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C00002 00002		COMMENTS ON THE LAYOUTS AND PLANS FOR THE MODEL M.I.T. ARM
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	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.