perm filename ARM1[1,VDS] blob sn#082278
filedate 1974-01-15 generic text, type T, neo UTF8
00100 THE CARE AND FEEDING OF THE STANFORD ARM
00300 BY VIC SCHEINMAN
00500 This paper explains some of the operating and maintainence
00600 details of the Stanford Arm.
00800 INSTALLING THE ARM
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.
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.
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
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.
04800 OPERATION IN MANUAL MODE
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.
06600 COMPUTER CONTROL
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).
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.
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.
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
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.
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.
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.
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!
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.
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
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.
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.
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".