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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
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
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.
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
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.
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.
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
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
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
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
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 errors are made in initially interfacing or programming
The manual controller plugs into the power supply, and the
arm connects to the power supply thru the 50 conductor 3M flat cable
provided. One end plugs into the arm, the other into the ARM
connector on the power supply. As one end of the cable is polarized,
it is not possible to plug it in wrong (so long as you don't try to
force things). It is best to lead the cable out of the power supply
underneath the carrying handle and under the supply. This way the
power supply acts as a strain relief should the cable recieve an
The manual controller permits computerless remote movement of
individual joints of the arm. It also selects the operating mode.
There are 5 brake switches on this control. They control the brakes
on joints 1 thru 5. (joint 6 and the hand have no brakes). Joint
numbering starts at the base (#1) and works out to the hand (#7).
These switches absolutely turn their proper brake off. They AND to
turn their proper brake on. The arm can be physically positioned
using only the brake switches (MODE switch in any position, including
OFF is OK). Just turn a joint brake off and move that joint with
your hands (execpt the hand which must be electrically operated).
The MODE switch selects the function. OFF means only the
brake switches function (Both OFF positions are the same). COMPUTER
means that the computer controls the ARM. The numbered and HAND
positions refer to the manual control mode where turning the VEL knob
+ and - makes the selected joint move + or -. Turning the VEL knob
does two things. It first turns off the selected joint brake and at
the same time commands a joint velocity. This commanded velocity is
proportional to the knob displacement, but also dependent on gravity
and load torques on the particular joint. To properly use this mode,
keep the brake switches in ON position, as their off position is an
There are two more buttons on this controller. The RED
button is the stop button when operating in COMPUTER mode. Pushing
it momentarily will disable the arm. This means the arm will stop
where it is and the brakes will turn on. To re-enable the arm, see
the section under computer control. The BLACK button is the impact
mode on the hand. Pushing this button WHILE turning the VEL knob,
with HAND mode selected will cause the hand to see a rectangualr wave
drive signal whose duty cycle is proportional to VEL knob
displacement. This will cause the hand to tighten or release its
grip. CAUTION- use this mode sparingly, as it heats the motor up and
also causes more rapid hand screw wear.
Some observations on the manual operation of the arm. Under
manual control, the maximum speeds of the arm are much slower than
the maximums under computer control. In addition, the joint
strengths are slightly reduced. Do not hold a joint against its stop
too long. If no motion is taking place, let the VEL knob center
itself. This reduces motor heating. The power supply has motor
temperature sensors in it. If you do keep a joint on too long,
especially at high current levels, the overtemp sensor will disable
the arm until the motor cools a bit. The hot joint will be indicated
by its correspondingly numbered LED on the power supply. DON'T run
the arm into its stops too frequently. The arm stops are primarily
to keep the joints from winding up. Properly operated the arm should
never run into its stops. Just like with a person- its painful to
move a joint to its limits of motion, expecially at high speed. This
caution is very important under computer control where maximum joint
velocity is much higher.
The other 50 pin 3M connector comming out of the power supply
is for connection to a computer interface, or other external
hardware. The accompanying pin chart lists the pin assignments for
this connector. A summary follows. Pot voltage means the pot
reference supply terminals. Customer must supply his own precision
supply. This should be compatible with the A/D used. Typically a
+-10 vdc supply is used. This supply should be capable of supplying
at least 250 ma. Ground or Gnd. or Ground Common terminals are all
tied together and represent the ground terminals. CC refers to a
Computer Command terminal. These are Motors (M1, etc) Brakes
(B1,etc.), and Enable and Disable. The Motors accept a +- 10 vdc.
signal range at 10 ma. max. This corresponds to full scale motor
current. The power supply contains CURRENT amplifiers. These
amplifiers output a motor current proportional to the voltage input
from the computer on the CC M1 (etc.) terminals. The BRAKE input
terminals accept a high (can be TTL or floating) or low (TTL low or
GND) at 1 ma. to turn the brakes off or on. LOW means the brakes
are ON. There is also the CC SET and CC RESET leads. These are used
to enable or disable the arm (all joints at a time). Grounding CC
SET enables the arm, while grounding CC RESET disables the arm. This
can be done with TTL logic, and only requires a momentary pulse. The
reset function is ORed with the RED button on the manual control.
It's a good idea to trun the brakes on with the computer before doing
your first ENABLE or else the arm may fall. Also, its advisable to
have a timeout on the ENABLE so that if the program dies the arm will
stop. Thus a 100 ms. timeout is a good thing. The software can give
enables every 50-100 ms. and a hardware timer can be set to trigger a
RESET in 100 ms. if no new RESET is recieved.
On the output side, The POT Wiper terminals (P1,P2,ETC.) are
the joint position signals comming directly from the potentiometers
on each joint. Their range will be about 90% of the pot supply
voltage, execpt for the hand which is only about 20% at present. On
joints 1,2,3,and 5 this corresponds to about 340 degrees maximum
rotation. On joints 4 and 6 there are two wipers spaced about 180
degrees apart. This allows for about 600 degrees of rotation as one
wiper is always in an active region on the pot element. The computer
must have an algorithm for selecting the proper wiper to use.
Starting algorithms must also be carefully thought out if both wipers
are to be properly used. Of course, many useful tasks can be done
with the more limited motion resolved by just a single wiper. The
pots are not highly linear devices. This doesn't matter if Unimate
type of operation is desired, but for computer planned motions, a
table lookup procedure must be stored in the computer.
Joints 1 thru 5 have electromechanical tachometers. Joints 6
and 7 have electronic tachometers. In all cases, the output signals
have been amplified inside the power supply. This scaling results in
larger than raw output. Due to electronic circuitry constraints, the
tach readings will be affected by turning the VEL knob even when in
COMPUTER mode. Thus, keep your hands off this knob when reading the
tachs, or running in computer mode. Joints 6 and 7 have electronic
tachometers. These are electronic circuits which measure the back
EMF of the motor and attempt to cancel out the armature losses.
Unfortunately, this is not too accurate a process, but it does serve
its purpose of providing a damping signal for use in improving servo
stability. The output of these electronic tachometers is about +- 3
volts full scale. The other tachs have 6-10 volt full scales.
The CC M7 HAMMER MODE terminal refers to the input for the
hand hammer mode. The hammer mode duty cycle is proportional to the
signal sign and amplitude. Actually, it is probably better to
operate the hammer mode from software directly into the CC M7 input.
This way the duty cycle AND frequency can be program changed easily.
One less output channel is needed too. Remember, take it easy in
using this mode.
The JOINT HOT output is normally low. it will go high when
the arm is hot and has stopped because of a hot motor. This is just
a status bit so the computer can keep track of what has happened. As
with all other digital inputs, it is TTL compatible.
You now have enough information to enable you to start using
your manipulator. Good luck.