COMMENT ⊗   VALID 00002 PAGES
C REC  PAGE   DESCRIPTION
C00001 00001
C00002 00002			THE CARE AND FEEDING OF THE STANFORD ARM
C00018 ENDMK
C⊗;
		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
should be.

	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
case.

	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.

COMPUTER CONTROL

	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
mode.

	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.


TINKERING

	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
suggestion.

	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".