ee301 answer4

What would be the effect of adding a zero to a control system?

Consider the second-order system given by

G(s) =1/(s+p1)(s+p2)

p1 > 0, p2 > 0

The poles are given by s = –p1 and s = –p2 and the simple root locus plot for this system is

shown in Figure.

When we add a zero at s = –z1 to the controller, the open-loop

transfer function will change to:

G1(s) =K( s+ z)/(s +p1)( s+ p2)

z1 > 0

We can put the zero at three different positions with respect to the poles:

1. To the right of s = –p1

Figure (b)

2. Between s = –p2 and s = –p1 Figure (c)

3. To the left of s = –p2

Figure (d)

We now discuss the effect of changing the gain K on the position of closed-loop poles

and type of responses.

(a) The zero s = –z1 is not present.

For different values of K, the system can have two real poles or a pair of complex

conjugate poles. This means that we can choose K for the system to be overdamped,

critically damped or underdamped.

(b) The zero s = –z1 is located to the right of both poles, s = – p2 and s = –p1.

In this case, the system can have only real poles and hence we can only find a value

for K to make the system overdamped. Thus the pole–zero configuration is even more

restricted than in case (a). Therefore this may not be a good location for our zero,

since the time response will become slower.

(c) The zero s = –z1 is located between s = –p2 and s = –p1.

This case provides a root locus on the real axis. The responses are therefore limited to

overdamped responses. It is a slightly better location than (b), since faster responses

are possible due to the dominant pole (pole nearest to jwaxis) lying further from the jw

axis than the dominant pole in (b).

(d) The zero s = –z1 is located to the left of s = –p2.

This is the most interesting case. Note that by placing the zero to the left of both

poles, the vertical branches of case (a) are bent backward and one end approaches the

zero and the other moves to infinity on the real axis. With this configuration, we can

now change the damping ratio and the natural frequency (to some extent). The

closed-loop pole locations can lie further to the left than s = –p2, which will provide

faster time responses. This structure therefore gives a more flexible configuration for

control design.

We can see that the resulting closed-loop pole positions are considerably influenced by

the position of this zero. Since there is a relationship between the position of closed-loop

poles and the system time domain performance, we can therefore modify the behaviour of

closed-loop system by introducing appropriate zeros in the controller.

Reference:

http://www.palgrave.com/science/engineering/wilkie/sample/0333_77129Xcha13sample.pdf

Control Systems Engineering, Nagrath & Gopal

ee301 answer1

What is a Synchro? Is it related in any way to a stepper motor?

A synchro or "selsyn" is a type of rotary electrical transformer that is used for measuring the angle of a rotating machine such as an antenna platform. In its general physical construction, it is much like an electric motor (See below.) The primary winding of the transformer, fixed to the rotor, is excited by a sinusoidal electric current (AC), which by electromagnetic induction causes currents to flow in three star-connected secondary windings fixed at 120 degrees to each other on the stator. The relative magnitudes of secondary currents are measured and used to determine the angle of the rotor relative to the stator, or the currents can be used to directly drive a receiver synchro that will rotate in unison with the synchro transmitter. In the latter case, the whole device (in some applications) is also called a selsyn .

On a practical level, synchros resemble motors, in that there is a rotor, stator, and a shaft. Ordinarily, slip rings and brushes connect the rotor to external power. A synchro transmitter's shaft is rotated by the mechanism that sends information, while the synchro receiver's shaft rotates a dial, or operates a light mechanical load. Single and three-phase units are common in use, and will follow the other's rotation when connected properly. One transmitter can turn several receivers; if torque is a factor, the transmitter must be physically larger to source the additional current. In a motion picture interlock system, a large motor-driven distributor can drive as many as 20 machines, sound dubbers, footage counters, and projectors.

A different type of receiver, called a control transformer (CT), is part of a position servo that includes a servo amplifier and servo motor. The motor is geared to the CT rotor, and when the transmitter's rotor moves, the servo motor turns the CT's rotor and the mechanical load to match the new position. CTs have high-impedance stators and draw much less current than ordinary synchro receivers when not correctly positioned.

Synchro transmitters can also feed synchro to digital converters, which provide a digital representation of the shaft angle.

Reference:

en.wikipedia.org

ee301 answer2

What are incremental encoders? Are they useful to us in any way?
Optical encoders are devices that convert a mechanical position into a representative electrical signal by means of a patterned disk or scale, a light source and photosensitive
elements. With proper interface electronics, position and speed information can be derived.

Incremental encoders

The disk of an incremental encoder is patterned with a single track of lines around its periphery. The disk count is defined as the number of dark/light linepairs that occur per revolution ("cycles / revolution" or "c/r").As a rule, a second track is added to generate a signal that occurs once per revolution (index signal), which can be used to indicate an absolute position.
To derive direction information, the lines on the disk are read out by two different photo-elements that "look" at the disk pattern with a mechanical shift of 1/4 the pitch of a linepair between them. This shift is realized with a "reticle" or "mask" that restricts the
view of the photo-element to the desired part of the disk lines. As the disk rotates, the two photo-elements generate signals that are shifted 90° out of phase from each other. These are commonly called the quadrature "A" and "B" signals. The clockwise direction for most encoders is defined as the "A" channel going positive before the "B" channel.




If the readout of the disk is obtained by a single photoelement for each of the A and the B channels, it is called a "single-ended" readout. This type of readout generates signals that are very susceptible to disk runout ("wobble"), slight imperfections in disk etching, etc. A much more effective and accurate readout system is called "push-pull" where the A and B channels are generated by two photo elements for each channel.

Incremental encoders are used to track motion and can be used to determine position and velocity. This can be either linear or rotary motion. Because the direction can be determined, very accurate measurements can be made.
Rotary encoders are often used to track the position of the motor shaft on permanent magnet brushless motor, which are commonly used on CNCmachines,robots, and other industrial equipment. In these applications, the feedback device (encoder) plays a vital role in ensuring that the equipment operates properly. The encoder synchronizes the relative rotor magnet and stator winding positions to the current provided by the drive.

Reference:
en.wikipedia.org

ee301 answer3

What do the poles and zeros contribute to in the control system?
The transfer function provides a basis for determining important system response characteristics without solving the complete differential equation. As defined, the transfer function is a rational function in the complex variable s = σ + jω.

It is often convenient to factor the polynomials in the numerator and denominator, and to write
the transfer function in terms of those factors:

H(s) = N(s)/D(s)
= K(s − z1)(s − z2) . . . (s − zm−1)(s − zm)/((s − p1)(s − p2) . . . (s − pn−1)(s − pn) )
where the numerator and denominator polynomials, N(s) and D(s), have real coefficients defined
by the system’s differential equation . Here,
the zi’s are the roots of the equation
N(s) = 0
and are defined to be the system zeros, and the pi’s are the roots of the equation
D(s) = 0
and are defined to be the system poles. The factors in the numerator and denominator
are written so that when s = zi the numerator N(s) = 0 and the transfer function vanishes, that is

lim H(s) = 0.
s→zi

and similarly when s = pi the denominator polynomial D(s) = 0 and the value of the transfer
function becomes unbounded,

lim H(s) = ∞.
s→pi

All of the coefficients of polynomials N(s) and D(s) are real, therefore the poles and zeros must
be either purely real, or appear in complex conjugate pairs. In general for the poles,
either pi = σi, or else pi, pi+1 = σi±jωi. The existence of a single complex pole without a corresponding conjugate pole would generate complex coefficients in the polynomial D(s). Similarly, the system zeros are either real or appear in complex conjugate pairs.

Control systems, in the most simple sense, can be designed simply by assigning specific values to the poles and zeros of the system.Physically realizable control systems must have a number of poles greater than or equal to the number of zeros.The locations of the poles, and the values of the real and imaginary parts of the pole determine the response of the system.

Reference:
Control Systems Engineering, Nagrath & Gopal

T3 Milacron Robot

CINCINNATI MILACRON T3 ROBOT ARM

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.

Servomechanism
Servomechanism, or servo is an automatic device that uses error-sensing feedback to correct the performance of a mechanism. The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position or other parameters. For example, an automotive power window control is not a servomechanism, as there is no automatic feedback which controls position—the operator does this by observation. By contrast the car's cruise control uses closed loop cruise control uses closed loop feedback which classifies it as a servomechanism.
A servomechanism is unique from other control systems because it controls a parameter by commanding the time-based derivative of that parameter. For example a servomechanism controlling position must be capable of changing the velocity of the system because the time-based derivative (rate change) of position is velocity. A hydraulic actuator controlled by a spool valve and a position sensor is a good example because the velocity of the actuator is proportional to the error signal of the position sensor.
Servomechanism may or may not use a servomotor. For example a household furnace controlled by thermostat is a servomechanism, yet there is no motor being controlled directly by the servomechanism.
Servomechanisms were first used in military fire-control and marine navigation equipment. Today servomechanisms are used in automatic machine tools, satellite-tracking antennas, remote control airplanes, automatic navigation systems on boats and planes, and antiaircraft-gun control systems. Other examples are fly-by-wire systems in aircraft which use servos to actuate the aircraft's control surfaces, and radio-controlled models which use RC servos for the same purpose. Many autofocus cameras also use a servomechanism to accurately move the lens, and thus adjust the focus. A modern hard disk drive has a magnetic servo system with sub-micrometre positioning accuracy.




Tyicpal servos give a rotary (angular) output. Linear types are common as well, using a screw thread or a linear motor to give linear motion.
Another device commonly referred to as a servo is used in automobiles to amplify the steering or braking force applied by the driver. However, these devices are not true servos, but rather mechanical amplifiers. (See also Power steering or Vacuum servo.)
In industrial machines, servos are used to perform complex motion.

In many applications, servomechanisms allow high-powered devices to be controlled by signals from devices of much lower power. The operation of the high-powered device results from a signal (called the error, or difference, signal) generated from a comparison of the desired position of the high-powered device with its actual position. The ratio between the power of the control signal and that of the device controlled can be on the order of billions to one.
All servomechanisms have at least these basic components: a controlled device, a command device, an error detector, an error-signal amplifier, and a device to perform any necessary error corrections (the servomotor). In the controlled device, that which is being regulated is usually position. This device must, therefore, have some means of generating a signal (such as a voltage), called the feedback signal, that represents its current position. This signal is sent to an error-detecting device. The command device receives information, usually from outside the system, that represents the desired position of the controlled device. This information is converted to a form usable by the system (such as a voltage) and is fed to the same error detector as is the signal from the controlled device. The error detector compares the feedback signal (representing actual position) with the command signal (representing desired position). Any discrepancy results in an error signal that represents the correction necessary to bring the controlled device to its desired position. The error-correction signal is sent to an amplifier, and the amplified voltage is used to drive the servomotor, which repositions the controlled device.
A typical system using a servomechanism is the communications-satellite–tracking antenna of a satellite Earth station. The objective is to keep the antenna aimed directly at the communications satellite in order to receive and transmit the strongest possible signal. One method used to accomplish this is to compare the signals from the satellite as received by two or more closely positioned receiving elements on the antenna. Any difference in the strengths of the signals received by these elements results in a correction signal being sent to the antenna servomotor. This continuous feedback method allows a terrestrial antenna to be aimed at a satellite 37,007 km (23,000 miles) above the Earth to an accuracy measured in hundredths of a centimetre.