perm filename MIT3[1,VDS] blob
sn#077475 filedate 1973-12-09 generic text, type T, neo UTF8
00100 SPECIFICATIONS FOR MIT ARM ELECTRONICS
00200
00300 By: Vic Scheinman
00400 December 6,1973
00500
00600
00700 The arm electronics will include the following major systems:
00800
00900 A power supply
01000 Seven D.C. Servo Amplifiers
01100 Seven Velocity Amplifiers
01200 Five brake drivers
01300 Seven motor temperature sensors
01400 Overcurrent protection circuitry
01500 FET switch enables for all seven power amplifiers
01600 Socket pins suitable for computer interfacing with flat cable
01700
01800 Here are the details of each system.
01900
02000 The Power Supply:
02100 The entire electronics package will operate on 115 vac. The
02200 power amplifiers require about +and- 30 vdc,or just +30 vdc if bridge
02300 power amps are used, at 8 amps filtered but not necessarily
02400 regulated. A power supply for the op amps and any switching logic
02500 must also be considered. The brake drivers use the same 30 volt
02600 amplifier supply. A 10 volt dc reference supply, providing about
02700 200 ma of smooth, well regulated and stable dc should also be
02800 included for running the potentiometer elements.All these supplies
02900 should be designed with low cost and light weight in mind.
03000
03100 D.C. Servo Amps:
03200 There are six joints on the arm and one degree of freedom in
03300 the hand which gives a total of seven permanent magnet d.c.
03400 motors.All of these motors have current limits which can never be
03500 exceeded. The motors should be driven with current drivers (current
03600 is commanded rather than voltage). The amplifiers can all be the same
03700 with provision for individually setting their current limit, and
03800 current gain. A maximum of 2.2 amps is required. The amplifiers are
03900 driven either from a computer DAC output, typically of 0 to -10
04000 volts, or +-10 volts, or 0 to +10 volts, or they are driven from a
04100 manual control amplifier which may also have the same output, or more
04200 typically +-14 volts or so if run on a 15 volt supply. Provision for
04300 setting the amplifier input to match the computer output should be
04400 included. Amplifier bandwidth must be at least 1 khz, Switchhing from
04500 computer to manual mode should also be included- like by using FET
04600 switches. There is one amplifier which is different from all the
04700 others. This is the hand driver. It must be able to operate in two
04800 modes. The first mode is a conventional mode, where current output
04900 is proportional to signal input. The second mode is what we can call
05000 a pulse mode. The amplifier must be capable of putting out + and -
05100 current pulses of a controlled width.This mode can be done with the
05200 computer, but a hardwaare alternative would make programming simpler.
05300 As a suggestion, a NE556 dual timer could poossible be used to drive
05400 the amplifier with pulse width being controlled by trim pots. FET
05500 swithes or other logic should be used to switch these two modes in
05600 and out.
05700
05800 Velocity Amplifers:
05900 The early versions of the arm will not have tachometers. This
06000 has been done for economy and design simplification. In lieu of these
06100 tachs, the velocity will have to be derived by electronic means.
06200 This involves the use of an amplifier which looks at both the motor
06300 current and the voltage across the motor. See reference data for a
06400 derivation of the amplifier gain, and other necessary details of the
06500 required network. In manual control mode, one will be commanding
06600 velocity rather than current. In computer mode, these amplifiers
06700 will be connected directly to an A-D channel because the servo loop
06800 is closed within the computer, and not in the electronics package.
06900
07000 Brake Drivers:
07100 Five of the joints have brakes. These electromechanical
07200 devices require about 100 ma at 28-32 vdc each. They are controlled
07300 from the computer by a logic level change (TTL), and thus the brake
07400 driver should be compatible with this output. In manual mode, it can
07500 be assumed that a switch from open to ground will control the brakes.
07600 As the brakes are inductive devices, the electronics should be
07700 protected from inductive spike damage (diode protection is required)
07800
07900 Motor Temperature Sensors:
08000 If operated at full current for too long a period of time,
08100 the servo motors will overhaeat and damage themselves. Some sort of
08200 protection must be included to prevent this from happening. A simple
08300 solution is to place a resistor in series with the motor and then
08400 tape a thermocouple or thermistor to the resistor. As the motor runs
08500 and heats up, so does the resistor. A threshold temperature is
08600 sensed but the thermistor and a warning light or sound comes on. At
08700 a second level, current is either switched off to the motor or it is
08800 reduced to a level low enough to prevent furthur heating. The motor
08900 thermal time constant is matched in the resistor-thermistor package
09000 by suitably wrapping the components in heat conductive and heat
09100 insulative material. Another way of doing this is to place a current
09200 integrator in the circuit. This is an op amp. set up as an integrator
09300 with a controlled loss in the loop. Current to the motor causes the
09400 integrator to integrate with a potential dependent loss. Thus the
09500 output of this special integrator would be an analog of the motor
09600 temperature. Unfortunately, switching the power supply off and then
09700 on would restart the device at an initial position rather than where
09800 it should be. In any event, as the sensor will be a set at a safe
09900 value, some provision can be included to prevent override of the
10000 device.
10100
10200 Overcurrent Protection:
10300 As mentioned in the section on Servo Amps., the motors are
10400 very overcurrent sensitive. This means that if the armature current
10500 ever rises above a certain level, the armature magnetic field
10600 strength will be large enough to demagnetize the field permanent
10700 magnets. In this event, the motor will then produce less torque for
10800 the same current, until the motor is removed and the field magnets
10900 recharged on a special magnetizing device. In current command mode
11000 this sort of thing should not happen, as full command should equal
11100 maximum allowable current. True- but accidents will happen, and
11200 protection features should be included.As an example of possible
11300 overcurrent modes. If you remove one of the supplies from a 741 op
11400 amp., it will latch up at full output. Besides causing a potential
11500 overcurrent mode, it can result in a wild and disasterous arm motion.
11600 So, if amplifiers of this sort are used, some sort of power supply
11700 protection circuitry should be included. By the way, there are
11800 amplifiers which don't do this bad thing... I'm not sure of the
11900 device numbers. Power supply protection means that the supplies are
12000 controlled so that they come up and go away at at the same time or at
12100 a rate so that both sides are reasonably close to one another. An
12200 alternative is to use bridge circuits with only one supply, but the
12300 increased component count may not be worth it. Another mode of
12400 overcurrent failure is latchup of a DAC output. Most DACs use a 741
12500 or equivalent as the output device. They produce a 0- to 10 volt
12600 swing, execpt if they loose one of their supplies, or else they fry
12700 themselves, in which case they put out 15 volts. Thus, a 10 volt
12800 zener on the inputs can be used to protect from this overcurrent
12900 mode. Another safety device is to have a device look at the inputs,
13000 and if they ever exceed the allowed maximum, they will open the FET
13100 switches which enable the power amplifiers. This way, an input
13200 failure can be prevented from causing disasterous arm motion.
13300
13400
13500 Switch Enables:
13600 The arm will operate in two modes. One is manual and the
13700 other is computer. In manual mode a manual control device will move
13800 the arm in velocity mode. I.E., direction and speed of the arm are
13900 controlled by the position of a control knob. Only one joint
14000 operates at a time in this mode. In computer mode, the servo loop is
14100 closed in the computer, and all joints can be controlled at one time.
14200 Seven DAC outputs run the seven servo motors, and the computer reads
14300 the potentiometers, and electronic tach signals, plus whatever else
14400 is fed back from the arm. FET switches provide an easy means of
14500 switching modes with high reliability and minimum mechanical
14600 switching. There are two kinds of FET switches, one is good for
14700 switching signals of all levels and the other good for switching
14800 signals which can allow the FET drain to remain at less than 200 mv.
14900 The latter are cheap and simple and are suitable for op- amp input
15000 control. The brake drivers must be wired up so that they can be
15100 enabled either by the computer or by manual mode. The override mode
15200 should be brake off. Both brake modes can be allowed to operate at
15300 the same time, so switching of modes is not required in this case.
15400
15500 Socket Pins:
15600 The following signals come from the arm to the electronics
15700 box, all in a single 50 conductor 3-m flat cable.
15800
15900 7 motor supply wires
16000 7 motor return wires-to current sense resistor
16100 5 brake supply wires
16200 1 brake common wire
16300 2 pot element wires- from precision 10 volt supply
16400 9 pot wiper wires
16500 11 wires reserved for future use with their possible use
16600 allocation as follows:
16700
16800 5 tach supply wires
16900 1 tach common wire
17000 5 wires for touch or force sensors,etc.
17100
17200 A single 26 conductor flat cable from the manual controller to the
17300 electronics box with the following signals:
17400
17500 7 brake wires
17600 1 brake common
17700 7 joint select signals
17800 1 pot signal for joint velocity
17900 2 pot element signals
18000 1 computer select signal
18100 2 emergency stop signals
18200 1 signal common
18300 4 spares
18400
18500 A single 50 conductor 3-m flat cable will run from the electronics
18600 box to the computer. This will carry the following signals.
18700
18800 7 DAC motor command signals.
18900 5 I.O. Buss Brake signals.
19000 1 DAC ground
19100 9 pot signals to the A-D.
19200 2 pot reference and gnd. signals
19300 7 derived tach signals to A-D.
19400 19 spare wires for any future applications such as touch
19500 sensors, etc.
19600
19700
19800
19900 General Design Guidelines:
20000 The electronic package should be designed to fit entirely
20100 into a single enclosed box. Its typical location will be on the
20200 floor below an arm, or on the table next to the arm. It should be
20300 light enough to be moved around easily, yet designed to be reliable
20400 and uncomplicated. Ideally, it should contain a minimum of wire wrap
20500 connections, or hand soldered wires, and a maximum of p.c. carded
20600 components. To keep costs down, the number of different cards should
20700 be minimized, and the package count should be kept low by use of
20800 multiple element packages. It should be designed to be preset so
20900 that components such as trimmer pots can be eliminated.