perm filename ARM1.S[1,VDS] blob sn#146925
filedate 1975-02-18 generic text, type C, neo UTF8
COMMENT ⊗ VALID 00002 PAGES
C REC PAGE DESCRIPTION
C00002 00002 THE CARE AND FEEDING OF THE STANFORD ARM
THE CARE AND FEEDING OF THE STANFORD ARM
BY VIC SCHEINMAN
This paper explains some of the operating and maintainence
details of the Stanford Arm.
INSTALLING THE ARM
The arm must be bolted to a solid table surface or other
suitable mounting plate. The 1/2 inch screw threads on the bottom of
the base plate are for this purpose. Use them all! The wires running
down the side of the main column indicate the out of range area of
motion for joint #1, thus, these should be placed away from the
workspace. The supply cable for joint #1 can exit either thru a hole
cut in the table surface, or thru the slot cut in the base of the
arm. The two wide cables running to the other joints should be strain
relieved in such a way that they do not get in the way of the arm
when it is operating in its normal workspace. A little
experimentation will easily show where a suitable clamping point
Place the power amplifier and control box such that all the
cables from the arm will reach the box. Do not add extender cables to
the arm, as this will increase the overall resistance of the motor
drive cables and will result in slower motions and increased response
times. A typical location for the amp. box is under the table with
the cables being fed thru the table surface. Plug the amp. box into
the power supply. Again, do not attempt to extend the cable length.
The power supply plugs into 117 v.a.c. and is fused for 8 amps. An
extension cord can be used here if necessary. For semi-portable
applications, where the arm is mounted on a dolly or cart, the amp.
box, and the power supply should be mounted on the same device.
The manual control box plugs into the front of the amplifier
box, with the cable orientation colorcoded as is the case with the
cables to the arm. DO NOT PLUG THE CABLES IN BACKWARDS OR CONFUSE
THEIR ORDER! If the hand held control box is not plugged in, the arm
will not operate as the "OFF" mode is automatically selected in this
Plug the computer into the computer plug using a 50 pin 3M
ribbon connector wired to the A-D channels and DAC channels as
described later. This cable need not be plugged in if the arm is to
be used in manual mode only.
OPERATION IN MANUAL MODE
Set the manual control switch to OFF, either one will do.
Turn on the power supply, indicated by the pilot light. Place all the
brake switches in the ON or LOCK position. To grab and place the arm
somewhere, release the brakes on the proper joints, grab the arm and
move it to where you want it. Then LOCK the brakes. To move the arm
remotely, Put all the brakes in the LOCK position and then select
which joint you want to move with the joint select switch on the
manual controller. Now turn the speed and direction control knob and
the selected joint will move slowly. If anything goes wrong, release
the knob immediately and it will return to center, turning off the
servo and locking the joint. DO NOT ATTEMPT TO INCREASE THIS MAXIMUM
VELOCITY-IT IS SET LOW FOR YOUR OWN PROTECTION. If the joint stalls
under too great a load or because it has hit its own stop (joints 3-7
only) do not hold the knob on any longer than necessary, as this may
cause excessive motor heating and possible motor damage.
To operate the arm in computer mode, the arm must first be
properly interfaced with the computer. Thirteen A/D channels, 7 DAC
channels , 7 brake bit outputs and 7 enable channel outputs are the
minimum interface requirements. For more than 300 degree rotation of
joints #4 and #6 you must have two more A/D channels. A potentiometer
element power supply is also necessary. The paralleled resistance of
all the pot elements is about 200 ohms, so a 10 volt supply must be
capable of supplying at least 50 ma. To reduce precision requirements
of this supply, it helps to use an extra A/D channel to read the
supply voltage. The tachometers have bi-polar outputs, with one side
common. Should your A/D be single ended you will have to provide an
offset voltage to keep them within A/D range. You may also want to
install external tach op. amps. to set the tach gain to provide full
scale A/D signals(see table of tach maximum output signals).
The output from the DAC must be limited to less than + and -
15vdc. If you have a single ended output, an offset must be provided.
It is best to do this in an output op. amp. Some means of clamping
the output to less than 15 vdc. should be installed to insure that
the motor current limits are never exceeded, even in the event of DAC
amplifier saturation or catastrophic failure. The power amplifier
input impedance is 10k ohms. Full scale current is 15 volts input,
for each joint.
The brake drivers require a TTL driver output. A low signal
turns the brakes off. To enable the power amplifiers, FET switch
gates are provided. These, too, require TTL high level logic signals
from the computer.
To operate the arm in computer mode, the manual control box
must be plugged in and the mode selector knob set in "COMPUTER" mode.
On the present model, the only built in way of stopping the arm in
emergency is to turn the mode select knob to OFF. The computer and
manual brake switches are ORed together. Thus the manual switches
should be in LOCK position when operating the arm in computer mode.
Likewise, the computer gates should be low when operating in manual
The arm should only be operated in computer mode with a
carefully debugged program. Some sort of duty cycle protection must
be included in the program to prevent overheating of the motors. This
will normally not be a worrysome problem, but if the arm stalls up
against a surface, or else holds a large load against gravity for too
long a time, motor heating can be damaging. Prevent this by putting a
timeout in the control routine. Experience has shown that no one
trajectory should take longer than 5 seconds.
The power amplifiers are current drivers. This means that
they provide a current proportional to DAC voltage. The servo motors
are very sensitive to overcurrents. Thus it is imperative that the 15
volt dac output level never be exceeded, otherwise demagnetization of
the field magnets will result with an associated reduced torque
constant (torque/current). Because of the freeness of all the joints,
current is proportional to joint torque. Thus, the computer command
can be interpreted as a joint torque command. This should be kept in
mind when developing the servo routines.
There are no stops on several of the joints. Thus, various
protection features must be built into the software. It is also
suggested that one hand always be kept on the mode select knob when
debugging programs, to permit almost instant emergency switch off. A
separate emergency stop button connected to the I-O bus of the
computer is a valuable accessory, as the mode select switch will only
turn the power drivers off. It will not insure that the brakes are
switched to LOCK position. This can only be done in the computer on
the present version of the hand controller.
No doubt there will come a time when you will want to do
something physical to the arm. Resist this temptation mightily!! But,
if the poor arm requires maintenance, and no one in the know is
around, proceed with great caution. What follows are some general
guidelines. Sometime in the great future, a service manual of sorts
will be issued. No promises as to when!
The first point to remember is to keep your eyes open. Look
over the situation very carefully and try to diagnose the possible
problem before opening things up or removing anything. Look at the
layout drawings carefully.
The second point to remember is that everything should come
apart easily- it went together that way! If you have to use force,
you probably haven't removed all the screws, or else you are not
supposed to be taking it apart there. The motors must never be taken
apart. This means that you must not remove the armatures from within
the fields of the open motors, or open the cases of the housed
motors. To do this will result in instant demagnetization,and
resulting torque constant reduction. Don't open the arm up just to
see how it works- you don't do it on your own arms, so take the
The third point to remember is that there are lots of wires
running around the arm. Be careful not to break too many of these
when taking things apart or you'll really have a mess on your hands.
Oh yes, if you must fool with the pots, keep your cottonpicking hands
off the elements unless you have some lilly white cotton gloves on.
And do things gently, the wiper elements are fragile and bend out of
shape easily-especially during assembly or disassembly.
Fourth- you probably will have no difficulty assuring
yourself that you can maintain the arm. In case you didn't measure it
when you took it apart, the brake armature spacing is about.010-.020
inches. Also, gears run smoother if there is a little bit of backlash
(free play) rather than none. Harmonic drives can accidentally be
installed anodal. This means that the flexible inner gear which has
two less teeth than the outer ring gear has been installed with all
the difference on one side, rather than one tooth difference on each
side. You can tell that something is wrong because it will be hard to
push the wave generator (the ball bearing like thing on the motor
shaft) into place, and then the drive will be hard to back drive.
That's about it for now, I hope you have read this far before
doing anything important. Actually, if you did read all the way thru
to here-congratulations, you are one of the few people who ever
bothers to completely read anybody's instructions before plugging in
a new "toy".