A robot control system was designed, developed and
tested at The University of Texas at Austin for a six degree of
freedom (T976) industrial robot. The new robotic control
system can be divided into two major subsections; the servo
amplifiers and the system controller. The original system
consisted of an inflexible system controller and SCR based
power ampliiers. The new system controller has a 32 bit highspeed
p " r that can be easily expanded, reprogrammed,
and interfaced to other computers. The servo amplifiers use a
PWM methodology to control an €I-bridge which utilizes state
of the art Insulated Gate Bi-polar Transistor (IGBT) power
modules, and is capable of both current and velocity control.
INTRODUCTION
A typical industrial robotic controller consists of two
controller levels, a system level controller and servo loop
controllers. The system level controller calculates the set
points for each servo loop controller. Various types of
control algorithms can be used to generate the set points
such as position, velocity or torque. The servo loop
controllers perform low level control on each of the robot's
actuators. A 6 degree of freedom robot requires 6 servo
controllers.
The University of Texas at Austin Robotics Research
Group is engaged in research which demonstrates that an
industrial robot can be utilized in precision light machining
without jigs and for other high value added processes.
The robot used by the Robotics Research Group is a
Cincinnati Milacfon Inc. (CMI) T3-776 heavy duty
industrial robot. The robot's factory controller consists of a
CMI ACRAMATIC version 4 system controller and CMI
Silicon Controlled Rectifier (SCR) servo loop controllers
which are manufactured by Siemens. It became apparent
that the existing system controller of the T3-776 was not
capable of executing complex control strategies.
Most current industrial robot system controllers are
designed for a particular machine. Each brand and, in some
cases, model of robot has a different controller architecture.
Thus control algorithms designed for one robot cannot
necessarily be ported to anothex. The Robotics Research
Group decided to replace the system controller with a highspeed
generic controller which can be interfad to almost
any robot with minimal changes to the system architecture.
The new system controller, the Multi-Channel Robotic
Controller (MCRC), by-passed the ACRAMATIC controller
by interfacing to the robot's sensors and outputting the
command set points directly U, the Semen servo loop
Controllers. After the initial upgrade of the system
controller, it became apparent that the servo controllers were
a major bottleneck in the robot's performance. The
remaining sections in this paper will discuss the new system
controller, the new servo controllers, and the robot's
performance enhancements due to these new controllers.
SYSTEM CONTROLLER
The Multi-Channel Robotic Controller (MCRC) is
designed to control a standard industrial robot insuring high
quality coordinated control of all axes. It allows for future
expansion to control multiple robots or other machines. The
architecture is designed around a VME bus with National
Semiconductor 32532 CPUs equipped with Weitek 3164
floating-point units (see Fig 1). Each processor unit is rated
with a peak performance of 15 million instructions per
second (MIPS) and 10 million floating-point operations per
second (MFLOPS). This produces a powerful distributed
processing system with high bandwidth interprocessor
communication and YO capabilities. The use of a VRTX
operating system kernel results in a powerful, flexible and
expandable machine controller. The fmt application of this
controller is to replace the logic side of a CMI कंट्रोलर.
BACKGROUND
Work on the MCRC was initiated in association with
the Precision Light Machining project at The University of
Texas at Austin. The goal of this project is to use an
industrial robot, a CMI T3-776, to perform precision, light
duty machining without templates. The T3-776 robot is a
large, 6 degree-of-freedom industrial robot capable of lifting
150 lbs with a repeatability of 0.01 inches. The Original
controller (a CMI ACRAMATIC version 4) employs a
multibus card cage with an Intel 8086 CPU as bus master.
Communication with peripheral devices is accomplished
with a serial communication board and digital YO strips. It
became apparent that there were limitations with this
controller that would create problems when we tried to
implement more advanced control algorithms. A system
with both higher computational performance and YO
bandwidth is required to produce tracking of sufficient
quality.
It was for the above reasons as well as the inflexibility
of the ACRAMATIC control program, its low processor
performance and poor communications, that the logic side of
the T3-776 was replaced. A set of minimum requirements to
successfully meet the goals of this project and future
projects was developed. The requirements were then
modified so that the controller would be expandable and
flexible enough to be interfaced to different robots in the
future with a minimum of effort.
MCRC Design and Features
The MCRC was required to be compatible with the T3-
776's hardware. Furthermore, the controller was required to
initially be connected to the CMI T3-776, leaving the robot's
servo controllers intact. The controller can be quickly
switched back to the CMI controller, and this has been
accomplished on several occasions in under 5 minutes. The
procedure requires swapping 7 plugs, and turning 2
switches. The MCRC design allows for easy migration to
other industrial robotic systems. This was accomplished by
using a filter box to scale the tachometers to acceptable
values, and a resolvers to digital box to convert the resolvers
into a digital representation of the angles. Interfacing to
other robots would require modification of these
components. For example, if an incremental encoder input
is required, an external counting circuit to make the
encoders appear as absolute encoders can be used, and the
resulting signal fed into the parallel VO board. Absolute
encoders can be interfaced directly. For robots with
different tachometer constants, a filter box with a different
scaling factor can be built. The tachometer signals can be
sampled at a resolution of 16 bits and a speed up to 4OOO Hz
continuously using a swapped buffer mode of the A/D
board. The output command to the amplifiers is also 16 bits
and has a settling time of 16 p-sec. The requirement for
position measurement specified, that all axes should be read
to a minimum of sixteen bits. This requirement was
completely satisfied. In fact, the base, shoulder, and elbow
have a resolution of better than 22-bits per axis revolution,
and the wrist axes have a resolution of 17-bits per motor
turn.
Several control routines were implemented on the
MCRC for general path following. The routines used the
CMI amplifier section in both its closed and open velocity
loop modes, and the new servo controllers in current and
velocity modes. The paths were downloaded to the
controller from files residing on a 386 compatibles'
harddkk. One of the most beneficial features of the MCRC
is its ability to record states during a process. The states are
stored in memory on the MCRC in real time and then
downloaded to a 386 for analysis after the process. The data
sets in this paper were collected by this procedure. A set of
minimum performance standards was developed, and the
controller was designed to meet those criteria. The
controller was then assembled and interfaced to an existing
industrial robot, a Cincinnati Milacron T3-776. This
involved, among other things, the design and
implementation of boards to convert resolver signals into
digital representations that can be utilized by the controller,
the cabling between robot and controller, the writing of lowlevel
device drivers, and the building of anti-aliasing filters.
Control code was implemented in C on the system controller
for a general 6 DOF arm. The current MCRC can be used to
control robots that very from 1 to 6 DOFs. Future system
controllers will be able to scale in both hardware and
software to match the mechanical structure.
SERVO CONTROLLERS
The CMI servo amplifiers manufactured by Siemens are
3-phase, four quadrant Silicon Controlled Rectifier (SCR)
controllers. The CMI amplifiers are divided into two drive
units: one for the lower three axes and one for the three wrist
axes. Each drive unit contains three axis controller boards, a
power supply board, a pulse transfonner board, and an SCR
amplifier board.
The axis controller boards perform the local control for
each motor. The controller utilizes a velocity Proportional-
Integral (PI) control loop which has a low and high speed
proportional gain. They can operate either in velocity or
open control mode. In velocity control mode, the input from
the system controller is a velocity command, and in open
control mode, the input from the system controller is the
SCR fving command. Additional features include speed
current limiting, overcurrent protection, and overspeed
detection. The fuing angle, a, for the SCRs ranges from 30"
to 150' of a cycle for each of the 6 SCRs. When the
controller has a zero velocity command, the SCRs are
altemately fired in opposite directions at a retarded firing
angle. This method, called crossfire, enhances the response
time of the system. When the system dynamically brakes the
motor, the energy stored in the motor's windings is returned
back into the main transmission lines. The pulse
transformer board is used to electrically isolate the axis
controller board from the high power of rhe SCR board. To
make the SCR switching amplifier a 4-quadrant controller,
the motor return is fed into the neutral of a 3-Phase
transformer wired in a wye configuration. The inputs to the
rectifiers are the three legs of the wye. The two output legs
of the 3-Phase rectifier bridge are tied together by a center
tapped inductor. This allows the switching amplifier to
produce bi-directional current and voltage necessary for four
quadsant operation .
Evaluation of the T3-776 after replacing the system
controller determined that the robot's performance was
substantially degraded due to limitations in the CMI
amplifiers. The SCR switching amplifiers created 180 Hz
harmonics on their motor outputs. Since the motors
bandwidth is above this frequency, it resulted in unwanted
harmonics superimposed on the desired motions. In addition,
the servo controllers do not contain current feedback loops
that are necessary for f d m q u e control. Also, the Eactory
CMI servo controllers did not allow for single axis control
which would allow us to study the robot's pafameters for a
particular axis.
The design objectives of the new CMI T3-776 power
amplifiers were to overcome the limitations of the CMI
power amplifiers. In addition, it was desirable to keep the
power connections to the robot the same as the CMI T3-776
factory power amplifiers. The removal of harmonics
introduced from the controlled rectifier circuit allows for
smoother motion at low speeds. The current control loop is
necessary for proper fdtorque control at the joint level,
and it also allows for quicker responses to velocity command
inputs. The single axis control aids in obtaining local axis
parameters and testing new control schemes. By keeping the
identid power connections to tbe robot, system performance
enhancement of the new Controllers is easily compared to the
CMI factory amplifiers.
The system features can be divided into two categories;
safety and design. The system safety features include Dc
bus overvoltage protection, capacitor bleeder resistors, safe
start-up, and safe shut down. Safety features for individual
axes include current limits, overspeed limits, individual gate
power supply undervoltage protection, and transistor short
circuit detection. The DC bus overvoltage protection circuit
protects the high power electronics that are attached to the
DC bus. The capacitor bleeder resistors drain the large DC
bus capacitors to insure tbe system is safe to work on after
shutdown. The safe startup insures that the user does not
bring up the robot with the brakes disabled or drives enabled.
The trip time far a current limit is set exponentially by a
resistor-capacitor network. The velocity limit is an
instantaneous limit that trips when the velocity surpasses a
set maximum. Undervoltage gate power supply protection
circuits protect the power transistors from operating in the
linear region which will cause unnecessary heating of the
transistors. The short circuit protection circuit is built into
tbe power transistor gate driver circuits.
The servo loop controller implements PI control
techniques in the current and velocity feedback loops. For
fordtorque control, the velocity loop can be switched out of
the control loop structure. The velocity and current analog
signals are available to the system controller independent of
which control loop structure is active. The switching
amplifier topology used for controlling the motors is a
chopper-wired H-Bridge configuration. The voltage output
of the H-bridge is controlled by Pulse Width Modulation
(PWM). The transistors used in the H-Bridge are Insulated
Gate Bi-polar Transistors (IGBTs). The IGBTs currently
switch at 16 kHz; however, the controller is designed to
switch at frequencies up to 20 kHz. The amplifier's output is
rated at 160 volts 40 amps continuous for the lower three
axes, and 160 25 amps continuous for the upper three axes.
The servo control system also allows for individual axis
control. When a velocity or current limit has occurred the
axis number is returned to the system controller for error
detection purposes.
The power amplifiers designed for the CMI T3-776 can
be broken down into the four sections; a power interface, a
sensor interface, a system controller interface, and closed
loop controller. The sensor interface and closed loop
controllers are both contained on the axis controller board.
Fig. 2 shows the basic layout of one servo controller, and the
interfaces between each section of the controller.
The power interface section contains the brake and
control relay logic, main power rectifier and conditioner,
Overvoltage Protection (OVP), the switching amplifiers, and
the control power supplies. The main power rectifm and
conditioner circuit implements a soft start strategy to protect
the three phase rectifiers from the high "in rush" of current
at the initial charging of the capacitor. The OVP circuit
protects the system from excess voltage that builds up f"
energy returned to the capacitor bank due to dynamic
braking of the motors and the switching of the amplifier
transistors (see Fig 3). The switching amplifier uses two
state PWM to control the output voltage. The switching
amplifier is designed to function up to 20 kHz before the
IGBT switching losses become significant. An amplifier's
peak switching frequency is limited by either the transistor
switching losses or the iron losses in the motor. In the case
of the CMI robot, the factor limiting the switching frequency
is the losses in the iron of the motors. At the initial design
stage, the factory was consulted to determine if the motor
could operate at frequencies up to 20 kHz. Kollmorgen Inc.,
the manufacture of the T3-776 motors, believed the motors
could operate at these frequencies but tests have shown that
there is considerable heating in the windings of axes 2 and 3
for a frequency of 16 kHz. A switching frequency that
minimizes the heating in the windings of axes 2 and 3 has
yet to be determined due to insufficient motor information.
The switching amplifiers use 20V floating gate power
supplies to drive the transistors. The 20V power supplies are
electrically isolated from each other and the rest of the
System.
The MCRC communicates to the axis controller system
through the system interface board. All power, with the
exception of the gate driver power, for the axis controller
board goes through the system interface board. The system
interface board also resets the axis controller boards when a
current or velocity limit is hipped and when the system is
initialized. When a velocity or current limit occurs, the
system interface board en& the axis number for the
system controller, and promptly deactivates
the amplifiers. The axis
controller gains for the current and velocity
loops are set by a capacitor and a
potentiometer. The board also features
signal filters, limit circuitry, and two state
PWM controller for the H-bridge amplifier.
The filters used on the feedback signals are
second order Bessel filters. .The designed cutaff Gipency
for the current feedback loops is 10 LHZ and
the designed cut-off frequency for the
velocity filter is 500 Hz. The PWM driver circuitry
contains a comparator, a triangular wave generator, gate
lock-out circuitry, undervoltage gate protection, IGBT gate
drivers, and IGBT short circuit detection.
System Analysis
The servo control system was packaged in a cabinet
donated by Motorola, Inc. The cabinet is 22 inches wide by
22 inches deep by 44 inches tall. The entire system is
contained inside the cabinet except for a control panel,
(situated on top of the cabinet), and a remote enable box.
The system is designed such that the ambient air enters at
the bottom of the cabinet and circulates over the controller
electronics before it passes over the switching amplifiers and
their heat sinks, and exits the cabinet. This allows the cool
air to pass over the temperature sensitive electronic
components before passing over the power amplifien which
can operate at a higher tempexature.
The inside of the cabinet can be divided into four layers.
Layer one contains the main power mMiers and
conditioners for the base and wrist axes, the 48OVAC to
12OVAC step down transformer, main power up relays, and
three terminal strips. Layer hkro contains all of the control
power supplies. Layer three contains some of the control
relay logic, all of the brake relay logic and brake power
circuitry, the system interface board, and the axis controller
boards. The axis controller boards slip into a modified
Multi-Bus card cage. Layer four contains all of the
switching amplifier circuitry.
The system interfaces to the robot through receptacles
on the back panel. The main disconnect is also on the back
panel. The main disconnect breaks the main power lines
when the system draws more than thirty amps from the
procedure.
The system start-up procedure is designed such that
when a detectable fault occurs anywhere in the system, the
system is shut completely down. The pracedure is also
designed such that the system cannot be brought up out of
sequence for safety reasons. The pracedure involves fmt
charging up the main JX bus systems for the wrist and base
axes. Then, after the system is charged and the components
reach a steady-state temperature to account for any drift due
to temperature, the brakes may be released and amplifiers
enabled.
System Testing and PerJbrmance Analysis
Testing was first peaformed on each of the components
to insure proper operation. The system testing began with
debugging the wrist controllers until they worked properly.
Then the design made to the wrist axis controllers were
made to the base axis controllers. After making the
appropriate modifications to base WO controllers, they
were functionally debugged. Finally, the entire system was
brought up. The new actuator controller system was then
compared to the CMI servo controller system to determine if
the original design objectives had been met.
The order in which the system components were tested
followed the order in which the system was assembled. The
components were tested as thoroughly as possible before
being integrated with other components in the system. The
system testing was a more difficult task. Many problems
were found in the system at this point. Two of the largest
contributors to the system problems were ground loops and
Electromagnetic Interference (EMI). The ground loops
proved to be an easier problem to solve than the EM1
because they were measurable and repeatable. The EMI, on
the other hand, was very unpredictable and often affected
other components unexpectedly.
Twist wires carrying current to minimize
inductance;
Run the DC bus bars as close together as
possible;
Shield the outside of the cabinet with a Ferromagnetic
material so Eh41 leakage meets federal
standards;
Minimize the amount the large current carrying
cable; and,
Isolate the large relays from components sensitive
to EMI.
The steps to eliminate the problems associated with the
ground loops were:
Isolate the digital relay returns from the
digital electronics returns;
Separate the signal grounds as much as
possible to eliminate crosstalk;
Shield all analog signals at one end
only;
Separate all digital and analog grounds,
and bring them together at one
common point, prefembly where the
ground is brought into the board or at
the power supply; and,
Ground all components such as the
cabinet, power supplies, etc.
The system performance analysis consisted of two parts:
a current step response test, and a velocity step response test
The current response test was only performed on the new
system since the CMI Conmller does not contain a current
loop
The velocity step test was performed on each of the
robot's axes using both controllers,
the new controller clearly reacts at a much higher rate than the
CMI factory controller. The complete results of the velocity
step tests for all the axes are presented in Table 1. The new
servo system had at least one and a half times faster rise and
fall times than the CMI controller for all axes, and was abaut
two times faster for all axes except axis 2. The new
controller reduced the delay time between changes in the
command signal and the actuator controller response by a
factor of three on all axes. The new system can react
considerably faster to extemal disturbances than the CMI
controller. The tests also showed that the new controller
eliminates the 180 Hz pulsations which are present when the
CMI controller is used (as shown in Fig. 5-8).
CONCLUSION
Several major accomplishments have been achieved.
The MCRC was used to successfully control the T3-776
Robot in a variety of demonstrations, and using several
different control algorithms including compliance in the
machining demonstration. Additional safety features that
were implemented in both the system and servo controllers
were successfully tested, and still allowed for a high
sampling rate of between 2 and 5 milli-seconds, The
MCRC was tested with both the original SCR amplifiers and
the new PWM amplifiers. The interchange between the two
amps requires only 5 minutes. This demonstrates how, with
little modification, the controller can be moved between
robots. The MCRC proved very useful in the debugging of
the new servo controllers. Its flexibility allowed the
individual axes to be tested in a variety of ways, and the
data to be downloaded and stored.
One of the largest improvements of the new servo
controllers over the CMI factory controllers arose from the
design of the switching amplifier. The switching amplifier
topology chosen for the new controllers was the chopper
wired in an H-bridge configuration. The H-bridge is
controlled by a two state PWM methodology which switches
at approximately 16 kHz. The power transistors used in the
H-bridge are IGBTs which are present state of the art
technology.
Many lessons were learned from this work. Particularly
the problems associated with EM1 and ground loops both at
the system and servo level. By considering possible
interference sources in the initial design specifications a
great deal of debugging can be eliminated. Only so much
improvement can be made to an existing robot before the
limitations in the robot, its motors, and its sensors prevent
any further improvements.
