SERA Actuator/Actuator Spec

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Contents 1 1 DoF Actuator Specifications (New) 1.1 Mechanical Size 1.2 Electrical Size 1.3 Mechanical Capabilities 1.4 Electrical Capabilities 1.5 Mechanical Bandwidth 1.6 Electrical and Computational Bandwidth 1.7 Inputs 1.8 Outputs 1.9 Communications 1.10 Failsafes 2 Old Actuator Spec 2.1 Overall 2.2 Biped to Actuator Stuff 2.3 Specific 1DoF 2.4 2 DoF 2.5 Questions

1 DoF Actuator Specifications (New)

Mechanical Size

• Length: < 7.5 cm from center of rotation to end of case
  • 1/2 shortest limb length, Upper Arm
• Width: < 125% motor diameter
• Weight: < .35 kg
  • Per mass distribution of 14 kg biped

Electrical Size

• Power Board: 1.5”x1.5”, double-sided, AS50xx centered on board
• Logic Board: 1.75”x1.5”, double-sided, ports located symmetrically on either side of 1.5” long edge

Mechanical Capabilities

• Peak Dynamic Torque: > 5.1 Nm
  • Deep knee bends with vertical movement ability
  • + Factor of Safety
• Peak Mechanical Output Power: > 20 Watts
  • 2 DoF knees powering 12 cm jump
• Peak Motor Power: > 40 Watts
  • Geartrain losses
  • + Factor of Safety
• Continuous Power Rating: 10 watts
• Worm Linear Displacement is 0.0625 in = 1.5875 millimeters in either direction from 0.

Electrical Capabilities

• Logic Heat Dissipation: 3 Watts
• Power Electronics Heat Dissipation: 40 Watts
• Temperature: 10 Celsius – 50 Celsius
• Position Resolution: > 1,024 counts per revolution at output
  • Better than Dynamixel servos
• Force Resolution: > 1,024 total counts per torque range
  • Better than Dynamixel servos

Mechanical Bandwidth

• Driving Bandwidth: > 100 Hz
• Force-Sensing Bandwidth: > 100 Hz

Electrical and Computational Bandwidth

• H-Bridge PWM Frequency: > 20 kHz
  • Above audible range
• Actuator Logic Clock Frequency: > 1 kHz
  • M2 update frequency
• Control System Update Frequency: 100 Hz
  • 2x hobby servo update frequency
  • Andreas’ suggestion

Inputs

• Downloaded Data: Spring set point, spring rate, damping set point, damping constant, force desired, position desired, velocity desired
  • Impedance, force, position and velocity control data
• Electrical Power: 120 Watts
  • 1/3 efficient power transfer of 40 watts mechanical power
• Nominal Voltage: 14.4 Volts
  • 4 lithium polymer cells
• Current Limit: 10 Amps (excluding spikes from startup)
  • Falls out of 120 watt and 14.4 volt specification

Outputs

• Rotational Output: Circular boss with tapped holes
  • Servo horns are always used to transform splines into a useful form
• Case Output: Counter-bored through-holes that allow for standoffs and SHCS
  • Allow for motor spacing and flush-mount fasteners
• Wiring: Port connectors and strain relief integrated into motor case
  • No pigtails to cut to length while still providing relief on all ports
• Port Requirements: Positive locking and keyed for both comms and power
  • Guarantee wire functionality under load after untrained installation
• Uploaded Data: Position, velocity, force, voltage, current, position, temperature
  • Impedance data, state feedback, status

Communications

• Control Data Transfer Rate: 640 kbps
  • Impedance control transfer requirements plus error checking
• Synchronous Communication
  • All actuators updated at the same time
• Daisychainable
  • Ease of wire routing and installation

Failsafes

• Grease-based worm/worm-gear lubrication
  • Lubricates mechanical parts without needing an oil bath
• Fully potted/conformally coated boards
  • Vibration and grease protection
• Current-limited torque unable to stress gear teeth past elastic limits
  • Ensures gear longevity
• Over-temperature protected
  • Ensures motor longevity
• Elastic travel unable to touch down on sensors
  • Ensures actuator longevity

Old Actuator Spec

Overall

'Good force-controlled actuators have several important measures: force bandwidth, mechanical output impedance, dynamic range, power density and force density' - Robinson 2000 phd thesis

Biped to Actuator Stuff

Part	Length	Mass
Head		0.973kg
Torso	34cm	6.09kg
Upper Arm	15cm	0.397kg
Lower Arm	17cm	0.227kg
Thigh	20cm	2.12kg
Shank	25cm	0.605kg
Foot	2cm tall	0.191kg

Specific 1DoF

Currently Undefined = (CUD)

• Size: CUD
• Weight: Under .35 kg
• Moving Torque: +/- 5.1 Nm (~3.7 ft*lb) (per Motor)
• Peak Power Rating: 20 Watts (per Motor)
• Continuous Power Rating: CUD
• Mechanical Bandwidth:
• PWM Freq: > 20 kHz
• Actuator Logic Update Freq: >= 1kHz
• Control Update Freq: >= 100 Hz
• Free Speed: 449 Rev/Min
• Slew Rate: CUD
• Comm Spec:
  • 640 kbps Minimum SYNCHRONOUS
  • Daisy Chainable
• EE INPUTS (from brain / robot)
  • Motor Power, Logic Power, Comms (TBD + RC PWM), (Impedance: K,B,X_0), Force Control (PWM), Position Control (PWM)
• EE OUTPUTS (to brain)
  • Temp, Voltage, Current, Position, Velocity, Force
• ME Outputs - Easy to use
  • 1 DoF
    • Shaft mounting: Circular Boss w/ 4-Hole attachment
    • Actuator mounting: Lots of attachments
    • Wire Ports: No Pigtails. Include Strain Relief Option.
  • 2 DoF
    • Shaft Mounting:
    • Actuator Mounting:
    • Wire Ports: No Pigtails. Include Strain Relief Option.
• Connector:
  • Logic: Falls out from Comm choice - Keyed, Positive Lock (except w/ good reason)
  • Power: Positive Lock & Keyed
• Power Max (2DoF-Spec'ed) (V/I): 40W Out @ 1/3 eff: 120W In (for 2 DoF) @ 14.4V & 8 A per 2DoF
  • At most Daisy Chained Actuators: 6 DoF, 3x 2DoF Actuators: No, 4DoF
  • Actuator itself only needs to take the 8A, daisy-chaining is in cabling.
• Power Dissipation (2DoF) - 3W for Logic. 40 Watts of dissapation (Peak)?
• Temperature: 10C MIN - 50C MAX
  • Cooling could be passive
• Lubrication?
• Thermal Potting
• Can't break itself
• Overtemp (and overcurrent?) Protection
• Externally Powered by DC

2 DoF

• Differential linkage between 2 side-by-side actuators
• angular range of the primary axis?

Questions

• How do we do homing?