as shown in Fig. 11. Two types of safety modules were constructed depending on the passive threshold torque and size (Types A and B). The specifications of the safe joint modules are listed in Table 1. 4.2. Combination of active and passive compliance method
Fig. 11. Prototype of safe joint module.
Table 1 Specifications of safe joint module. Type
A
B
Weight Dimension Passive threshold torque Operation range Torque sensing
1 kg /86 63 mm 7.2 N m 25° 10 N m
1.5 g /98 77 mm 15 N m 20° 20 N m
and the passive compliance method to provide collision safety with high reliability. The robot arm constructed of the safe joint modules can detect collisions and react to reduce the collision force. The active threshold torque for the collision detection needs to be set low when the robot arm is operating around humans, but set high for tasks that require heavy payloads. Therefore, both collision safety and high task efficiency can be achieved at the same time. Moreover, the robot arm with the proposed safe joint modules can absorb the collision force when the robot collides with the environment at a high speed, a situation too fast for only the active method to perform a proper reaction. When the safety mechanism is not applied, due to the low bandwidth of the controller and the inertial effect of the robot arm, there will be a time delay before the controller stops or reacts against the collision. During this delay, due to the position controller of the robot, the desired position would be placed inside the environment, causing the robot to continuously push the environment as shown in Fig. 12a. This would induce a contact force between the robot arm and the environment, damaging both the robot and the environment (e.g., human). However, the contact force during the time delay between collision detection and reaction can be absorbed by the safe joint mechanism. Then, after the time delay, the active compliance method would perform an appropriate reaction against the collision. Fig. 12b shows the operation of both safety systems when a collision occurs. Upon a collision, the stiffness of the robot arm decreases abruptly by the passive safety mechanism and it stops completely after the delay by the active compliance method. Thus,
Motor Speed
Stiffness
This paper proposes a novel design of a safe joint module to combine the advantages of both the active compliance method
Rear view
(a) Fig. 10. Structure of proposed safety joint module.
(b)
Fig. 12. Collision of robot arm: (a) without passive compliance method, and (b) with passive and active compliance methods.
H.-S. Kim et al. / Mechatronics 22 (2012) 1023–1030
II
Fig. 13. Safety region with respect to the configuration of robot arm.
through the combination of the active and passive compliance methods, the proposed joint module can provide reliable collision safety against various types of collisions. Also, the proposed safe joint module can provide a larger safety region against a collision than when the passive compliance method alone is used. As mentioned, the passive compliance method may fail near singular points. As shown in Fig. 13, inside region I, both the active and passive compliance methods can be applied. On the other hand, inside region II, the passive compliance method alone can no longer ensure human safety since the robot is near its singular points. However, with the proposed safe joint module composed of the active and passive compliance methods, this region is covered by the active compliance method.
Angular position of robot arm Torque (w/o safe joint module) 30 Torque (w/ safe joint module) Tthp (7.1Nm)
8.0 6.0
Commanded position
4.0
Collision
2.0
Motor stop
Robot arm
Safe joint module I
10.0
0 0
10
20
20
10
Actual position 30
Angular Position (deg)
Reachable distance Operational limitation
External torque (Nm)
1028
0
40
Time (s) Fig. 15. Experimental results with static collision to verify nonlinear stiffness characteristic of proposed mechanism.
As shown in Fig. 15, the external torque of the robot arm without the safe joint module increased more than 10 N m due to the high stiffness of the harmonic drive, which corresponds to the contact force between the robot arm and the wall. However, the joint torque of up to only 7.1 N m was generated by the robot arm with the safe joint module, which was similar to the pre-determined passive threshold torque Tthp of 7.2 N m because the excess torque was absorbed by the safe joint module.
5. Experiments To verify collision safety and force control performance of the safe joint module, various experimental results are presented in this section. Section 5.1 verifies the nonlinear stiffness of the safe joint module. Section 5.2 shows the result of the force estimation with the safe joint module in comparison with that with a commercial force sensor. Finally, Section 5.3 discusses the collision safety of a 3-DOF robot arm with and without the safe joint module. 5.1. Performance test of safety mechanism with static collision The experimental setup shown in Fig. 14 was constructed to evaluate the nonlinear stiffness of the safety mechanism. The 1-DOF robot arm consists of the safe joint module, a motor, a link and a belt-pulley connection was prepared. The motor was connected to the input shaft of the safety joint module using a timing belt. Therefore, the motor torque can be transmitted to the robot link via the safety mechanism, which is included in the safe joint module. To press the wall continuously after a collision, the desired position of the robot was placed 20° inside the wall. In the experiments without the safe joint module, only a harmonic drive was used for speed reduction. During this experiment, collision detection was turned off.
Fig. 14. Experimental setup for 1-DOF robot arm with and without the safety joint module.
5.2. Force control with 3-DOF robot arm constructed of three safe joint modules To show that the proposed module can be used for force control, a wall-following experiment was conducted using the 3-DOF robot arm constructed using three safe joint modules. The experimental setup shown in Fig. 16a was constructed to evaluate the force control performance of the robot. A ball caster was attached at the end-effector of the 3-DOF robot arm so that the arm could follow the wall smoothly. The wall was mounted on a linear guide, and springs were installed to provide the stiffness. A commercial F/T sensor was attached on the wall to measure the contact force between the ball caster and the wall. As shown in Fig. 16b, the robot arm followed the wall in two directions as it maintained a certain contact force with the wall. Fig. 17 shows the experimental results for the case when the robot arm was controlled to move in the vertical direction and the horizontal direction at constant contact force of 22 N. As shown in Fig. 17a, the contact force at the wall was kept at 22 N during the wall following in both horizontal and vertical directions. It should be noted that the measured force from the F/T sensor agrees with the estimated force using the three embedded torque sensors
Fig. 16. Experimental setup for force control (wall following): (a) 3-DOF robot arm, and (b) direction of wall following.
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30
Estimated force Measured force
25
Force in y direction (N)
Force in y direction (N)
30
20 15
Force in y direction
10
Wall following (x direction) Contact
5 0 -5
Force in -y direction 0
5
10
15
Estimated force Measured force
25 20 15
Force in y direction
10
Wall following (z direction)
5
Contact
0
Force in -y direction
-5 20
5
0
15
10
Time (s)
20
Time (s)
(a)
(b)
Fig. 17. Experimental results with force control applied to wall following: (a) horizontal direction, and (b) vertical direction.
The experimental setup shown in Fig. 18 was constructed to evaluate the safety achieved by the 3-DOF robot arm. The experiments on a dynamic collision were conducted to investigate the collision safety of the 3-DOF robot arm with and without the safety joint modules. In this experiment, the impactor of the robot arm collided at a constant velocity of 2 m/s. To detect collisions, the active threshold torque Ttha was set to 6 N m for joints 1 and 3, and 14 N m for joint 2, as joint 2 had a longer moment arm and heavier weight. For the reaction strategy, the robot was programmed to stop when the external torque reached the active threshold torque (i.e., when collision was detected). Fig. 19 shows the resulting external torques at joints 2 and 3 of the 3-DOF robot arm when the robot arm collided with the block at a constant velocity of 2 m/s. The external torques exceeded 20 N m and 10 N m, respectively, without the safe joint module. With the safe joint modules, however, the robot arm stopped when the external torque reached 14 N m/6 N m due to the active compliance method. However, a time delay occurred because of the limited bandwidth. Due to this delay, the external torque increased continuously
Impactor
3-DOF safe robot arm
Joint 3 (Type A) Motor Silicon cover Block
Joint 2 (Type B) Joint 1 (Type A) Motor
Fig. 18. Experimental setup for 1-DOF robot arm with and without safety joint module.
Angular pos. (°) External torque (Nm)
5.3. Collision safety with 3-DOF robot arm constructed of three safe joint modules
until the motor stopped. However, using the safe joint module, the safety mechanism inside the module absorbed the external torque during this delay by abruptly dropping its stiffness. The areas B/C shown in Fig. 19 represent the safe zone, which clearly shows that the safety mechanism was able to absorb the impact from the
20 Tthp (15.5 Nm)
15
Ttha (14 Nm)
Collision detection
10 5 0
Collision
External torque
-5 -10 55
w/o safe joint module w/ safe joint module
Motor stop s. ed po mand Com
50 45 0
20
40
g
B Collision Delay 60 80 100
l
120
Time (ms)
(a) Angular pos. (°) External torque (Nm)
installed inside the safe joint modules shown in Fig. 17b, which indicates that force control was performed at high precision with the embedded joint torque sensors.
10 Tthp (7.6 Nm) Ttha (6 Nm)
8 6
Collision 4 detection 2
Collision
External torque
0 -2
w/o safe joint module w/ safe joint module
-4 75
Motor stop pos. ded 70 omman C 65 0
20
40
g
C Collision Delay 60
80
100
l
120
Time (ms)
(b) Fig. 19. Experimental results on dynamic collision with and without safe joint module in 3-DOF robot arm: (a) joint 2, and (b) joint 3.
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collision during the delay of the actuator. Angles hg and hl shown in Fig. 19 are the rotation angles of the input and output parts of the safety mechanism, respectively, which have the same values before the collision due to the high stiffness of the safety mechanism. However, after the collision and the activation of the safety mechanism, the stiffness of the safety mechanism drops abruptly, and hg and hl are no longer the same. This allows the springs to absorb the external torque. Therefore, the maximum torque was limited to 15.5 N m/7.6 N m whereas the pre-determined passive threshold torques Tthp of the safety mechanism were 15 N m/7.2 N m. 6. Conclusion This paper proposed a safe joint module composed of a speed reducer, a torque sensor and a safety mechanism. With the proposed safety joint module, collision safety was guaranteed by the active and passive compliance methods. Therefore, reliable collision safety against various types of collisions was achieved using the safe join module. Based on our analysis and experiments, the following conclusions are drawn: 1. The limited bandwidth of the active compliance method is compensated for by the passive compliance method which can provide a fast response since it consists of purely mechanical elements. 2. Using the active compliance method, a robot can effectively react to collisions. Moreover, the lack of flexibility of the passive compliance method can be compensated by merging it with the active compliance method. 3. The proposed safe joint module enables low-cost force control and easy construction of a safe robot arm.
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Acknowledgments This work was supported by the Center for Autonomous Intelligent Manipulation under Human Resources Development Program for Convergence Robot Specialists (Ministry of Knowledge Economy) (NIPA-2011-C7000-1001-0003) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2012-0000792). References
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