Compliant Neck

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Compliant neck.png

The compliant neck uses force sensing to allow for force contolled movements on 3 axes. It is designed for use with SociBot and RoboThespian model RT4 onwards as well as stand alone use. The RT4 head shell, internal projector and webcam is combined with the neck to create an advanced head and neck design.

Mechanical

The Compliant Neck is a 3DoF, RRR robot deriven with 3 Series Elastic Actuators (SEA).
Each joint and each SEA are equipped with encoders to allow precise position and force control.

Kinematics

Simplified kinematic diagram of compliant neck. The Roll and Pitch axis are driven by 2 Parallel Linear Actuators

The naming convention adopted for the joint axis are:

Roll tilt head left and right
Pitch vertical nod: "YES"
Yaw sideways shake: "NO"

All 3 axis (Roll, Pitch and Yaw) of the neck joint intersect in 1 point (Ball joint).
The roll and pitch axis are driven by 2 Parallel Linear Actuators. The Yaw axis is driven by a single rotary actuator.


Parallel Roll & Pitch SEA

The parallel linear Roll/Pitch actuation act as a differential mechanism: the sum of the actuator lengths governs the Pitch angle, the difference between the actuator lengths governs the Roll angle.

Work Space Limitation

The Roll/Pitch work space is limited by 4 pairs of lines : min and max of Roll, Pitch, SEA0 and SEA1

The works space is limited by the maximum travel of each joint and/or actuators.
The Yaw axis is limited only by the maximum joint travel.
The Roll and Pitch axis are limited both by their joint travels and actuator travels. See the table below for details


minimum value middle value maximum value
Yaw 130 180 230
Left Right
Pitch 155 180 193
Up Down
Roll 165 180 195
Left Right
SEA0 18 30 37
SEA1 18 30 37

! Note that these are only the default values. They are highly dependent on the length of the two "double-rod-end links" connecting the SEA to the head ( Default Length = 58.5 mm ).
Length of the "double-rod-end links" measured from the center of the ball

Electrical

Electronic Boards

Wiring

Motor Power

  • 1 off 245mm
  • Roll and Pitch motors have JST PH0 connector crimped directly to flying leads

See low power motor cable

Encoders

  • 1 off 60mm
  • 1 off 48mm
  • 2 off 15mm
  • 1 off 130mm
  • 1 off 100mm

See Encoder Cable (Standard)

6 way, 0.5mm pitch FPCs:

  • 2 off 150mm (6")
  • 2 off 75mm (3")

The encoders are connected in series on an I2C bus and are arranged as shown below. Compliant head encoder chain.jpg

Software/Firmware

Control

The 3 axis Compliant Neck uses hardware elasticity for high fidelity compliance and force measurement.
A triple control loop structure is used: we have a motor speed loop within a force (torque) loop within a position loop. A motor speed control loop uses a motor encoder. The target motor speed is driven by a actuator force loop. The force loop output is fed back by measuring the defection on the series elastic element. The target force is defined by the position control loop.
The compliance of the system is defined by the position loop PID and force loop PID. A tight PID control allows faster and preciser position control. A gain called "follow mode/position mode" varying form 0 to 1 switches between a passive high compliance mode and a precise position mode.

Control diagram of 3 axis compliant neck.

Encoder Addresses

Encoder Type Address (HEX) Address Dec GUI Name
Pitch/Roll Left SEA ICMU 0x32 50 Encoder 0
Pitch/Roll Right SEA ICMU 0x33 51 Encoder 1
Yaw Force ICMU 0x30 48 Encoder 2
Yaw Position ICMU 0x31 49 Encoder 3
Roll Position I2C Rotary Encoder 0x40 64 Encoder 4
Pitch Position I2C Rotary Encoder 0x41 65 Encoder 5
Pitch/Roll Left Speed I2C Rotary Encoder 0x42 66 Encoder 6
Pitch/Roll Right Speed I2C Rotary Encoder 0x43 67 Encoder 7

CAN Open

The CANOpen protocol runs on the 3 axis CAN servo board.

The CAN pin-out can be found here.

Object Dictionary

The EDS (Electronic Data Sheet) for the Object Dictionary can be found here File:Compliant Head EDS.txt

This Object Dictionary table shows only the manufacturer specific entries. Other mandatory and optional entries set out in the canOpen specification are also implemented. check out the EDS to confirm all Object Dictionary entries.

Index Subindex Entry Description Length RW
2000 1..3 Axis Controller - Position P Gain Position proportional gain f32 rw
2001 1..3 Axis Controller - Position I Gain Position integral gain f32 rw
2002 1..3 Axis Controller - Position D Gain Position differential gain f32 rw
2003 1..3 Axis Controller - Force P Gain Force proportional gain f32 rw
2004 1..3 Axis Controller - Force I Gain Force integral gain f32 rw
2005 1..3 Axis Controller - Force D Gain Force differential gain f32 rw
2010 1..3 Axis Controller - Position I Limit Position integrator limit f32 rw
2011 1..3 Axis Controller - Force I Limit Force integrator limit f32 rw
2020 1..3 Axis Controller - Position Following Error Not implemented f32 rw
2021 1..3 Axis Controller - Position Following Error Time Not implemented f32 rw
2022 1..3 Axis Controller - Force Following Error Not implemented f32 rw
2023 1..3 Axis Controller - Force Following Error Time Not implemented f32 rw
2030 1..3 Axis Controller - Position Deadband Low The deadband value below the target position f32 rw
2031 1..3 Axis Controller - Position Deadband High The deadband value above the target position f32 rw
2032 1..3 Axis Controller - Force Deadband Low The deadband value below the target Force f32 rw
2033 1..3 Axis Controller - Force Deadband High The deadband value above the target Force f32 rw
2040 1..3 Axis Controller - Position Liveband Low The integrator only acts when it is between the target position - this value and the target position + Position Liveband High. This helps stop excessive integrator windup acceleration f32 rw
2041 1..3 Axis Controller - Position Liveband High The integrator only acts when it is between the target position + this value and the target position - Position Liveband Low. This helps stop excessive integrator windup acceleration f32 rw
2042 1..3 Axis Controller - Force Liveband Low The integrator only acts when it is between the target Force - this value and the target Force + Force Liveband High. This helps stop excessive integrator windup acceleration f32 rw
2043 1..3 Axis Controller - Force Liveband High The integrator only acts when it is between the target Force + this value and the target Force - Force Liveband Low. This helps stop excessive integrator windup acceleration f32 rw
2070 1..3 Axis Controller - Position Demand The position demand f32 rw
2071 1..3 Axis Controller - Force Demand The Force demand f32 rw
2072 1..3 Axis Controller - Position Demand Post Slew The position demand is internally calculated to include a slew rate (a maximum acceleration), the resultant position demand is written here f32 ro
2080 1..3 Axis Controller - Position Low Limit Software position limit, the axis will hard stop at this position. The demand only can be greater than this limit. f32 rw
2081 1..3 Axis Controller - Position High Limit Software position limit, the axis will hard stop at this position. The demand only can be less than this limit. f32 rw
2082 1..3 Axis Controller - Position Limit Overrange The measured position value can exceed the limits with the overrange. In case the position is larger than High Limit + Overrange, then the output can't driven so that the position moves more far from this limit. Same is for the Low Limit - Overrange. f32 rw
2083 1..3 Axis Controller - Position Slew Rate Up Maximum positive slew. Amount/sample time f32 rw
2084 1..3 Axis Controller - Position Slew Rate Down Maximum negative slew. Amount/sample time f32 rw
2085 1..3 Axis Controller - Force Low Limit Software Force limit, the axis will not exert any more force (in a negative direction) than this limit f32 rw
2086 1..3 Axis Controller - Force High Limit Software Force limit, the axis will not exert any more force (in a positive direction) than this limit f32 rw
2090 1 Mode 0 - All Axis Controllers are disabled. The motors can be controlled freely via Axis Speed Demands. 4 - Neck mode: The Axis Controller 0,1 are controlling the Pitch and Roll via the SEAs. Axis Controller 2 is controlling the Yaw. Other values are for other purposes. u8 rw
2091 1 Combi Factor A factor that is used, when the Axis Controllers are in combi mode (force loop within the position loop). Values can be between 0...1. The output of the position controller is multiplied by this value, before it goes into the force controller. When this value is 0, then the input for the force loop is always zero, so it is working in force following mode. When 1 it is working in combination mode (the output of the position controller is the input of the force controller). This value is the same for all Axis Controllers. f32 rw
2092 1 Enable Global enable, only the axes which have their enable set will be enable when this is set. All axes are disabled when this is cleared. This value is not saved to EEPROM, has to be enabled after reset. u8 rw
20A0 1..8 Encoder Value Encoder value (assuming encoder attached and working) r32 ro
20A1 1..8 Encoder Zero Offset Zero offset for the encoder f32 rw
20A2 1..8 Encoder Address I2C bus address of encoder u8 rw
20A3 1..8 Encoder Type Type of encoder, 0:None, 1:I2C Rotary, 2:ICMU, 3:Speed. If the encoder cannot be found the value of this register automatically set to zero. u8 rw
20AB 1..3 Axis - Speed Encoder Channel The channel, where the speed encoder is located (0 - 7). -1 means no speed encoder on this axis i8 rw
20AC 1..3 Axis - Current P Motor current proportional gain. Not implemented f32 rw
20AD 1..3 Axis - Current I Motor current integral gain f32 rw
20AE 1..3 Axis - Current D Motor current differential gain f32 rw
20AF 1..3 Axis - Current I Limit Motor current integrator limit f32 rw
20B0 1..3 Axis - Enable Separate enable for the Axis. AND relation with the global enable. u8 rw
20B1 1..3 Axis - Current Value of Motor Current f32 ro
20B2 1..3 Axis - Output Value of motor demand output f32 ro
20B3 1..3 Axis - Torque Limit Low The amount of torque which is needed to spin the motor. The minimum value of the output if it is not zero. (Behaves like an offset, this value is added to the output if it is not zero) f32 rw
20B4 1..3 Axis - Torque Limit High Limit the output. f32 rw
20B5 1..3 Axis - Slew Rate Limit Limit the slew rate of the output f32 rw
20B6 1..3 Axis - Low pass filter Low pass filter on the motor output. 0 means no filter 1-50 uses a 1Hz, 2Hz, ... 50Hz IIR Bessel low pass filter (http://www-users.cs.york.ac.uk/~fisher/mkfilter/trad.html) with 1...50 Hz corner frequency f32 rw
20B7 1..3 Axis - Current Limit Limit the motor current f32 rw
20B8 1..3 Axis - Overcurrent Time Allowable time for the current to be too high (on motor startup for example) f32 rw
20B9 1..3 Axis - Output Inverted Inverts the output of the motor. If enabled negative demand for the Axis provides positive output and positive demand negative output. u8 rw
20BA 1..3 Axis - Speed Demand Speed Demand f32 rw
20BB 1..3 Axis - Speed P Motor Speed proportional gain f32 rw
20BC 1..3 Axis - Speed I Motor Speed integral gain f32 rw
20BD 1..3 Axis - Speed D Motor Speed differential gain f32 rw
20BE 1..3 Axis - Speed I Limit Motor Speed integrator limit f32 rw
20C0 1..3 Axis Controller - Control Mode 0 - Position / 1 - Force / 2 - Combi mode (force loop within a position loop) u8 rw
20C1 1..3 Axis Controller - Encoder Rotary Rotary encoder value. f32 ro
20C2 1..3 Axis Controller - Encoder Linear Linear encoder value. f32 ro
20C3 1..3 Axis Controller - Position The position of the Axis Controller f32 ro
20C4 1..3 Axis Controller - Linear Encoder Zero Offset Offset for the linear encoder. This can be used to calibrate the Force measurement. f32 rw
20C5 1..3 Axis Controller - Rotary Encoder Zero Offset Offset for the rotary encoder. This can be used to calibrate the Angle measurement. f32 rw
20C6 1..3 Axis Controller - Linear Encoder Inverted Invert the output of the linear encoder. u8 rw
20C7 1..3 Axis Controller - Rotary Encoder Inverted Invert the output of the rotary encoder. u8 rw
20C8 1..3 Axis Controller - Ignore Linear Encoder Absent Ignores the missing force encoder. The output will not be zero if the linear encoder is missing u8 rw
20C9 1..3 Axis Controller - Rotary Channel Select The rotary encoder channel select u8 rw
20CA 1..3 Axis Controller - Linear Channel Select The linear encoder channel select u8 rw
20CB 1..3 Axis Controller - Axis Channel Select Select which Axis is controlled by this Axis Controller (0-3) u8 rw
20CC 1 Demand Type The type of the demand. 0 - The demand for the Axis Controllers is the position. 1 - The position demand is calculated from the angle demands. u8 rw
20CD 1..3 Angle The actual angle of the Axis Controllers f32 ro
20CE 1..3 Angle Demand The angle demand of the Axis Controllers (only has effect, when the Demand Type is 1.) f32 rw
20CF 1..3 Angle Low Limit The low limit of the Angle Demand f32 rw
20D0 1..3 Angle High Limit The high limit of the Angle Demand f32 rw
20D1 1..3 Angle Limit Overrange The overrange of the Angle, with that amount it can exceed the limits. f32 rw
20E0 1..4 FETs 1 FET 1 output status u8 rw
20E1 1..4 FETs 2 FET 2 output status u8 rw
FFFF 1 EEPROM Save OD 1 - It saves the current values to the EEPROM. If write was successful green led blinks. If failed red led blinks. After saving the value is set back to zero. u8 rw

User Guide

Find a 'Getting Started' guide in compliant head usage.

Resources

[CAN in Automation Website] - CiA, the body who oversee CANopen and its related standards