Investigation into soft-start techniques for driving servos

Mechatronics 24 (2014) 79–86

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Investigation into soft-start techniques for driving servos Robert Ross ⇑ Department of Electronic Engineering, La Trobe University, Melbourne, Australia

a r t i c l e

i n f o

Article history: Received 26 March 2013 Accepted 30 November 2013 Available online 22 December 2013 Keywords: Servo control Soft start Voltage profiling Servo Motor control

a b s t r a c t Servos (as commonly used in radio controlled vehicles and small scale robotics) are DC actuators which use a potentiometer to provide built-in feedback to localise an actuator arm. To minimise jerky control movements, many servo controllers include velocity and acceleration control which operate using a defined velocity profile resulting in the Pulse Width Modulation (PWM) control value being changed over time. On power-up the initial position of the servo arm is unknown to external controller—resulting in the arm moving to the starting value at maximum speed (which may be mechanically hazardous). In this paper two different techniques for performing a soft-start for servos are described and evaluated, namely: voltage profiling and intermittent drive. The different techniques are implemented and evaluated using four different servos based on measured velocity profiles, current consumption, and peak torque. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Remote Control (RC) servo motors are small feedback actuators consisting of a driver, motor and a feedback potentiometer which is used to precisely control the position of an armature connected to the motor via a series of gears [1]. For the purpose of clarity, these internal feedback servo motors will be referred to as ‘servos’ throughout this paper as distinct from traditional servomotors which tend to use an encoder to provide feedback of velocity and armature position rather than simply position [2,3]. Servos offer a high torque revolute control system in an inexpensive, small package allowing simple control using a Pulse Width Modulated (PWM) signal. These servos have many applications including manipulating control surfaces for Unmanned Vehicles [4,5], haptic feedback [6], panning sensors in robotics [7] and general robotic motion control for locomotion and manipulation [8–10]. The internal servo driver uses a H-Bridge to provide bi-directional motor control. The current position of the motor is provided to the internal driver by way of a potentiometer which is connected to the servo gearing system (Fig. 1). A PWM signal is used to drive the servo, typically with a period of 20 ms and a pulse width varying from 1 ms to 2 ms corresponding to the desired servo armature position (see Fig. 2) [11,12]. More recently, digital servos have been developed which use a microcontroller to interpolate control signals to drive the DC motor at a frequency of 300 Hz (as opposed to 50 Hz for analogue servos).

⇑ Tel.: +61 3 9479 1593. E-mail address: [email protected] 0957-4158/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mechatronics.2013.11.014

This increased frequency reduces the servo dead-band, provides faster response, constant torque and more holding power when stationary [12]. Additionally some digital servos are programmable—allowing factors such as acceleration, end-point, centre-point and velocity to be controlled. This programming can significantly reduce the functionality required from the servo controller (as the microcontroller in the servo driver regulates these parameters). Servos tend to have a reasonably high rotational velocity (in the order of 350 °/s). Although this relatively fast rotational speed is useful in some applications (e.g. UAV control surfaces), some applications including precisely panning robotic sensors or controlling a robotic manipulator may require a slower velocity and/or acceleration. Many dedicated servo controllers provide programmable functionality for both velocity profiling and acceleration profiling. Velocity profiling can be of particular use when the servo is driving a relatively heavy load by allowing a slower velocity to minimise potential mechanical wear and reduce large current in-rush through the system. Commercial servo controllers perform profiling by periodically changing the PWM position between the previous desired position and the next desired position [11]. Using this method ensures that the servo does not attempt to move instantaneously to the end position but rather moves through a series of programmed intermediate steps before settling at the final desired position. One problem faced by servo controllers is that the initial position of the servo (although known internally to the servo) is not known by the system driving the servo. Hence, a servo controller may have acceleration and velocity profiling configured, but as the starting position is unknown it cannot slowly step the servo to the new desired position—resulting in the servo snapping to

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R. Ross / Mechatronics 24 (2014) 79–86

Fig. 1. Exploded picture of servo showing control circuit which provides internal feedback of the servo armature position. Fig. 3. Circuit to implement servo voltage profiling.

Fig. 2. Servo armature angle controlled by PWM input.

the position at full velocity. For some applications this may result in additional wear on components, but for some applications (like hexapod walking robots) the start-up routine may cause some legs to damage other legs if various legs or joints collide on high speed at start-up. This paper proposes two soft-start techniques, voltage profiling and intermittent drive, to overcome this significant soft-start problem. Section 2 describes the proposed techniques and provides some background as to how these techniques are used in other areas. The test setup is then described in Section 3 with details of velocity profile and current measurements. Section 4 then presents the results of the measurements with the soft-start techniques compared against baseline measurements over a range of different servos. The results are then discussed in Section 5 before concluding remarks in Section 6.

2. Proposed soft-start techniques In this section two proposed soft-start methods (voltage profiling and intermittent drive) are described—both theoretically and in terms of measurement variables required for evaluation. Due to significant differences between servo controllers (relating to dead-band, damping, torque and start-up time) empirical testing was used to develop and verify the efficacy of the different proposed soft-start solutions across a range of different servos.

As the voltage is controlled by the microcontroller using PWM (a common technique for Digital to Analog Conversion), a wide variety of voltages can be produced to supply power to the servo. This paper evaluates three different voltage profiles; Steady-State, Ramping and Segmented-Ramping (as shown in Fig. 4). The Steady-State voltage profile seeks to supply the servo with a fixed lower voltage for a short period of time before stepping up the voltage to the full supply voltage of 6 V. Empirical testing over a range of servos found that a minimum no-load supply voltage of 3.5 V was required for the servo to move. For a practical system, a higher voltage should be chosen to ensure that the servo will have enough torque to drive any load that is connected to it. For the experiments, a start-up voltage of 4 V was used for a period of 2 s before switching to the full supply voltage. The Ramping voltage profile seeks to steadily increase the voltage (from 2 V) over a period of two seconds until it reaches the full supply voltage of 6 V. The ramping technique is designed to have the servo start movement at the lowest possible voltage, hence minimising the initial acceleration as the servo begins to move. Although the ramping model should have the lowest initial acceleration, the servo will ramp up to a significantly higher velocity towards the end of the soft-start period. Finally, the Segmented-Ramping profile seeks to combine the very low initial acceleration of the Ramping profile with a relatively low overall velocity by using segments. For the first segment the voltage is ramped up for 1 s until the voltage used for the SteadyState profile is reached. This voltage will then be maintained for a further second before assuming the full servo supply voltage. A fixed two second time period for each of the voltage profiling examples was chosen based on empirical observations related to the best case angular velocity measurements. The best case empirical results (using the ramping profile) showed a mean velocity of 171°/s. As the full range of movement for a typical servo is in the order of 170 °, the servo will have settled (even in the worst case of a full deflection) over approximately 1 s—comfortably within the 2 s soft-start period.

2.1. Voltage profiling 2.2. Intermittent drive The proposed voltage profiling technique seeks to limit the supply voltage provided to the servo for a short period of time as the servo turns on. This is similar to traditional digital motor control techniques where some form of modulation allows proportional control over the motor [13]. The implementation of the voltage profiling uses a P-Channel Metal–Oxide–Semiconductor Field-Effect Transistor (MOSFET) which is driven by a PWM output on the microcontroller with a frequency of 200 kHz. A LC (Inductor–Capacitor) first order Low Pass Filter (LPF) (as shown in Fig. 3) with a cut-off frequency of 66 Hz is attached to the output of the MOSFET to filter the switching noise in order to provide the servo with a sufficiently stable DC voltage.

The proposed intermittent drive soft-start servo technique seeks to implement a soft-start without using any additional circuitry (unlike the voltage profiling technique). The servo motors essentially internally use a bi-directional form of bang–bang control as is common for some motor control operations [14,15]. The intermittent drive technique essentially inserts a fixed length delay between the on-periods of the controller to reduce the servo angular velocity. This was achieved by maintaining a constant full scale supply voltage (of 6 V) whilst intermittently sending the control signal to the servo. Rather than sending the PWM signal to the servo with a 20 ms period as specified in the servo data sheet, this method

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R. Ross / Mechatronics 24 (2014) 79–86

Fig. 4. Voltage profiles for soft-start voltage profiling.

Fig. 5. Waveform demonstrating intermittent drive where variable number of pulses are skipped.

allows multiple control pulses to be skipped. So tests were performed skipping 2, 5 and 10 pulses (henceforce labelled as Skip-2, through to Skip-10). Rather than slowing down the actual speed of the motor as is done for voltage profiling, the intermittent drive technique seeks to pulse the motor so that the motor will run at full speed, but only for a short time. This results in a decreased mean velocity as the servo moves to the desired position in a series of short jumps. For a soft-start time of 2 s, there are a total of 100 servo control periods (20 ms each). For Skip-2, only 33 control signals are be sent, and for Skip-10 only 10 control pulses are be sent. 2.3. Measurement variables This section describes the essential measurement variables of velocity and current which are used to evaluate the different soft-start methods in terms of velocity, current consumption and torque. An encoder wheel can be used to simply determine the servo angular displacement. A single differentiation can be performed on this displacement to determine the angular velocity (x in Eq. 1). The aim of both of the soft-start techniques described in this paper is to reduce the velocity and acceleration on start-up to a lower, safer level. The relevant equation pertaining to angular motion is [16]:

1 h ¼ h0 þ ðx0 þ xÞt 2

a significant load), an additional motivation for providing velocity profiling may include the control of large instantaneous current spikes. On start-up, without any soft-start procedures standard sized servos tend to draw in excess of 0.5 A for a short period of time. This instantaneous current spike becomes more significant for multi-servo systems. Current may be measured by sampling the voltage across a shunt resistor of known value before using Ohm’s law to compute the current. As the motor torque is related to the current and voltage through Eqs. (2) and (3), reducing the large instantaneous current peaks should regulate the torque and limit unnecessary high instantaneous torque. Such forces may damage either the servo gears or the control surface attached to them. Torque is defined as the cross product of a force with the length of a lever-arm [16]. The following equations relate torque to the other relevant motor characteristics which are measured or computed within the experiments [17,18]:

ss ¼ V

kT R

ð2Þ

and

Is ¼

ss kT

¼

V R

ð3Þ

where ss denotes the stall torque and kT is the motor torque constant. V represents the motor supply voltage with R as the motor winding resistance and Is as the motor stall current. As both the angular velocity and the current consumption are significant in evaluating the effectiveness of the soft-start solutions the following section describes the test parameters and setup used to measure each of these variables.

ð1Þ

where h is the final angle of rotation, h0 is the initial angle of rotation, x is the final angular velocity, x0 is the initial angular velocity and t corresponds to the time interval. Besides some applications where servos need to be driven at a relatively low velocity or reduced acceleration (e.g. they may bear

Fig. 6. Four servos used for experiments.

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Table 1 Manufacturer rated servo velocities and torque for a 6 V supply voltage. Servo

Velocity (°/s)

Max torque (N m)

HS-422 HX5010 HK15138 MG996R

375 375 300 429

0.30 0.64 0.42 1.08

3. Test procedure for soft-start evaluation To verify the effectiveness and suitability of the proposed softstart techniques, measurements relating to the velocity and current profiles were performed. To ensure repeatability, each of the following soft-start experiments were repeated 10 times. To determine the uniformity over different models of servos, four standard sized servos were used (Hitec HS-422, Hextronik HX5010, HobbyKing HK15138 and TowerPro MG996R) (shown in Fig. 6). The manufacturer rated velocities (converted to °/s) and maximum torque at a supply voltage of 6 V is summarised for each of the servos in Table 1. 3.1. Velocity profile measurements A successful soft-start servo driving mechanism will result in a comparatively slow initial velocity as the servo-arm moves towards its initial set-point value—hence validation of the proposed techniques require characterisation of the servo angular velocity. To characterise the soft-start velocity profile of each of the servos, a slot based encoder wheel was fitted to the output of the servo horn (see Fig. 7). The encoder wheel has a total of 50 slots, resulting in an angular resolution of 7.2°. The motion of this encoder wheel is read by an optical encoder, which ensures contact free measurement and provides a frictionfree determination of the servo angular velocity. The optical encoder is interfaced with an interrupt enabled microcontroller pin which facilitates accurate recording of the time taken between successive encoder wheel slots. These time measurements are sent back to a computer for analysis and for interpretation as velocity profiles. 3.2. Current profile measurements Measurement of the motor current provides two useful pieces of information. Firstly, knowing the motor current allows computation of the steady-state motor peak torque sp (with peak torque

calculated from motor start-up current) from Eq. 3. Secondly, the magnitude and profile of the current provide insight to the power supply requirements and filtering required—a factor of particular importance as the number of servos in the system increases. To measure the current a 0.05 X shunt resistor was placed in series with the servo voltage supply lines. This small value resistor ensures very low voltage drop to ensure minimal disruption to the servo being tested. An INA195 current shunt monitor was then used to amplify the very low voltage formed across the shunt resistor. A second order Low Pass Filter (LPF) was applied to the output of the shunt monitor to remove the switching noise generated when using voltage profiling soft-start control. This filtered signal was then sampled by a 10-bit ADC and logged at 5 ms intervals. 3.3. Servo movement experiments Experiment 1 – Baseline evaluation To provide some baseline test data, each of the four servos were individually connected to the test circuit with a supply voltage of 6.0 V. A separate microcontroller was used to control the position of the servo. The microcontroller first moved the servo to the starting (full deflection) position before powering down the servo to prepare it for the start-up test where it was moved to the alternate full deflection position. During this start-up test period the servo angular velocity and current were measured. The baseline test was repeated 10 times for each servo to ensure repeatability and provide a reasonable comparison for the soft-start techniques to improve on. Experiment 2 – Voltage profile soft start For the voltage profile experiments, each servo was tested with each of the different voltage profiles (Steady-State, Ramp and Segmented Ramping). These tests were repeated 10 times for each servo. Angular velocity and current were logged and are recorded in the following section. Experiment 3 – Intermittent drive soft start For the intermittent drive soft-start, a series of Skip-2 through to Skip-10 intervals (as shown in Fig. 5) were performed for each servo. Angular velocity and current consumption were measured for each test (which were each repeated 10 times). Mean results and a representative oscilloscope current measurement are presented in Section 4. 3.4. Stall torque measurements The stall torque (ss ) describes the torque load on the servo at the point where it causes the rotational velocity to become zero, hence causing the motor to stall [17]. The stall torque provides a measure of the size of the mass which can be moved by the motor at a certain distance. It is useful to characterise ss for each of the soft-start implementations as it provides a measure of the expected operational performance under load. The baseline stall torque was measured for each of the servos by attaching a series of calibrated weights to an armature connecting

Fig. 7. Test assembly used to measure servo velocity using a 50-slot encoder wheel.

Fig. 8. Test assembly used to measure stall torque (ss ).

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R. Ross / Mechatronics 24 (2014) 79–86 Table 2 Baseline measurements for mean maximum velocity (v max ), velocity torque ss .

v max

HS-422 HX5010 HK15138 MG996R

353 375 353 316

v

(°/s)

and the stall

HK15288A HS422 HX5010 MG996R

500

ss (N m)

(°/s)

316 333 316 272

0.25 0.41 0.38 0.91

400

Current (mA)

Servo model

600

v

to the servo horn. The armature (Fig. 8) was 100 mm in length, thus reducing the mass required to stall the servo. Additional weights were added in 10 g increments until the motor was made to stall and hence could not move its arm into the h region of Fig. 8. These measurements were performed for motor supply voltages of 6 V and 4 V to ensure linearity of kT and to allow a direct verification of the steady-state soft-start implementation. Once the stall torque for each of the servos was measured, the motor torque constant (kT ) was computed. Knowing kT facilitates calculation of the peak torque for the soft-start schemes using the motor current (Ip ) as the peak start-up current using Eq. (4). This current is measured at a point where the motor is stationary, and, hence back-EMF has not been generated to reduce the overall current consumption.

sp ¼ I p k T

ð4Þ

300

200

100

0

0

This section presents the results of each of the soft-start servo experiments. The mean and maximum values for velocity are tabulated and the raw velocity and current consumption for each experiment is presented graphically to give an understanding of the profile over time. Experiment 1 – Baseline evaluation The purpose of the baseline evaluation was to record the servo performance in terms of velocity and current consumption without any soft-start methods being applied. Table 2 records the baseline results which include the mean of the maximum angular velocity recorded for each cycle and the mean velocity across all the 10 cycles. The peak torque sp (calculated from the peak start-up current) are then recorded. The velocity profiles from representative baseline tests for each servo are shown in Fig. 9. The profiles show rapid acceleration, followed by a relatively stable velocity before rapid deceleration to a rest position.

200

300

400

500

600

700

800

Time (ms) Fig. 10. Representative baseline current measurements from each of the four servos synchronised with when servo started movement.

Table 3 Soft-start maximum velocity v max , velocity v and peak torque State (SS), Ramping (R) and Segmented-Ramping (SR) profiles.

v max

Servo

HS-422 HX5010 HK15138 MG996R

4. Experimental results

100

v

(°/s)

sp results for Steady-

sp (N m)

(°/s)

SS

R

SR

SS

R

SR

SS

R

SR

214 222 222 207

200 214 214 316

200 214 214 316

188 207 207 194

176 171 171 273

171 171 171 261

0.12 0.20 0.19 0.39

0.10 0.16 0.16 0.21

0.11 0.15 0.16 0.22

Fig. 10 shows a current plot with representative results from each servo overlaid. The current shows a sharp peak at the start of servo movement in the order of 500 mA, before stabilizing to a lower stable current. For each of the repeated baseline tests the servo was observed to reach its final position within 500 ms. Experiment 2 – Voltage ramp soft start Three different voltage profiles (SS, R and SR) were characterised for each of the four servos. Table 3 summaries the results starting with the mean of the maximum velocities from each cycle v max and the mean velocity (across 10 movement cycles) v . Also tabulated is the peak torque sp for each of the different voltage profiles.

350 400

R(HS−422) SR(HS−422) SS(HS−422) R(MG996R) SR(MG996R) SS(MG996R)

300

Angular Velocity °/s

350

Angular Velocity °/s

300 250 200 150 100

HK15288A HS422 HX5010 MG996R

50

250 200 150 100 50 0

0 0

50

100

150

200

250

300

350

400

450

0

500

1000

1500

2000

2500

Time (ms)

Time (ms) Fig. 9. Baseline representative velocity profiles for each of the four servos synchronised with when servo started movement.

Fig. 11. Results from voltage profiling soft-start comparing an analogue servo (HS422) with a digital servo (MG996R) showing that the digital servo exhibits a startup time in the order of 1.6 s.

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R. Ross / Mechatronics 24 (2014) 79–86 900 800 700

Skip−10(HS−422) Skip−5(MG996R)

600

500

600 500

400

Current (mA)

Current (mA)

Skip−2(HS−422) Skip−5(HS−422)

R(HS−422) SR(HS−422) SS(HS−422) R(MG996R) SR(MG996R) SS(MG996R)

400 300 200

300

200

100 100

0 0

500

1000

1500

2000

2500

Time (ms)

0

Fig. 12. Results of the measured current from voltage profiling soft-start experiment comparing an analogue servo (HS-422) with a digital servo (MG996R) showing that the digital servo exhibits a start-up time in the order of 1.6 s.

0

500

1000

1500

2000

2500

Time (ms) Fig. 14. Results of the measured current from intermittent drive soft-start experiment.

Table 4 Intermittent drive soft-start measurements for the mean of the maximum velocity (v max ), the velocity (v ) and the peak torque sp .

v max

Servo

HS-422 HX5010 HK15138 MG996R

v

(°/s)

sp (N m)

(°/s)

S-2

S-5

S-10

S-2

S-5

S-10

S-2

S-5

S-10

273 353 222 316

214 316 222 316

171 286 222 316

118 146 103 136

72 125 103 136

43 102 103 133

0.30 0.43 0.37 0.53

0.28 0.44 0.38 0.53

0.28 0.44 0.37 0.55

scenarios tested. For clarity, only the HS-422 and the MG996R are graphed to show representative results from both an analog and digital servo. Fig. 14 shows current profiles for each of the intermittent drive schemes for the HS-422 and MG996R servos (which were representative of the results obtained from the other servos). This image highlights the periodic current spikes induced by the intermittent drive when applied to analogue servos. 5. Discussion

350 Skip−2(HS−422) Skip−5(HS−422) Skip−10(HS−422) Skip−2(MG996R) Skip−5(MG996R)

Angular Velocity °/s

300 250 200 150 100 50 0 0

200

400

600

800

1000

1200

1400

1600

1800

2000

Time (ms) Fig. 13. Results from intermittent drive soft-start comparing an analogue servo (HS-422) with a digital servo (MG996R).

Fig. 11 overlays representative measured velocity profiles from each of the three voltage profiles tested. For clarity only the HS-422 and the MG996R results are shown to illustrate representative results from an analog (HS-422) and a digital (MG996R) servo. Representative current profiles for the HS-422 and MG996R servos for each of the different voltage profiles are shown in Fig. 12. Experiment 3 – Intermittent drive soft start Three different intermittent drive skip periods (2, 5 and 10) were characterised and recorded for each of the four servos with a summary of the results listed in Table 4. Fig. 13 shows representative measured velocity profiles for the four different intermittent drive (Skip-2, Skip-5 and Skip10)

This section discusses the servo velocity and current measurements recorded in the previous section in order to evaluate the relative merits of the soft-start servo approaches for implementation into a range of mechatronic systems. The relative overall performance in reduction of rotational velocity is summarised in Table 5. Experiment 1 – Baseline evaluation The baseline measurements (from Table 2) exhibit similar velocity performance compared to the manufacturer’s stated values (Table 1), although the MG996R is significantly slower than manufacturer claims. Fig. 9 overlays the velocity measurements from a representative baseline test for each servo, showing similar performance and a similar average speed between the different servo models. The baseline current results (Fig. 10) show three distinct regions in the servo movement. Firstly, at start-up, the servo current peaks in the order of 500 mA. This initial peak, when the motor is stationary, is caused by the zero back-EMF generated when stationary [19]. The second region is punctuated by the current significantly decreasing before remaining near constant—occurring as the motor starts moving, generating back-EMF and hence lowering the current drawn. Finally, when the motor comes to a stop, the current peaks again (sometime multiple times) as the motor performs a series of direction reversals as it overshoots the set point and moves towards the final armature position. The stall torque shows lower results to the manufacturer stated values, with the Hitec servo having the closest performance to the stated stall torque. Experiment 2 – Voltage ramp soft start The voltage profiling results from Table 3 show that the analogue servos (HS-422, HX5010 and HK15138) all produce slower velocity for all the different ramping profiles. The results indicate that the Segmented Ramp (SR) and Ramp (R) profiles tends to

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R. Ross / Mechatronics 24 (2014) 79–86 Table 5 Comparison of the maximum velocity reduction for each of the different soft-start techniques. Intermittent drive techniques

SS (%)

R (%)

SR (%)

Skip-2 (%)

Skip-5 (%)

Skip-10 (%)

39 41 37 34

43 43 39 0

43 43 39 0

23 6 37 0

39 16 37 0

51 24 37 0

produce similar velocity results—an expected result as Segmented Ramp matches the Ramp profile for the first half of the start-up period (which includes the start-acceleration and most of the servo movement). For the analogue servos, both the Ramp and Segmented-Ramp profiles have significantly slower acceleration than the Steady-State profile—caused by the lower start-up voltage used by these profiles. The simplest of the profiles, the Steady-State, showed significant improvement over the baseline cases, typically reducing velocity in the order of 40%. The Steady-State profile was also the only profile which made a significant velocity reduction for the digital servo. It was observed that the servo had a significant start-up time (in the order of 1600 ms—evident in Fig. 12) from when the voltage was above the minimum voltage and when the servo arm started to move. This start-up delay explains the significantly faster velocities for the Ramp and Segmented-Ramp cases as the servo does not start moving until near the end of the soft-start period. The start-up delay for the digital servos is caused by the startup and initialisation time required by the internal servo microcontroller. In normal practice (where a steady voltage is supplied to the servo), this start-up time goes unnoticed—except that the servos would not move immediately when power is applied. Hence, a successful Ramp or Segmented Ramp profile would require a significantly longer start-up period for digital servos. The implementation of the ramping profiles requires additional hardware (a FET coupled with a LPF). Testing was performed to determine if the FET alone would be sufficient to produce the required voltage profiling results but the servos behaved erratically. As the servos draw a significant current, the LPF needs to be of adequate size, which, depending on the number of servos used will occupy significant PCB real-estate. Experiment 3 – Intermittent drive soft start The experimental results shown in Table 4 indicate that the intermittent drive method significantly reduces the mean velocity for the analogue servos tested. These servos tend to move in small jumps each time a pulse is received when the intermittent technique is used. This technique does not work for the TowerPro MG996R which is a digital servo. In comparison, other digital servos were also tested (HKSCM9-5 and TGY-113MG) with the same result—after the servo receives a second pulse (even when separated by large time intervals) it locks to this pulse and does not change until power is removed or a different pulse-width is received. For the analogue servos however, the intermittent drive softstart method produces a series of small jumps rather than a fast swing to the final destination point. These short burst of movement result in a lower mean velocity, although it is augmented by short bursts of motion (with high velocity) which are of small duration. Hence, they are less likely to damage linkages attached to the servo. As the pulsed voltage is of the same magnitude, the peak torque is close to the baseline tests (although it has a far shorter duration over which movement is performed). In addition, these short bursts of motion result in a series of current spikes (see Fig. 14) which are undesirable as they may induce noise into the system and will need to be well managed around digital control circuitry, particularly as the number of servos increase. These

600 Baseline Voltage Ramping Skip−5

500

400

Current (mA)

HS-422 HX5010 HK15138 MG996R

Voltage profiling techniques

300

200

100

0

0

200

400

600

800

1000

1200

Time (ms) Fig. 15. Results for the HS-422 servo comparing the current for Baseline, Ramp and Skip-5 controllers.

350 Baseline Voltage Ramping Skip−5

300

Angular Velocity °/s

Servo

250 200 150 100 50 0 0

200

400

600

800

1000

1200

Time (ms) Fig. 16. Results for the HS-422 servo comparing angular velocity for Baseline, Ramp and Skip-5 controllers.

current spikes are each of a similar size to the motor start-up current spike for the baseline measurement (Fig. 15). In this section, the performance, in terms of velocity reduction and current consumption, for the two soft start methods was compared with the baseline servo performance where no soft-start was used. Both soft-start methods showed good velocity reduction performance (Fig. 16) and have trade-offs related to the amount of additional hardware required compared to the use of digital servos and current spikes. 6. Conclusion This paper has demonstrated and evaluated two different techniques in order to provide soft-start functionality to servos. Firstly,

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using voltage profiling, which requires additional hardware to control the supply voltage used to power the servo. The segmented ramping approach seemed to be the most effective profile, providing a good balance between keeping acceleration to a minimum and maintaining a moderate overall velocity. The second technique, intermittent drive, requires no additional hardware but only works for analogue servos and results in significant current spikes being induced into the system. Using either of these soft-start techniques, servo motors should no longer need to move at full speed to their starting position on power up—potentially damaging what they are connected to. Just as many commercial servo controllers have programmable velocity and acceleration control, the techniques presented in this paper enable soft-start for servos, an advance which gives significantly enhanced operational control. References [1] Orton K. RC Servo. US Patent 4914368, April 3 1990. [2] Ohishi K, Nakao M, Ohnishi K, Miyachi K. Microprocessor-controlled DC motor for load-insensitive position servo system. IEEE Trans Indust Electron 1987:44–9. [3] Montague R, Bingham C, Atallah K. Servo control of magnetic gears. IEEE/ASME Trans Mechatron 2012;17(2):269–78. [4] Liang W. Research on electrical servo actuators based on DSP for UAV. Micromot Servo Tech 2007;2:013. [5] Cai G, Feng L, Chen BM, Lee TH. Systematic design methodology and construction of {UAV} helicopters. Mechatronics 2008;18(10):545–58.

[6] Wagner C, Lederman S, Howe R. A tactile shape display using RC servomotors. In: Proceedings of the 10th symposium on haptic interfaces for virtual environment and teleoperator systems, 2002, HAPTICS 2002. IEEE; 2002. p. 354–5. [7] Ume I, Kita A, Liu S, Skinner S. Graduate mechatronics course in the school of mechanical engineering at Georgia Tech. Mechatronics 2002;12(2):323–35. [8] Rogers JR. Low-cost teleoperable robotic arm. Mechatronics 2009;19(5):774–9. [9] Dittrich P, Buergel A, Banzhaf W. Learning to move a robot with random morphology. In: Evolutionary robotics. Springer; 1998. p. 165–78. [10] Baltes J, McGrath S, Anderson J. The use of gyroscope feedback in the control of the walking gaits for a small humanoid robot, RoboCup 2004: Robot Soccer World Cup VIII 2005;628–35. [11] Pololu. Micro serial servo controller – user’s guide [October 2005]. [12] Futaba. Technical report: futaba digital FET servos [November 2000]. [13] Ohnishi K, Shibata M, Murakami T. Motion control for advanced mechatronics. IEEE/ASME Trans Mechatron 1996;1(1):56–67. [14] Ho H. Fast servo bang–bang seek control. IEEE Trans Magnet 1997;33(6):4522–7. [15] Ning K, Worgotter F. Control system development for a novel wire-driven hyper-redundant chain robot, 3D-trunk. IEEE/ASME Trans Mechatron 2012;17(5):949–59. [16] Tipler P, Mosca G. Physics for scientists and engineers, vol. 1. Wh Freeman; 2007. [17] Movellan J. Technical report – DC motors [March 2010]. [18] de Silva C. Mechatronics: a foundation course. CRC mechanical engineering series. Taylor & Francis; 2010. [19] Iizuka K, Uzuhashi H, Kano M, Endo T, Mohri K. Microcomputer control for sensorless brushless motor. IEEE Trans Indust Appl 1985(3):595–601.