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C00002 00002			SPECIFICATIONS FOR MIT ARM ELECTRONICS
C00021 00003		COMMENTS ON THE LAYOUTS AND PLANS FOR THE MODEL M.I.T. ARM
C00030 00004		INSTALLING AND INTERFACING THE MODEL M.I.T. ARM
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		SPECIFICATIONS FOR MIT ARM ELECTRONICS

			By:  Vic Scheinman


The arm electronics will include the following major systems:

	A power supply
	Seven D.C. Servo Amplifiers
	Seven Velocity Amplifiers
	Five brake drivers
	Seven motor temperature sensors
	Overcurrent protection circuitry
	FET switch enables for all seven power amplifiers
	Socket pins suitable for computer interfacing with flat cable
	
Here are the details of each system.

The Power Supply:
	The  entire electronics package will operate on 115 vac.  The
power amplifiers require about +and- 30 vdc,or just +30 vdc if bridge
power  amps  are  used,  at  8  amps  filtered  but  not  necessarily
regulated.   A power supply for the op amps and any  switching  logic
must  also  be  considered.   The  brake drivers use the same 30 volt
amplifier supply.    A 10 volt dc reference supply,  providing  about
200  ma  of  smooth,  well  regulated  and  stable  dc should also be
included for running the potentiometer  elements.All  these  supplies
should be designed with low cost and light weight in mind.

D.C. Servo Amps:
	There are six joints on the arm and one degree of freedom  in
the  hand  which  gives  a  total  of  seven  permanent  magnet  d.c.
motors.All of these motors have current limits  which  can  never  be
exceeded.  The  motors should be driven with current drivers (current
is commanded rather than voltage). The amplifiers can all be the same
with  provision  for  individually  setting  their current limit, and
current gain.  A maximum of 2.2 amps is required. The amplifiers  are
driven  either  from  a  computer  DAC  output, typically of 0 to -10
volts, or +-10 volts, or 0 to +10 volts, or they are  driven  from  a
manual control amplifier which may also have the same output, or more
typically +-14 volts or so if run on a 15 volt supply.  Provision for
setting  the  amplifier  input to match the computer output should be
included. Amplifier bandwidth must be at least 1 khz, Switching  from
computer  to  manual  mode should also be included- like by using FET
switches.   There is one amplifier which is different  from  all  the
others.   This is the hand driver.  It must be able to operate in two
modes.   The first mode is a conventional mode, where current  output
is proportional to signal input.  The second mode is what we can call
a pulse mode.  The amplifier must be capable of putting out +  and  -
current  pulses  of a controlled width.This mode can be done with the
computer, but a hardwaare alternative would make programming simpler.
As  a suggestion, a NE556 dual timer could poossible be used to drive
the amplifier with pulse width being controlled by  trim  pots.   FET
swithes  or  other  logic should be used to switch these two modes in
and out.

Velocity Amplifers:
	The early versions of the arm will not have tachometers. This
has been done for economy and design simplification. In lieu of these
tachs,  the  velocity  will  have  to be derived by electronic means.
This involves the use of an amplifier which looks at both  the  motor
current  and the voltage across the motor.   See reference data for a
derivation of the amplifier gain, and other necessary details of  the
required  network.  In  manual  control  mode, one will be commanding
velocity rather than current.  In  computer  mode,  these  amplifiers
will  be  connected directly to an A-D channel because the servo loop
is closed within the computer, and not in the electronics package.

Brake Drivers:
	Five  of  the  joints  have brakes.   These electromechanical
devices require about 100 ma at 28-32 vdc each.  They are  controlled
from  the  computer by a logic level change (TTL), and thus the brake
driver should be compatible with this output.  In manual mode, it can
be assumed that a switch from open to ground will control the brakes.
As the brakes  are  inductive  devices,  the  electronics  should  be
protected from inductive spike damage (diode protection is required)

Motor Temperature Sensors:
	If operated at full current for too long a  period  of  time,
the  servo motors will overheat and damage themselves.   Some sort of
protection must be included to prevent this from happening.  A simple
solution  is  to  place  a resistor in series with the motor and then
tape a thermocouple or thermistor to the resistor.  As the motor runs
and  heats  up,  so  does  the resistor.   A threshold temperature is
sensed but the thermistor and a warning light or sound comes on.   At
a  second level, current is either switched off to the motor or it is
reduced to a level low enough to prevent furthur heating.  The  motor
thermal  time  constant is matched in the resistor-thermistor package
by suitably wrapping the  components  in  heat  conductive  and  heat
insulative  material. Another way of doing this is to place a current
integrator in the circuit. This is an op amp. set up as an integrator
with  a controlled loss in the loop.  Current to the motor causes the
integrator to integrate with a potential dependent loss.    Thus  the
output  of  this  special  integrator would be an analog of the motor
temperature.  Unfortunately, switching the power supply off and  then
on  would restart the device at an initial position rather than where
it should be.  In any event, as the sensor will be a set  at  a  safe
value,  some  provision  can  be  included to prevent override of the
device.

Overcurrent Protection:
	As  mentioned  in  the section on Servo Amps., the motors are
very overcurrent sensitive.  This means that if the armature  current
ever  rises  above  a  certain  level,  the  armature  magnetic field
strength will be large enough  to  demagnetize  the  field  permanent
magnets.   In this event, the motor will then produce less torque for
the same current, until the motor is removed and  the  field  magnets
recharged  on  a special magnetizing device.  In current command mode
this sort of thing should not happen, as full  command  should  equal
maximum  allowable  current.    True-  but accidents will happen, and
protection features should be  included.As  an  example  of  possible
overcurrent  modes.   If you remove one of the supplies from a 741 op
amp., it will latch up at full output.  Besides causing  a  potential
overcurrent mode, it can result in a wild and disasterous arm motion.
So, if amplifiers of this sort are used, some sort  of  power  supply
protection  circuitry  should  be  included.   By  the way, there are
amplifiers which don't do this bad  thing...  I'm  not  sure  of  the
device numbers.   Power supply protection means that the supplies are
controlled so that they come up and go away at at the same time or at
a  rate  so  that both sides are reasonably close to one another.  An
alternative is to use bridge circuits with only one supply,  but  the
increased  component  count  may  not  be  worth it.  Another mode of
overcurrent failure is latchup of a DAC output.  Most DACs use a  741
or  equivalent  as  the  output device.  They produce a 0- to 10 volt
swing, execpt if they loose one of their supplies, or else  they  fry
themselves,  in  which  case  they put out 15 volts.  Thus, a 10 volt
zener on the inputs can be used  to  protect  from  this  overcurrent
mode.   Another safety device is to have a device look at the inputs,
and if they ever exceed the allowed maximum, they will open  the  FET
switches  which  enable  the  power  amplifiers.   This way, an input
failure can be prevented from causing disasterous arm motion.


Switch Enables:
	The arm will operate in two modes.  One  is  manual  and  the
other  is computer.  In manual mode a manual control device will move
the arm in velocity mode.  I.E., direction and speed of the  arm  are
controlled  by  the  position  of  a  control  knob.   Only one joint
operates at a time in this mode. In computer mode, the servo loop  is
closed in the computer, and all joints can be controlled at one time.
Seven DAC outputs run the seven servo motors, and the computer  reads
the  potentiometers,  and electronic tach signals, plus whatever else
is fed back from the arm.  FET switches  provide  an  easy  means  of
switching   modes   with  high  reliability  and  minimum  mechanical
switching.  There are two kinds of FET  switches,  one  is  good  for
switching  signals  of  all  levels  and the other good for switching
signals which can allow the FET drain to remain at less than 200  mv.
The  latter  are  cheap and simple and are suitable for op- amp input
control.  The brake drivers must be wired up  so  that  they  can  be
enabled  either by the computer or by manual mode.  The override mode
should be brake off.  Both brake modes can be allowed to  operate  at
the same time, so switching of modes is not required in this case.

Socket Pins:
	The following signals come from the arm  to  the  electronics
box, all in a single 50 conductor 3-m flat cable.

	7 motor supply wires
	7 motor return wires-to current sense resistor
	5 brake supply wires
	1 brake common wire
	2 pot element wires- from precision 10 volt supply
	9 pot wiper wires
	11  wires  reserved  for  future  use with their possible use
allocation as follows:

		5 tach supply wires
		1 tach common wire
		5 wires for touch or force sensors,etc.

A single 26 conductor flat cable from the manual  controller  to  the
electronics box with the following signals:

		7 brake wires
		1 brake common
		7 joint select signals
		1 pot signal for joint velocity
		2 pot element signals
		1 computer select signal
		2 emergency stop signals
		1 signal common
		4 spares

A  single  50  conductor 3-m flat cable will run from the electronics
box to the computer.  This will carry the following signals.

	7 DAC motor command signals.
	5 I.O. Buss Brake signals.
	1 DAC ground
	9 pot signals to the A-D.
	2 pot reference and gnd. signals
	7 tach signals to A-D.
	19 spare wires for any  future  applications  such  as  touch
sensors, etc.



General Design Guidelines:
	The electronic package should be  designed  to  fit  entirely
into  a  single  enclosed  box.   Its typical location will be on the
floor below an arm, or on the table next to the  arm.  It  should  be
light  enough  to be moved around easily, yet designed to be reliable
and uncomplicated. Ideally, it should contain a minimum of wire  wrap
connections,  or  hand  soldered wires, and a maximum of p.c.  carded
components.  To keep costs down, the number of different cards should
be  minimized,  and  the  package  count should be kept low by use of
multiple element packages.   It should be designed to  be  preset  so
that components such as trimmer pots can be eliminated.

	COMMENTS ON THE LAYOUTS AND PLANS FOR THE MODEL M.I.T. ARM


	
	The  manipulator  is  basically a  seven  degree  of  freedom
electromechanical  device.  Each  degree of freedom  is essentially a
separate complete  servo system.   For  convenience  in referring  to
these  different degrees  of  freedom,  they are  numbered  1 thru  7
starting with the degree of freedom in the base (the rotation about a
vertical axis) numbered "#1 JOINT". The last degree of freedom is the
hand which is labeled the "#7 JOINT".  

	Each  servo system  ,execpt  the hand,  consists  of a  D.C.  
permanent magnet type motor, two  stages of gear reduction (giving  a
reduction of 30-40/1),  a position potentiometer on the  output shaft
(the  joint axis), an  electromechanical brake which  is energized to
hold (execpt joints  6 and  7 which  have no brakes),  and an  analog
tachometer   to   measure  velocity.      Joints   1  thru   5   have
electromechanical tachometers, and joints 6 and 7 have an "electronic
tachometer" which  is  an electronic  circuit  which looks  at  motor
voltage and current to derive velocity. 

	Going over  the layout drawings you will  note that this type
of arm is characterized by the complete servo system being placed  at
or  near the  corresponding joint.    This results  in  a stiff,  low
response  time system.   The layout of  each joint is  such that each
joint  can  operate  independently   of  all  other  joints.     This
facilitates programming. 

	The motors  used are permanent  magnet d.c. motors  which are
characterized  by  high torques  at modest  power  levels.   The high
performance level is obtained by using premium grade magnet material,
and complex armature winding  patterns.  Although producing very high
torques for their size, these motors are sensitive to overcurrents to
the armature.  Thus never run the arm on just a plain D.C. supply, as
you run  the risk of exceeding the  maximum allowable current through
the motor.   If you  do, even  for 1 ms.,  you will  reduce the  peak
torque output of the motor, and reduce  the strength of the arm.  The
motor  magnets will have to be  recharged- a procedure which requires
removing the motor from the arm. 

	The gear trains generally  consist of two meshes  of hardened
stainless steel pinion gears on  aluminum spur gears.  In some cases,
you will note the the output spur gear is actually machined into  the
arm structure itself.  This produces a  more accurately located gear,
and saves on weight and space too. 

	All  the position sensing potentiomenters  used in this model
arm are special elements  custom tailored to each particular  joint. 
In most cases the potentiometer  element is assembled into the output
gear  or  member.   The dull  black surface  of  this element  is the
conductive plastic material of the pot itself.  Do not touch this, as
your finger  nails may scratch the  surface, or your  finger oils may
change the resistance.  

	The joint brakes are electromechanical devices which  attract
a rotor  to a stator  when energized.   This allows  the joint to  be
locked in any  position without the need for continuous motor current
which can cause  excessive motor heating. In  general the brakes  are
about as  strong as  the joint motor.   Thus if  a brakes  slips when
energized, it  probably means that you are trying to handle too heavy
a load.    At maximum  load, the  brakes  must be  used  as often  as
possible, as the motors  are not capable of continuous output torques
at these levels.   Refer to the specification  sheet for the  maximum
intermittent and continuous torque levels for each joint. 

	Tachometers are used on  each joint to give an  indication of
motor  or joint velocity, for  use in velocity servoing  and also for
damping in position servoing.  Joints 1 thru 5 have electromechanical
tachometers.  These  are either directly connected to  each motor, or
are  geared, as in the case  of joints 2 and 4.   Joints 6 and 7 have
electronic tachometers.  The  derivation of motor speed is  done with
circuitry located in the power supply.  

	The  hand is  interfaced to  the arm  with a  threaded ring.  
Unscrewing this  ring  and then  pulling  lightly on  the  hand  will
release the hand.   You will then see the printed  circuit borad type
of  connector which is the  electrical interface between  the arm and
hand.  The hand has a motor,  a set of internal keys, a screw  thread
drive  shaft and  driven nut,  and a  potentiometer element  position
sensor imbedded in the hand structure. 

	All  joints  on  the arm  are  wired the  same.    Inside the
shoulder there is  a p.c.   board connector  manifold with 6  sockets
corresponding to joints 2 thru  7.  The socket for joint 1 is located
in the underside of the base of  the arm.  Although not all the  pins
are used,  a 16  pin polarized  plug is  associated with each  joint.
Color coding and pin assignments are the same for each of these plugs
and sockets on  the master  manifold.  This  facilitates tracing  and
debugging.   To provide good  flex life,  very thin stranded  wire is
used  in the outer  sections of the  arm.  Use care  in handling this
wire.  You will also note the the wire is not  tightly cabled, and in
some areas it is actually just loosly laid in place.  This produces a
more felxible and adaptive bundle which flexes longer. 

	If you must open the arm,  do so with care, as the  structure
of the  main  links is  characterized by  a very  stiff complete  box
section  made up  of two  halves,  which are  very flexible  when not
screwed together. 

	INSTALLING AND INTERFACING THE MODEL M.I.T. ARM


	The arm package consists  of three units, the arm,  the power
supply, and the  manual controller.
	
	The  arm must  be  clamped or  screwed  to a  suitable  rigid
support such  as a table, large plate or  rigid bracket of some sort.
For debugging purposes it is wise to place some flexible polyurethane
foam  (like  that used  in  the  arm  shipping container)  over  hard
surfaces within  the range of the arm.  This will help prevent damage
in the event error