;)
;)
;)
;)
« Project Plan Completed (lablog4-17-05) | Main | Setting up a Java Environment (Lablog4-25-05 #2) »
April 25, 2005
Arm R&D (Lablog4-25-05)
The arm and body R&D has been a learning experience. 13 days went by fast. We have been doing research since March so we had some idea what size characters we wanted. Michelle leans toward the miniature and I keep thinking bigger is likely easier up to a point, as mini- and micro options for actuators are limited and expensive.
But I planned on baselining shape memory alloy (SMA) actuators and stepper motors into the research as they both had desirable qualities and I am already pretty familiar with servo and hobby servo motor performance and control.
SMA was desirable from the standpoint of cost, quiet operation, compact size, strength and similarity to the human muscle actuation. Basically, you apply a current across muscle wire, a brand name pre-shaped SMA, and you get a small thermally-triggered contraction in the wire at about 70C or 90C depending on the type. Once the circuit is opened, the wire cools and begins to return to its normal length with some hysteresis depending on the mechanical loading and thermal environment on the wire.
An almost comic link on SMAs:
http://www.robotstore.com/shapememoryalloys.asp?afid=home
I set up two experiments using muscle wire from the robot store (see our current baseline document for ordering information). I built a 5 degree-of-freedom (DOF) arm from Fimo clay and wire. Michelle had already built me two previous arms in Fimo in order to determine the ideal DOFs we thought we would need to make a generally expressive arm with a range of motion visually similar to a human arm. I saw two potential areas where SMAs might be useful on the arm: the full contraction of the deltoid muscle and the forearm pronation/supination via the pronator teres muscle.
The Deltoid
The Pronator Teres
The Supinator
The muscle wire was to provide a single degree of freedom, e.g. lifting the arm to the horizontal plane at the shoulder or forearm pronation respectively, while a spring was to enable the arm or forearm to return to a loaded home position.
After multiple tests with poor results in terms of both desired motion and two fried slivers of large diameter, high-temp muscle wire, I simplified the test set-up to just experiment with muscle wire control and sensitivity:
I have concluded that given the time we have SMA will be a pain in my ass for three major reasons:
1. They contract almost enough to achieve the desired motion but not quite. 3-5% of the total wire length is the suggested contraction in order to get the millions of cycles promised. You can achieve as much as 8% contraction but the lifetime of the actuator will be diminished.
2. They are hard to mechanically connect to the worksite due to their small diameter and operational temperature. They operate at 70C + and ABS plastic is only spec’ed to 60C. This could be remedied by thermally isolating the wire from the plastic mounting and connection features. With the Fimo arm, I used a barrel crimp and crimped a thin-gauge wire (26 AWG) in one side of the barrel with a doubled-up piece of muscle wire. I crimped a bent staple in the other end of the barrel. The staple connected to the eyelet features on the arm. This was clugey no doubt and broke many times during the research. A better solution would be to use a potting compound or epoxy to mechanically fix the wires, SMA and mechanical connections, but this would make it hard to impossible to replace burnt out wires.
3. SMAs are hard to control. One guiding principle in this effort has been to afford each actuator a method of keeping the control closed-loop. This means each actuator should have sufficient position feedback to implement a PID or other control algorithm. This is the minimum needed to make smooth, controllable and expressive motions. With the SMA, I would have to implement feedback via a strain-gauge or flex-sensor and it would need to be very responsive in order to keep contraction within the 3-5% range. And these guys are really sensitive in general to mechanical and electrical damage.
So no muscle wire.
This menas the overall minimum arm diameter and length need to be larger than we had previously thought. The motor and motor assembly diameter will be the primary discriminator in terms of arm diameter.
Now, we have three types of motors to work with: steppers, servos and hobby servos.
Steppers are desirable from the perspective of control and size. Even without feedback, you can construct a type of closed-loop control so you can adjust your step rate to create a movement envelope that is aesthetically pleasant. If you loose your place though, position error would build up fast and it would be hard to correct unless you just routinely returned to a home position identified via limit switch. Steppers are also commonly flat and compact. I found and purchased two Seiko 10mm, 20 steps per revolution stepper motors. These guys are “ahh cute” tiny and worked very well when tested. I used a stepper motor controller chip that Mike Passaretti, a CE colleague of mine at Honeybee, suggested. He let me use the chip evaluation board to test the performance of the motor.
http://allegromicro.com/sf/3967/
http://allegromicro.com/demo/apek3967slb-01.htm
The board worked great. The motor performed pretty well too. It could half-, quarter- and eighth-step and moved up to 1k hz across the full stepping range at under 3 Volts. Here are the problems with the stepper for me:
(I'll insert an image of board and test set-up here tomorrow)
1. They are really coggy when they rotate at the speed at which we would need them to move. Cogging means you can see or feel the rotational movement in discrete intervals corresponding to the internal magnet positions vs. continuous rotational movement. 20 steps per rev is good for such a small motor and even while eight-stepping, it looked jumpy and mechanical. Realism isnt necessary, but it didn’t look believable.
2. These motors have really bad documentation and some really undesirable minimum purchase amounts. I got these 10mm motors at a surplus store. They cant really be relied upon for future use. They don’t have any documentation in terms of torque, operation power, etc. I have found other similar size motors but they can only be ordered in 10,000 + units. The next size up in available stepper motors jumps to 1” + diameter sizes and twice the mass.
While steppers presented an attractive potential in terms of simple control with internal feedback, they also could be difficult to correct in terms of position error caused by slippage and the available steppers were not as compact as I had hoped.
So we arrive where we began. I have used servos with precision encoder feedback, both custom made and COTS, and hobby servos for actuation of less sensitive DOFs. This is an approach used pretty commonly by hobbyists and research scientists alike. Kismet uses maxon motors with optical encoder feedback for the sensitive DOFs, like the eyes, mouth and neck, but relegates the eyebrows, ears and lips to Futabo RC servos.
I know the range of sizes for RC servos:
http://www.aeromicro.com/Catalog/servos_108602_products.htm
I am going to continue investigating servo motors, particularly those with thru shafts, and add-on encoders like these:
http://www.usdigital.com/products/optical-encoders.shtml
In terms of the other arm DOFs and a general summary of the arm R&D:
1. The arm will have 5 DOF:
A. The equivalent to the Deltoid in order to raise the arm out from the body to the horizontal plane
B. The equivalent to the Coracobrachialis for raising the arm out in front of and perpendicular to the body also to the horizontal plane.
C. The equivalent to the Biceps brachii in order to enable pronation and supination of the upper arm
D. The equivalent to the Tricep and Brachialis for flexion and extension of the arm at the elbow.
E. And the equivalent to the Supinator and Pronator Teres for supination and pronation of the forearm.
The wrist will likely also have a single DOF (likely a pulley type assembly) that attempts to simulate to a limited extent the muscles controlling wrist extension/flexion and the fingers (Flexor digitorum sublimis, Flexor carpi ulnaris, Palmaris longus, Flexor carpi radialis)
The wrist and hand DOF and DOF D will require a right-angle conversion, likely a beveled gear and pinion assembly.
Posted by powderly at April 25, 2005 03:48 PM
