00100			THE CARE AND FEEDING OF THE STANFORD ARM
00200	
00300				BY VIC SCHEINMAN
00400	
00500		This  paper  explains  some of the operating and maintainence
00600	details of the Stanford Arm.
00700	
00800		INSTALLING THE ARM
00900	
01000		The arm must be bolted to a  solid  table  surface  or  other
01100	suitable  mounting plate. The 1/2 inch screw threads on the bottom of
01200	the base plate are for this purpose. Use them all! The wires  running
01300	down  the  side  of the main column indicate the out of range area of
01400	motion for joint #1, thus, these  should  be  placed  away  from  the
01500	workspace.  The supply cable for joint #1 can exit either thru a hole
01600	cut in the table surface, or thru the slot cut in  the  base  of  the
01700	arm. The two wide cables running to the other joints should be strain
01800	relieved in such a way that they do not get in the  way  of  the  arm
01900	when   it   is   operating   in   its   normal  workspace.  A  little
02000	experimentation will easily show  where  a  suitable  clamping  point
02100	should be.
02200	
02300		Place  the  power amplifier and control box such that all the
02400	cables from the arm will reach the box. Do not add extender cables to
02500	the  arm,  as  this will increase the overall resistance of the motor
02600	drive cables and will result in slower motions and increased response
02700	times.  A  typical  location for the amp. box is under the table with
02800	the cables being fed thru the table surface. Plug the amp.  box  into
02900	the  power  supply. Again, do not attempt to extend the cable length.
03000	The power supply plugs into 117 v.a.c. and is fused for  8  amps.  An
03100	extension  cord  can  be  used  here  if necessary. For semi-portable
03200	applications, where the arm is mounted on a dolly or cart,  the  amp.
03300	box, and the power supply should be mounted on the same device.
03400	
03500		The  manual control box plugs into the front of the amplifier
03600	box, with the cable orientation colorcoded as is the  case  with  the
03700	cables  to  the  arm.  DO NOT PLUG THE CABLES IN BACKWARDS OR CONFUSE
03800	THEIR ORDER! If the hand held control box is not plugged in, the  arm
03900	will  not operate as the "OFF" mode is automatically selected in this
04000	case.
04100	
04200		Plug the computer into the computer plug using a  50  pin  3M
04300	ribbon  connector  wired  to  the  A-D  channels  and DAC channels as
04400	described later. This cable need not be plugged in if the arm  is  to
04500	be used in manual mode only.
04600	
04700	
04800	OPERATION IN MANUAL MODE
04900	
05000		Set the manual control switch to OFF,  either  one  will  do.
05100	Turn on the power supply, indicated by the pilot light. Place all the
05200	brake switches in the ON or LOCK position. To grab and place the  arm
05300	somewhere,  release the brakes on the proper joints, grab the arm and
05400	move it to where you want it. Then LOCK the brakes. To move  the  arm
05500	remotely,  Put  all  the  brakes in the LOCK position and then select
05600	which joint you want to move with the  joint  select  switch  on  the
05700	manual  controller. Now turn the speed and direction control knob and
05800	the selected joint will move slowly. If anything goes wrong,  release
05900	the  knob  immediately  and it will return to center, turning off the
06000	servo and locking the joint. DO NOT ATTEMPT TO INCREASE THIS  MAXIMUM
06100	VELOCITY-IT  IS  SET LOW FOR YOUR OWN PROTECTION. If the joint stalls
06200	under too great a load or because it has hit its own stop (joints 3-7
06300	only)  do not hold the knob on any longer than necessary, as this may
06400	cause excessive motor heating and possible motor damage.
06500	
06600	COMPUTER CONTROL
06700	
06800		To operate the arm in computer mode, the arm  must  first  be
06900	properly  interfaced  with the computer. Thirteen A/D channels, 7 DAC
07000	channels , 7 brake bit outputs and 7 enable channel outputs  are  the
07100	minimum  interface requirements. For more than 300 degree rotation of
07200	joints #4 and #6 you must have two more A/D channels. A potentiometer
07300	element  power supply is also necessary. The paralleled resistance of
07400	all the pot elements is about 200 ohms, so a 10 volt supply  must  be
07500	capable of supplying at least 50 ma. To reduce precision requirements
07600	of this supply, it helps to use an extra  A/D  channel  to  read  the
07700	supply  voltage. The tachometers have bi-polar outputs, with one side
07800	common. Should your A/D be single ended you will have to  provide  an
07900	offset  voltage  to  keep them within A/D range. You may also want to
08000	install external tach op. amps. to set the tach gain to provide  full
08100	scale A/D signals(see table of tach maximum output signals).
08200	
08300		The  output from the DAC must be limited to less than + and -
08400	15vdc. If you have a single ended output, an offset must be provided.
08500	It  is  best  to do this in an output op. amp. Some means of clamping
08600	the output to less than 15 vdc. should be installed  to  insure  that
08700	the motor current limits are never exceeded, even in the event of DAC
08800	amplifier saturation or catastrophic  failure.  The  power  amplifier
08900	input  impedance  is  10k ohms. Full scale current is 15 volts input,
09000	for each joint.
09100	
09200		The brake drivers require a TTL driver output. A  low  signal
09300	turns the brakes off. To enable  the  power  amplifiers,  FET  switch
09400	gates  are provided. These, too, require TTL high level logic signals
09500	from the computer.
09600	
09800		To  operate  the arm in computer mode, the manual control box
09900	must be plugged in and the mode selector knob set in "COMPUTER" mode.
10000	On  the  present  model, the only built in way of stopping the arm in
10100	emergency is to turn the mode select knob to OFF.  The  computer  and
10200	manual  brake  switches  are  ORed together. Thus the manual switches
10300	should be in LOCK position when operating the arm in  computer  mode.
10400	Likewise,  the  computer gates should be low when operating in manual
10500	mode.
10600	
10700		The arm should only be  operated  in  computer  mode  with  a
10800	carefully  debugged  program. Some sort of duty cycle protection must
10900	be included in the program to prevent overheating of the motors. This
11000	will  normally  not  be a worrysome problem, but if the arm stalls up
11100	against a surface, or else holds a large load against gravity for too
11200	long a time, motor heating can be damaging. Prevent this by putting a
11300	timeout in the control routine. Experience  has  shown  that  no  one
11400	trajectory should take longer than 5 seconds.
11500	
11600		The  power  amplifiers  are  current drivers. This means that
11700	they provide a current proportional to DAC voltage. The servo  motors
11800	are very sensitive to overcurrents. Thus it is imperative that the 15
11900	volt dac output level never be exceeded, otherwise demagnetization of
12000	the  field  magnets  will  result  with  an associated reduced torque
12100	constant (torque/current). Because of the freeness of all the joints,
12200	current  is  proportional to joint torque. Thus, the computer command
12300	can be interpreted as a joint torque command. This should be kept  in
12400	mind when developing the servo routines.
12500	
12600		There  are  no  stops on several of the joints. Thus, various
12700	protection features must be built  into  the  software.  It  is  also
12800	suggested  that  one hand always be kept on the mode select knob when
12900	debugging programs, to permit almost instant emergency switch off.  A
13000	separate  emergency  stop  button  connected  to  the  I-O bus of the
13100	computer is a valuable accessory, as the mode select switch will only
13200	turn  the  power  drivers off. It will not insure that the brakes are
13300	switched to LOCK position. This can only be done in the  computer  on
13400	the present version of the hand controller.
13500	
13600	
13700	TINKERING
13800	
13900		No doubt there will come a time when  you  will  want  to  do
14000	something physical to the arm. Resist this temptation mightily!! But,
14100	if the poor arm requires maintenance, and  no  one  in  the  know  is
14200	around,  proceed  with  great  caution. What follows are some general
14300	guidelines. Sometime in the great future, a service manual  of  sorts
14400	will be issued. No promises as to when!
14500	
14600		The  first  point to remember is to keep your eyes open. Look
14700	over the situation very carefully and try to  diagnose  the  possible
14800	problem  before  opening  things up or removing anything. Look at the
14900	layout drawings carefully.
15000	
15100		The second point to remember is that everything  should  come
15200	apart  easily-  it  went together that way! If you have to use force,
15300	you probably haven't removed all the screws,  or  else  you  are  not
15400	supposed  to be taking it apart there. The motors must never be taken
15500	apart. This means that you must not remove the armatures from  within
15600	the  fields  of  the  open  motors,  or  open the cases of the housed
15700	motors.  To  do  this  will  result  in  instant  demagnetization,and
15800	resulting  torque  constant  reduction. Don't open the arm up just to
15900	see how it works- you don't do it on  your  own  arms,  so  take  the
16000	suggestion.
16100	
16200		The  third  point to remember is that there are lots of wires
16300	running around the arm. Be careful not to break  too  many  of  these
16400	when  taking things apart or you'll really have a mess on your hands.
16500	Oh yes, if you must fool with the pots, keep your cottonpicking hands
16600	off  the  elements unless you have some lilly white cotton gloves on.
16700	And do things gently, the wiper elements are fragile and bend out  of
16800	shape easily-especially during assembly or disassembly.
16900	
17000		Fourth-   you  probably  will  have  no  difficulty  assuring
17100	yourself that you can maintain the arm. In case you didn't measure it
17200	when  you took it apart, the brake armature spacing is about.010-.020
17300	inches. Also, gears run smoother if there is a little bit of backlash
17400	(free  play)  rather  than  none. Harmonic drives can accidentally be
17500	installed anodal. This means that the flexible inner gear  which  has
17600	two  less  teeth than the outer ring gear has been installed with all
17700	the difference on one side, rather than one tooth difference on  each
17800	side. You can tell that something is wrong because it will be hard to
17900	push the wave generator (the ball bearing like  thing  on  the  motor
18000	shaft) into place, and then the drive will be hard to back drive.
18100	
18200		That's about it for now, I hope you have read this far before
18300	doing anything important. Actually, if you did read all the way  thru
18400	to  here-congratulations,  you  are  one  of  the few people who ever
18500	bothers to completely read anybody's instructions before  plugging in
18600	a new "toy".