perm filename PLANS.M[1,VDS] blob sn#207061
filedate 1974-12-19 generic text, type C, neo UTF8
COMMENT ⊗ VALID 00002 PAGES
C REC PAGE DESCRIPTION
C00002 00002 COMMENTS ON THE LAYOUTS AND PLANS FOR THE MODEL M.I.T. ARM
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
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