perm filename MIT3[1,VDS] blob sn#077475 filedate 1973-12-09 generic text, type T, neo UTF8
00300				By:  Vic Scheinman
00400				December 6,1973
00700	The arm electronics will include the following major systems:
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
01800	Here are the details of each system.
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.
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.
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.
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)
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.
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.
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.
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.
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:
16800			5 tach supply wires
16900			1 tach common wire
17000			5 wires for touch or force sensors,etc.
17200	A single 26 conductor flat cable from the manual  controller  to  the
17300	electronics box with the following signals:
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
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.
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.
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.