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ASSIGNMENT:2 question1

October 2, 2009

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

The term synchro is a generic name for a family of inductive devices which work on the principle of a rotating transformer. A synchro is an electromagnetic transducer commonly used to angular position of a shaft into an electrical signal. It is commercially known as a selsyn or an autosyn.

The basic synchro unit is usually called a synchro transmitter. Its construction is similar to that of a three phase alternator. The stator is of laminated silicon steel and is slotted to accommodate a balanced three-phase winding which is usually of concentric coil type that is, three identical coils are placed in the stator with their axis 120° apart and is star connected. The rotor is of dumbbell construction and is wound with a concentric coil. An ac voltage is applied to the rotor winding through slip rings. This voltage causes a flow of magnetizing current in the rotor coil which produces a sinusoidally time varying flux directed along its axis and distributed sinusoidally in the air gap along the stator periphery. The synchro transmitter acts like a single phase transformer in which the rotor coil is the primary and stator coils form the three secondary’s.

The stepper motor is a special type of synchronous motor which is designed to rotate through a specific angle ,called a step, for each electrical pulse received from its control unit. Typical step sizes are 7.5°, 15° or larger. The stepper motor is used in digitally controlled position control systems in open loop mode. The input command is in the form of a train of pulses to turn a shaft through a specified angle.

The advantages of using stepper motors are:

  1. Their compatibility with digital systems.
  2. No sensors are needed for position and speed sensing as these are directly obtained by counting input pulses and periodic counting if speed information is needed.

Since stepper motors are essentially digital actuators, there is no need to use analog to digital or digital to analog converters in digital control systems. They are used in paper feed motors in type writers and printers, positioning of print heads, pens in X-Y plotters and recording heads in computer disk drives.

Reference: control systems.(nagrath and gopal)

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ASSIGNMENT:2 question2

October 2, 2009

What are incremental encoders? Are they useful to us in any way?

  • A rotary encoder or shaft encoder is an electro-mechanical device that converts the angular position of a shaft or axle to an analog or digital code, making it an angle transducer.
  • There are two main types: absolute and incremental (relative).
  • An incremental rotary encoder (quadrature encoder) consists of two tracks and two sensors whose outputs are called channels A and B.
  • Outputs are either mechanical or optical.
  • As the shaft rotates, pulse trains occur on these channels at a frequency proportional to the shaft speed, and the phase relationship between the signals yields the direction of rotation.
  • By counting the number of pulses and knowing the resolution of the disk, the angular motion can be measured.
  • The channels are used to determine the direction of rotation by assessing which channel “leads” the other.
  • The signals from two channels are a 1/4 cycle out of phase with each other and are known as quadrature signals.
  • Often a third output channel, called INDEX, yields one pulse per revolution, which is useful in counting full revolutions.

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  • They are used to track motion and can be used to determine position and velocity,  track the position of the motor shaft on permanent magnet brushless motors.

ASSIGNMENT:2 question 4

October 2, 2009

What would the effect of adding a zero to a control system? Consider the second-order system, G(s) = 1/((s+p1)*(s+p2)) , p1>0 , p2>0 The poles are s = –p1 and s = –p2 and the simple root locus plot for this system is shown in (a). When we add a zero at s = –z1 to the controller, the open-loop transfer function will be: G1(s)= K(s+z1)/((s+p1)*(s+p2)) , z1>0graph We can put the zero at three different positions with respect to the poles: 1. To the right of s = –p1 (b) 2. Between s = –p2 and s = –p1 (c) 3. To the left of s = –p2 (d) (a) The zero s = –z1 is not present. the system can have two real poles or a pair of complex conjugate poles ,may be overdamped, critically damped or underdamped. 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.System can have only real poles and system is 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 system is overdamped. 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 jw axis) 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. 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, the damping ratio and the natural frequency can be changed 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. Since there is a relationship between the position of closed-loop poles and the system time domain performance, the behavior of closed-loop system can be modified by introducing appropriate zeros in the controller.

ASSIGNMENT:2 question3

October 2, 2009

POLES AND ZEROS The transfer function provides a basis for determining important system response characteristics without solving the complete differential equation. A transfer function is defined as a ratio of two polynomials: H(S)= N(S)/D(S) Where N(s) and D(s) are simple polynomials These can be written as: N(s)=(s − z1)(s − z2) . . . (s − zm−1)(s − zm) D(s)=(s − p1)(s − p2) . . . (s − pn−1)(s − pn) , Zeros are the roots of N(s) (the numerator of the transfer function) obtained by setting N(s) = 0 and solving for s. Poles are the roots of D(s) (the denominator of the transfer function), obtained by setting D(s) = 0 and solving for s. 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. Poles and Zeros of a transfer function are the frequencies for which the value of the transfer function becomes infinity or zero respectively.As s approaches a zero, the numerator of the transfer function (and therefore the transfer function itself) approaches the value 0. When s approaches a pole, the denominator of the transfer function approaches zero, and the value of the transfer function approaches infinity.The values of the poles and the zeros of a system determine whether the system is stable, and how well the system performs. Control systems, in the simplest sense, can be designed simply by assigning specific values to the poles and zeros of the system. A system is characterized by its poles and zeros .Because the transfer function completely represents a system differential equation, its poles and zeros effectively define the system response. In particular the system poles directly define the components in the homogeneous response. The locations of the poles, and the values of the real and imaginary parts of the pole helps to determine the response of the system. The stability of a linear system may be determined directly from its transfer function. An nth order linear system is asymptotically stable only if all of the components in the homogeneous response from a finite set of initial conditions decay to zero as time increases.In order for a linear system to be stable, all of its poles must have negative real parts. Reference: http://en.wikibooks.org/wiki/Control_Systems/Poles_and_Zeros

Cincinnati Milacron T3 Robotic Arm

July 27, 2009

CINCINNATI MILACRON T3 ROBOTIC ARM

OPP

An industrial robot is officially defined by ISO as an automatically controlled, reprogrammable, multipurpose manipulator programmable in three or more axes.
At Cincinnati Milacron Corporation, Richard Hohn developed the robot called The Tomorrow Tool or T3. Released in 1973, the T3 was the first commercially available industrial robot controlled by a microcomputer as well as the first U.S. robot to use the revolute configuration.
This robot is a more classically designed industrial robot. Designed as a healthy compromise between dexterity and strength this robot was one of the ground breakers, in terms of success, in factory environments. However, while this robot was a success in industry its inflexible interfacing system makes it difficult to use in research.
The Cincinnati Milacron T3 robot is an example of jointed arm robot which most closely resembles the human arm. This type of arm consists of several rigid members connected by rotary joints. In some robots, these members are analogous to the human upper arm, forearm and hand; the joints are analogous to the human shoulder, elbow and wrist.
The T3 robot arm is mounted on a rotary joint whose major axis is perpendicular to the robot mounting plate. This axis is known as the base or waist. Three axes are required to emulate the movement of the wrist and they are called: pitch, yaw and roll.
CONTROL SYSTEM
The T3 robotic arms are controlled using a Hierarchical Control System. A Hierarchical control system is partitioned vertically into levels of control.
The basic command and control structure is a tree, configured such that each computational module has a single superior, and one or more subordinate modules. The top module is where the highest level decisions are made and the longest planning horizon exists. Goals and plans generated at this highest level are transmitted as commands to the next lower level where they are decomposed into sequences of sub goals. These sub goals are in then transmitted to the next lower control decision level as sequences of less complex but more frequent commands.
HIERARCHICAL CONTROL SYSTEM
The hierarchical control structure serves as an overall guideline for the architecture and partitioning of a sensory interactive robot control system.
The system is configured in the hierarchical manner and includes five major subsystems:(1) The Real-Time Control System (RCS)(2) The commercial. T3 Robot equipment(3) the End-Effector System(4) The Vision System(5) The Watchdog Safety System
The Real-Time Control System as shown in figure is composed of four levels:(1) The Task Level(2) The Elemental-Move Level(3) The Primitive Level(4) The T3 Level
The Task, Elemental-Move and Primitive levels of the controller are considered to be Generic Control Levels which would remain essentially the same regardless of the particular robot (commercial or otherwise) being used.
The T3 Level, however, uses information and parameters particular to the T3 Robot and is, therefore, unique to the T3 Robot. The Joystick shown provides an alternate source of commands to the Primitive Level for manual control of the robot and is not used in conjunction with the higher control levels .The T3 controller is subordinate to the T3 Level of the RCS and communicates through a special interface.
The End-Effector System consists of a two fingered gripper equipped with position and force sensing .The gripper is pneumatically actuated and servo controlled by a controller which is subordinate to the Primitive Level of the RCS.
There are three sensory systems on the robot:
1. The finger force and position sensors on the gripper which report data to the End Effector Controller2. The 3 point Angle Acquisition System which reports data to the T3 Controller, the T3 Level of the RCS and to the Watchdog Safety System3. The Vision System which reports data to the Elemental-Move Level of the RCS.Of the sensor systems, the vision system is obviously the most complex. It performssophisticated image processing which requires substantial computational time.
The Watchdog Safety System does not fit directly into the hierarchical control structure. It is an independent system which monitors robot motions and compares them to previously defined limits in position, velocity and acceleration. The Watchdog System has the power to stop the robot if any limits are exceeded and consequently monitors both the mechanical and control systems of the robot.
PARTS OF THE REAL TIME CONTROL SYSTEM(1) Task LevelThe Task Level interfaces with the Workstation Level above it and the Elemental-Move Level below it. The Task Level receives commands from the Workstation Level in terms of objects to be handled and named places in the workstation.For example, the task might be to find a certain part on the tray at the load/unload station, pick it up and put it in the fixture on the machine tool. This task could be issued as one command from the Workstation Level to the Task Level of the RCS.
(2)Elemental-Move LevelThe E-Move Level interfaces with the Task Level above it and the Primitive Level below it. In addition, the E-Move Level interfaces with the Vision System from which it acquires part position and orientation data. The E-Move Level receives commands from the Task Level which are elemental segments of the Task Level command under execution. These are generally single moves from one named location to another. If a part acquisition is involved, data from the Vision System is requested to determine the exact location of the next goal point. The E-Move Level then develops a trajectory between the new goal point and its current position.
(3)Primitive LevelThe Primitive Level interfaces with the E-Move Level above it and the T3 Level and End-Effector Controller below it. The Primitive Level is the lowest level in the RCSwhich is robot or device independent. Subsystems subordinate to the Primitive Level are considered to be at the device level in the control hierarchy. In this system, these subsystems or devices are the robot and the end-effector. The Primitive Level interfaces with the Joystick. The Joystick is a peripheral device which is used for manual operation of the robot. Using the Joystick, the operator can control robot motion in several coordinate systems (world, tool or individual joint motions).
(4) T3 LevelThe T3 Level interfaces with the Primitive Level above it and the commercial Cincinnati Milacron T3 Robot Controller below it. In addition there is a sensory interface which supplies the six individual joint angles. The T3 Level is so named because elements of it are peculiar to the T3 Robot. From a control hierarchy point of view the T3 Level does not constitute a logical control decision level but is infact a “gray box” necessary to transform command and feedback formats between the Primitive level and T3 controller.
APPLICATIONS
Hydraulically actuated, the T3 is used in applications such as welding automobile bodies, transferring automobile bumpers and loading machine tools. In 1975, the T3 was introduced for drilling operations and in the same year T3 became the first robot to be used in the aerospace industry.

ServoMechanisms

July 27, 2009

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A system for the automatic control of motion by means of feedback. The term servomechanism, or servo for short, is sometimes used interchangeably with feedback control system (servosystem). In a narrower sense, servomechanism refers to the feedback control of a single variable (feedback loop or servo loop). In the strictest sense, the term servomechanism is restricted to a feedback loop in which the controlled quantity or output is mechanical position or one of its derivatives (velocity and acceleration). See also Control systems.

The purpose of a servomechanism is to provide one or more of the following objectives: (1) ac­curate control of motion without the need for human attendants (automatic control); (2) maintenance of accuracy with mechanical load variations, changes in the environment, power supply fluctuations, and aging and deterioration of components (regulation and self-calibration); (3) control of a high-power load from a low-power command signal (power amplification); (4) control of an output from a remotely located input, without the use of mechanical linkages (remote control, shaft repeater).

The illustration shows the basic elements of a servomechanism and their interconnections; in this type of block diagram the connection between elements is such that only a unidirectional cause-and-effect action takes place in the direction shown by the arrows. The arrows form a closed path or loop; hence this is a single-loop servomechanism or, simply, a servo loop. More complex servomechanisms may have two or more loops (multiloop servo), and a complete control system may contain many servomechanisms. See also Block diagram.

Servo loop elements and their interconnections. Cause-and-effect action takes place in the directions of arrows. (After American National Standards Institute, Terminology for Automatic Control, ANSI C85.1)
Servo loop elements and their interconnections. Cause-and-effect action takes place in the directions of arrows. (After American National Standards Institute, Terminology for Automatic Control, ANSI C85.1)

Servomechanisms were first used in speed governing of engines, automatic steering of ships, automatic control of guns, and electromechanical analog computers. Today, servomechanisms are employed in almost every industrial field. Among the applications are cutting tools for discrete parts manufacturing, rollers in sheet and web processes, elevators, automobile and aircraft engines, robots, remote manipulators and teleoperators, telescopes, antennas, space vehicles, mechanical knee and arm prostheses, and tape, disk, and film drives.

History

James Watt’s steam engine governor is generally considered the first powered feedback system. The windmill fantail is an earlier example of automatic control, but since it does not have an amplifier or gain, it is not usually considered a servomechanism.

The first feedback position control device was the ship steering engine, used to position the rudder of large ships based on the position of ship’s wheel. This technology was first used on the SS Great Eastern in 1866. Steam steering engines had the characteristics of a modern servomechanism: an input, an output, an error signal, and a means for amplifying the error signal used for negative feedback to drive the error towards zero.

Electrical servomechanisms require a power amplifier. World War II saw the development of electrical fire-control servomechanisms, using an amplidyne as the power amplifier. Vacuum tube amplifiers were used in the UNISERVO tape drive for the UNIVAC I computer.

Modern servomechanisms use solid state power amplifiers, usually built from MOSFET or thyristor devices. Small servos may use power transistors.

The origin of the word is believed to come from the French “Le Servomoteur” or the slavemotor, first used by J. J. L. Farcot in 1868 to describe hydraulic and steam engines for use in ship steering.

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July 19, 2009

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