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Humanoid Robot: A Machine That Walks
Humanoid Robot: A Machine That Walks Jun Ho Oh KAIST (Korea Advanced Institute of Science and Technology)
Abstract: World widely, various types of humanoid robots are being on the research, and already became well acquainted with human beings. Also, many people expect these robots will be living together with mankind at work or home in the foreseeable future. Nevertheless, many researchers yet have a pessimistic point of view about the ultimate form of humanoid robot would be impossible to be realized in the real at the technology view. Moreover, they insist that it probably be less practical than other forms of robots. Then, the dilemmas come to us with these questions; "why do we study about the humanoid and why should we do?" "And how would the biped walking be made and what kind of tantalizing problems do we have to face for it?" These issues will be presented throughout this article. Copyright © 2006 IFAC Keywords: Humanoid robot, biped walking
1. WHY IS THE BIPEDAL WALKING SO IMPORTANT? Most of mobile robots, including service robots, are wheel type now in the world. These robots would be categorized into two groups. One is the low and flatbottomed mobile platform, usually used as the industrial mobile robot. This form insures the best stability of robot with very big kinematic stability margin due to very low center of mass. However, it is improper to be used as a service robot since the broad bottom occupies too much area and the body frame is too low. The other is the column style robots, which have from 1m to 1.5m high, mostly use for guidance or home-care. Why it was made to be relatively high, even sacrificing the kinematic stability, is easy to get along with human being's life style. Then, which ratio of the bottom length to the height of the robot would be proper? Figure 1 shows an instance of human. The ratio of the foot length to the stature is about 1:6. Based on this fact, the proper size of the service robot would be 40cm of base radius and from 1m to 1.2m high in practical way of use in the real. However, this kind of mobile platform has a big problem, the lack of stability, innately. Shown in figure x, it is impossible to accelerate or decelerate over 1/4g since the stability angle margin is only tan(b/h), also it is quite anticipated that the robot would fall over a ramp or a small doorsill. Therefore, this type of mobile robot is used very slowly only on the flat floor. To solve this
problem, the active control method has to be applied for securing dynamic stability. Unfortunately, it is almost impossible or too hard to apply the dynamic stability to mobile platforms of any tri, quad wheel or caterpillar, which already got secured the static stability. The bi-wheel mobile platform such as a Segway was proposed to solve this dilemma. While the Segway is statically unstable, it could be accelerated or decelerated unlimitedly as long as the dynamic stability is secured with proper control methods, then it could recovery the stability from any ramp, doorsill, or some pit. Still, this case has a weak point. Like other wheel-types, it cannot be omnidirectional and ascend the stairs. Somewhat high doorsill also be unchallengeable. Then, what can be any alternative proposal? The biped walking is the key to solve all above matters. It is statically stable (obviously, stability margin is very small), also can secure the dynamic stability through the control methods. Omni-directional movement is absolutely possible. Moreover, any stairs, doorsills, or slope cannot obstruct the movement. Especially, it has the best suitable structure and outfit in the human's living environment. 2. THE BIPED WALKING ROBOT Walking is the repetition of processes, to step forward and to land the lifted foot, moving the center of mass to each foot alternatively. At this time,
2
people shift the center of mass unconsciously, which was educated for so long. These walking patterns are believed to be programmed hereditarily from when the mankind started to walk erect in the early. It shows that the walking, complex stabilizing motion is not the education from the zero which has been applied from the birth. On the contrary, it is extremely intelligent ability represented as kinematically and dynamically being optimized throughout long term evolution and most of its part is originally. Thus, to walk like human being, the robot has to achieve the skills and information as much as the ones that human has achieved from the evolution of walking erect. However, these kinds of biological evolution process are hard to understand yet, the muscular or skeletal structure as a completion, even the methods of walking are not yet defined clearly. Human consists of about 650 muscles, 206 bones, and around 100 joints. In addition, hip joints related to the walking guarantee free motions, such as rotation, lift, and stretch, as a ball joint. Ankles and feet are also very complex structures combined with joints, ligaments, and muscles organically for interaction with various circumstances. Especially, a big toe takes an important roll in walking as an independent joint which produces dynamic walking motions minimizing energy consumption. But it is impossible to make the humanoid robot into such human organism through present technical methods of any mechanical, electric-electronical, or computer engineering methods. Furthermore, it is incongruent in the practical way. Therefore, minimized kinematical structures were selected for copying the human's motions at the humanoid robots in the real. First of all, the degree of freedom (DOF) is determined by minimized independent movements and actuation for the biped walking. Each leg has to get structured with minimum 6 DOF for the walking. Shown in figure 1, there are 3 DOF at the hip joint, 1 DOF at the knee, 2 DOF at the ankle. For more freely movement, the rotation would be added at the ankle and the big toe can be added at the foot, those are neglected in most of cases. Figure 1 shows the case of the HUBO. The standpoints of a robot design for the biped walking are abbreviated to two points commonly. One is the minimizing the backlash kinematically, and the other is minimizing the weight as long as structure stiffness is maintained. The harmonic driven reducer is usually chosen at humanoid robots which is over 1m high for reducing uncertainty by backlash. It has weak points at the stiffness of gears, durability, and friction yet; there is no such substitution in the high deceleration, zero backlash, compact size, and various types of standard products at this moment. The structure of robots should be designed to have box type legs to maintain the high stiffness, increasing the inertia moment against its weight. Every joint should guarantee the stiffness with a both ends support; avoid a cantilever type (one end support). Particularly, joints must be designed delicately so as not to be loose by clearances of bearings. The biped walking is the combination of two big steps. The first one is making standard pathway of walking using legs, ankles, arms along the pre-
planed patterns (named "the gait pattern design"), the other is maintaining the dynamic stability through continuous delicate movements of each part of the body at every single time (named "the postural stabilization"). The latter is conducted or calculated at on-line as a real-time control during the walking, and the former is conducted at off-line as a batch process before the walking. 3. FORMULATION OF THE GAIT PATTERN DESIGN For walking robots, the gait pattern design is the most fundamental part which needs much effort and time. In biped walking, every single joint is preprogrammed for the position of actuators along the planed gait patter design so can repeat the programmed motion during walking. At gait pattern design, patterns of every joint motion is designed for the zero moment point (ZMP) always being in the landed foot during a period of walking. For the first step, the ZMP is shifted to into the left foot from a phase supported by two legs, then the right foot is lifted and stretched forward. When the lifted foot is moved to a desired stride, it will be landed. The landing is done, so both feet are located diagonally for supporting the body together, and then the ZMP is shifted to the right foot. Next, the left foot is lifted to step forward in the desired stride location. Defining this pattern design, the ZMP of inside of foot has always changed continuously depends on the motion of the lifted foot, so the location of the ZMP must be controlled to be in the supporting foot by appropriately planning the motion of upper body. If the ZMP get reached the boundary of the supporting foot with the wrong pattern design, the robot would fall down. There are two methods used for completing the pattern design. One is the experimental way, and the other is using a computer simulation. This needs the equations of motions with the mathematical modeling of entire robot kinematics, and that needs the real robot model. The pattern design is not achieved automatically from either case. The pattern maintaining the walking without any falling down only be gotten by a lot of trial-and-error. In the toy robot actuated by a mechanical spring and also be quite cheap, the ZMP location is always be in the supporting feet so the walking can be made enough with simple path planning. At the small walking robot, the ZMP is almost same as the projected point of the statically center of mass at the surface so the way of generating the patterns experimentally where the point supports the statically center of mass at every steps. However, this simple way cannot be applied for bigger robots which are over 80cm high. Various changes of the ZMP by the motion throughout every single time cannot be understandable instinctively. Moreover, only way to predict the position of the ZMP is the computer simulation at the wilder motion cases such as kicking the ball. As the robot is bigger and heavier, mechanical damages and defects occur more and heavier, the pattern designs of wilder motions of
3
these robots only experimentally are followed by several dangerous factors. 4. UNCERTAINTY Although the satisfied results are achieved through defining the pattern design by experiments or computer simulations, the question of reconstruction follows at experiments after the results. Namely, the robot cannot walk satisfactorily along the planned path even though the defined above patterns are applied. The first reason of it is uncertainty by the backlash. Actuators of robots are usually electric motors combined with the reducers, oil or air pressure systems. The backlash exists always at any instance. If the very precise reducers or joints are used, looseness could be negligible but too expensive and heavy; besides, there is still a disadvantage that inner friction deteriorates the transmitting efficiency. Especially in an open structure of series links like humanoid robots, the error is accumulated more and more, therefore, backlashes and frictions at lower body joints (i.e. knees, an waist) where decide most of the walking pattern, take a important roll in repetition and accuracy of the biped walking system in which the precise and repeated actuation is needed. The second uncertainty is the friction and uniformity of ground. If the slope of ground is in the range of ±2°, the ground is considered as "flat" to people. The case in which the sole of the foot slips due to lack of friction with the ground is the same. That makes the robot's landing unstable and the accurately designed path plan meaningless. Because of a characteristic of repeated motions of the walking, the disturbed last posture of the one step (it usually become the initial condition of the next step) would make the next step unstable and disordered. The last reason of it is vibration and sag by elasticity of a structure which was not considered at the pattern design process. The structure of the robots are usually very complex and every joints are connected each other as a series, so is very vulnerable form of structure. In most of cases, it is impossible or very hard to consider these factors at the process of mathematical modeling. On this account, there could be a trouble of reconstruction though pre-planned pattern design is applied. 5. THERE ARE NO ARTIFICIAL MUSCLES The muscle of human being is the best ideal actuator as we know. 1) It can be a force source. 2) Range of the force can be from very small to very large. 3) Back drive is possible. 4) Muscle can generate the big force for its size and weight. 5) There are no friction and backlash. 6) Muscle is a linear actuator. 7) Inertial force of itself is very small.
8) It is elastic with ligaments, which can store and restore energy. etc. As one satisfies all these conditions, we can call it "the artificial muscle." As a matter of fact, this is only ideal case, namely, still it is impossible to be true as research and development. If the direct drive torque motor is used at joints, it operates as a force source and has many advantages while it does not generate enough moment yet, and also it is too large and heavy, so electric motors combined with reducers are used in most of humanoid robots. However, it has very different characteristic from human's muscles. Electric motors with reducers is position source or velocity source, and hard to generate inverse actuation, and it also has very big rotational inertial force with high ratio of reduction. These differences make difficult to move robots like human beings. The robot cannot loosen the force because inverse actuation is impossible. Energy consumption is too large when the direction and magnitude of the movement of joints changes rapidly like walking due to big inertial force of itself. And then, kinematic energy cannot be restored due to no elasticity. There is no adaptation when collision happens at landing, or bump. Consequently, maintaining of immovable posture of robots is the simplest and most natural issue. Think about hypothetically human do not move at all for 10 hours in a posture of horse riding. It can be an extreme drudge, almost impossible to do for human beings. By contrast, the relaxing and leaning on external force (easiest task for human) is one of the hardest tasks for robots. 6. THE POSTURAL STABILIZATION Natural walking is not only completed by repeated motions through the pattern design because there are various forms of uncertainty exist in the robots. Unevenness of ground is the biggest factor which infects reconstruction of the pattern design. There are two methods used for proper control of this. One is the compensation method of patterns with measuring a slope of ground by a tilt sensor at the sole of the foot, and the other is maintaining the center of body using accelerometer signals (or inclination sensors) with an attached rate gyro. It is known that every developer is using their own methods for detail control algorithms and signal processes, and their papers and patents only include general aspects. In the case of friction, if the slippery happens, then the patterns should be modified to prevent too much moment or landing force exerted on the foot. Sag of structure by elasticity could be calculated through repeated computer simulations. The robot can maintain stable and natural posture compensating with calculated sag. Robots have weak stiffness and almost no attenuation in structures. In accordance with this, there are too much residual vibration happens by the motion of arms/legs during landing. Especially, the vibration of the lifted leg could cause disturbance to landing position and timing, furthermore, affect
4
7. ADDITIONAL REMARKS
entire stability of the walking. This kind of vibration is controlled by active diminishment using signals of a rate gyro and accelerometer attached at legs and arms. And also, a force-moment sensor, which measures moment and contact force of the landed foot, and a inertia sensor, which measures inclination of the body and angular velocity, are necessary to control the posture. According to the achieved information, the landing position and timing will be calculated. And this information also be used for maintaining stability continuously, preventing from falling down or vibrating of one leg supported or two legs supported. The postural stabilization is an essential element guaranteeing stability of repeated walking along with circumstance changes. For making simpler algorithm and sensor systems, the postural stabilization is usually omitted in small size robots. As a result, it shows very unstable walking motions. Figure 2 shows the brief flow chart of posture control algorithm of HUBO.
More innovative theory, technology and idea beyond present technical limit are needed to appear more stable and safer walking robots which are more alike human beings than now. The research about the pattern design or the postural stabilization, using the self learning theory based on human neuroscience, has been conducted constantly to aim for the better control algorithm which is simpler and more humanfriendly. And essential technology for next generation humanoid biped robots has also been carried out such as actuators like the artificial muscle, light and strong materials or structures, small batteries of mass storage, high speed central process unit consuming low energy, more delicate sensor systems, artificial intelligent algorithm, and so on. If these theory or more innovative methods are completed and applied successfully, I may predict that the more natural, safer, and various walking and motions can be embodied.
Switch 1100
Doorknob 900 Kitchen Sink 830
Table 700
Coffee Table 500
Stairs 180
Figure 1. Service robot in human life environment. Units are in mm. Drawing is modified from Honda’s Homepage Walking Parameters Setting
Torso Roll & Pitch Controller
Walking Type Selection
Pelvis Swing Amp. Controller
Walking Pattern Generation
Landing Timing Controller Inverse Kinematics Ref. Position
PD controller
Upright Pose Controller
Damping controller
ROBOT
Landing Orientation controller
M om ent at Foot
Force at Foot Position
Landing D etection Attitude of Torso
( Walking control algorithm )
Figure 2. Flow chart of the walking control
5
Abstract: World widely, various types of humanoid robots are being on the research, and already became well acquainted with human beings. Also, many people expect these robots will be living together with mankind at work or home in the foreseeable future. Nevertheless, many researchers yet have a pessimistic point of view about the ultimate form of humanoid robot would be impossible to be realized in the real at the technology view. Moreover, they insist that it probably be less practical than other forms of robots. Then, the dilemmas come to us with these questions; "why do we study about the humanoid and why should we do?" "And how would the biped walking be made and what kind of tantalizing problems do we have to face for it?" These issues will be presented throughout this article. Copyright © 2006 IFAC Keywords: Humanoid robot, biped walking
1. WHY IS THE BIPEDAL WALKING SO IMPORTANT? Most of mobile robots, including service robots, are wheel type now in the world. These robots would be categorized into two groups. One is the low and flatbottomed mobile platform, usually used as the industrial mobile robot. This form insures the best stability of robot with very big kinematic stability margin due to very low center of mass. However, it is improper to be used as a service robot since the broad bottom occupies too much area and the body frame is too low. The other is the column style robots, which have from 1m to 1.5m high, mostly use for guidance or home-care. Why it was made to be relatively high, even sacrificing the kinematic stability, is easy to get along with human being's life style. Then, which ratio of the bottom length to the height of the robot would be proper? Figure 1 shows an instance of human. The ratio of the foot length to the stature is about 1:6. Based on this fact, the proper size of the service robot would be 40cm of base radius and from 1m to 1.2m high in practical way of use in the real. However, this kind of mobile platform has a big problem, the lack of stability, innately. Shown in figure x, it is impossible to accelerate or decelerate over 1/4g since the stability angle margin is only tan(b/h), also it is quite anticipated that the robot would fall over a ramp or a small doorsill. Therefore, this type of mobile robot is used very slowly only on the flat floor. To solve this
problem, the active control method has to be applied for securing dynamic stability. Unfortunately, it is almost impossible or too hard to apply the dynamic stability to mobile platforms of any tri, quad wheel or caterpillar, which already got secured the static stability. The bi-wheel mobile platform such as a Segway was proposed to solve this dilemma. While the Segway is statically unstable, it could be accelerated or decelerated unlimitedly as long as the dynamic stability is secured with proper control methods, then it could recovery the stability from any ramp, doorsill, or some pit. Still, this case has a weak point. Like other wheel-types, it cannot be omnidirectional and ascend the stairs. Somewhat high doorsill also be unchallengeable. Then, what can be any alternative proposal? The biped walking is the key to solve all above matters. It is statically stable (obviously, stability margin is very small), also can secure the dynamic stability through the control methods. Omni-directional movement is absolutely possible. Moreover, any stairs, doorsills, or slope cannot obstruct the movement. Especially, it has the best suitable structure and outfit in the human's living environment. 2. THE BIPED WALKING ROBOT Walking is the repetition of processes, to step forward and to land the lifted foot, moving the center of mass to each foot alternatively. At this time,
2
people shift the center of mass unconsciously, which was educated for so long. These walking patterns are believed to be programmed hereditarily from when the mankind started to walk erect in the early. It shows that the walking, complex stabilizing motion is not the education from the zero which has been applied from the birth. On the contrary, it is extremely intelligent ability represented as kinematically and dynamically being optimized throughout long term evolution and most of its part is originally. Thus, to walk like human being, the robot has to achieve the skills and information as much as the ones that human has achieved from the evolution of walking erect. However, these kinds of biological evolution process are hard to understand yet, the muscular or skeletal structure as a completion, even the methods of walking are not yet defined clearly. Human consists of about 650 muscles, 206 bones, and around 100 joints. In addition, hip joints related to the walking guarantee free motions, such as rotation, lift, and stretch, as a ball joint. Ankles and feet are also very complex structures combined with joints, ligaments, and muscles organically for interaction with various circumstances. Especially, a big toe takes an important roll in walking as an independent joint which produces dynamic walking motions minimizing energy consumption. But it is impossible to make the humanoid robot into such human organism through present technical methods of any mechanical, electric-electronical, or computer engineering methods. Furthermore, it is incongruent in the practical way. Therefore, minimized kinematical structures were selected for copying the human's motions at the humanoid robots in the real. First of all, the degree of freedom (DOF) is determined by minimized independent movements and actuation for the biped walking. Each leg has to get structured with minimum 6 DOF for the walking. Shown in figure 1, there are 3 DOF at the hip joint, 1 DOF at the knee, 2 DOF at the ankle. For more freely movement, the rotation would be added at the ankle and the big toe can be added at the foot, those are neglected in most of cases. Figure 1 shows the case of the HUBO. The standpoints of a robot design for the biped walking are abbreviated to two points commonly. One is the minimizing the backlash kinematically, and the other is minimizing the weight as long as structure stiffness is maintained. The harmonic driven reducer is usually chosen at humanoid robots which is over 1m high for reducing uncertainty by backlash. It has weak points at the stiffness of gears, durability, and friction yet; there is no such substitution in the high deceleration, zero backlash, compact size, and various types of standard products at this moment. The structure of robots should be designed to have box type legs to maintain the high stiffness, increasing the inertia moment against its weight. Every joint should guarantee the stiffness with a both ends support; avoid a cantilever type (one end support). Particularly, joints must be designed delicately so as not to be loose by clearances of bearings. The biped walking is the combination of two big steps. The first one is making standard pathway of walking using legs, ankles, arms along the pre-
planed patterns (named "the gait pattern design"), the other is maintaining the dynamic stability through continuous delicate movements of each part of the body at every single time (named "the postural stabilization"). The latter is conducted or calculated at on-line as a real-time control during the walking, and the former is conducted at off-line as a batch process before the walking. 3. FORMULATION OF THE GAIT PATTERN DESIGN For walking robots, the gait pattern design is the most fundamental part which needs much effort and time. In biped walking, every single joint is preprogrammed for the position of actuators along the planed gait patter design so can repeat the programmed motion during walking. At gait pattern design, patterns of every joint motion is designed for the zero moment point (ZMP) always being in the landed foot during a period of walking. For the first step, the ZMP is shifted to into the left foot from a phase supported by two legs, then the right foot is lifted and stretched forward. When the lifted foot is moved to a desired stride, it will be landed. The landing is done, so both feet are located diagonally for supporting the body together, and then the ZMP is shifted to the right foot. Next, the left foot is lifted to step forward in the desired stride location. Defining this pattern design, the ZMP of inside of foot has always changed continuously depends on the motion of the lifted foot, so the location of the ZMP must be controlled to be in the supporting foot by appropriately planning the motion of upper body. If the ZMP get reached the boundary of the supporting foot with the wrong pattern design, the robot would fall down. There are two methods used for completing the pattern design. One is the experimental way, and the other is using a computer simulation. This needs the equations of motions with the mathematical modeling of entire robot kinematics, and that needs the real robot model. The pattern design is not achieved automatically from either case. The pattern maintaining the walking without any falling down only be gotten by a lot of trial-and-error. In the toy robot actuated by a mechanical spring and also be quite cheap, the ZMP location is always be in the supporting feet so the walking can be made enough with simple path planning. At the small walking robot, the ZMP is almost same as the projected point of the statically center of mass at the surface so the way of generating the patterns experimentally where the point supports the statically center of mass at every steps. However, this simple way cannot be applied for bigger robots which are over 80cm high. Various changes of the ZMP by the motion throughout every single time cannot be understandable instinctively. Moreover, only way to predict the position of the ZMP is the computer simulation at the wilder motion cases such as kicking the ball. As the robot is bigger and heavier, mechanical damages and defects occur more and heavier, the pattern designs of wilder motions of
3
these robots only experimentally are followed by several dangerous factors. 4. UNCERTAINTY Although the satisfied results are achieved through defining the pattern design by experiments or computer simulations, the question of reconstruction follows at experiments after the results. Namely, the robot cannot walk satisfactorily along the planned path even though the defined above patterns are applied. The first reason of it is uncertainty by the backlash. Actuators of robots are usually electric motors combined with the reducers, oil or air pressure systems. The backlash exists always at any instance. If the very precise reducers or joints are used, looseness could be negligible but too expensive and heavy; besides, there is still a disadvantage that inner friction deteriorates the transmitting efficiency. Especially in an open structure of series links like humanoid robots, the error is accumulated more and more, therefore, backlashes and frictions at lower body joints (i.e. knees, an waist) where decide most of the walking pattern, take a important roll in repetition and accuracy of the biped walking system in which the precise and repeated actuation is needed. The second uncertainty is the friction and uniformity of ground. If the slope of ground is in the range of ±2°, the ground is considered as "flat" to people. The case in which the sole of the foot slips due to lack of friction with the ground is the same. That makes the robot's landing unstable and the accurately designed path plan meaningless. Because of a characteristic of repeated motions of the walking, the disturbed last posture of the one step (it usually become the initial condition of the next step) would make the next step unstable and disordered. The last reason of it is vibration and sag by elasticity of a structure which was not considered at the pattern design process. The structure of the robots are usually very complex and every joints are connected each other as a series, so is very vulnerable form of structure. In most of cases, it is impossible or very hard to consider these factors at the process of mathematical modeling. On this account, there could be a trouble of reconstruction though pre-planned pattern design is applied. 5. THERE ARE NO ARTIFICIAL MUSCLES The muscle of human being is the best ideal actuator as we know. 1) It can be a force source. 2) Range of the force can be from very small to very large. 3) Back drive is possible. 4) Muscle can generate the big force for its size and weight. 5) There are no friction and backlash. 6) Muscle is a linear actuator. 7) Inertial force of itself is very small.
8) It is elastic with ligaments, which can store and restore energy. etc. As one satisfies all these conditions, we can call it "the artificial muscle." As a matter of fact, this is only ideal case, namely, still it is impossible to be true as research and development. If the direct drive torque motor is used at joints, it operates as a force source and has many advantages while it does not generate enough moment yet, and also it is too large and heavy, so electric motors combined with reducers are used in most of humanoid robots. However, it has very different characteristic from human's muscles. Electric motors with reducers is position source or velocity source, and hard to generate inverse actuation, and it also has very big rotational inertial force with high ratio of reduction. These differences make difficult to move robots like human beings. The robot cannot loosen the force because inverse actuation is impossible. Energy consumption is too large when the direction and magnitude of the movement of joints changes rapidly like walking due to big inertial force of itself. And then, kinematic energy cannot be restored due to no elasticity. There is no adaptation when collision happens at landing, or bump. Consequently, maintaining of immovable posture of robots is the simplest and most natural issue. Think about hypothetically human do not move at all for 10 hours in a posture of horse riding. It can be an extreme drudge, almost impossible to do for human beings. By contrast, the relaxing and leaning on external force (easiest task for human) is one of the hardest tasks for robots. 6. THE POSTURAL STABILIZATION Natural walking is not only completed by repeated motions through the pattern design because there are various forms of uncertainty exist in the robots. Unevenness of ground is the biggest factor which infects reconstruction of the pattern design. There are two methods used for proper control of this. One is the compensation method of patterns with measuring a slope of ground by a tilt sensor at the sole of the foot, and the other is maintaining the center of body using accelerometer signals (or inclination sensors) with an attached rate gyro. It is known that every developer is using their own methods for detail control algorithms and signal processes, and their papers and patents only include general aspects. In the case of friction, if the slippery happens, then the patterns should be modified to prevent too much moment or landing force exerted on the foot. Sag of structure by elasticity could be calculated through repeated computer simulations. The robot can maintain stable and natural posture compensating with calculated sag. Robots have weak stiffness and almost no attenuation in structures. In accordance with this, there are too much residual vibration happens by the motion of arms/legs during landing. Especially, the vibration of the lifted leg could cause disturbance to landing position and timing, furthermore, affect
4
7. ADDITIONAL REMARKS
entire stability of the walking. This kind of vibration is controlled by active diminishment using signals of a rate gyro and accelerometer attached at legs and arms. And also, a force-moment sensor, which measures moment and contact force of the landed foot, and a inertia sensor, which measures inclination of the body and angular velocity, are necessary to control the posture. According to the achieved information, the landing position and timing will be calculated. And this information also be used for maintaining stability continuously, preventing from falling down or vibrating of one leg supported or two legs supported. The postural stabilization is an essential element guaranteeing stability of repeated walking along with circumstance changes. For making simpler algorithm and sensor systems, the postural stabilization is usually omitted in small size robots. As a result, it shows very unstable walking motions. Figure 2 shows the brief flow chart of posture control algorithm of HUBO.
More innovative theory, technology and idea beyond present technical limit are needed to appear more stable and safer walking robots which are more alike human beings than now. The research about the pattern design or the postural stabilization, using the self learning theory based on human neuroscience, has been conducted constantly to aim for the better control algorithm which is simpler and more humanfriendly. And essential technology for next generation humanoid biped robots has also been carried out such as actuators like the artificial muscle, light and strong materials or structures, small batteries of mass storage, high speed central process unit consuming low energy, more delicate sensor systems, artificial intelligent algorithm, and so on. If these theory or more innovative methods are completed and applied successfully, I may predict that the more natural, safer, and various walking and motions can be embodied.
Switch 1100
Doorknob 900 Kitchen Sink 830
Table 700
Coffee Table 500
Stairs 180
Figure 1. Service robot in human life environment. Units are in mm. Drawing is modified from Honda’s Homepage Walking Parameters Setting
Torso Roll & Pitch Controller
Walking Type Selection
Pelvis Swing Amp. Controller
Walking Pattern Generation
Landing Timing Controller Inverse Kinematics Ref. Position
PD controller
Upright Pose Controller
Damping controller
ROBOT
Landing Orientation controller
M om ent at Foot
Force at Foot Position
Landing D etection Attitude of Torso
( Walking control algorithm )
Figure 2. Flow chart of the walking control
5