What is synchro?How is it related with stepper motor?

SYNCHRO

Normally, the rotor windings of a wound rotor induction motor are shorted out after starting. During starting, resistance may be placed in series with the rotor windings to limit starting current. If these windings are connected to a common starting resistance, the two rotors will remain synchronized during starting.This is usefull for printing presses and draw bridges, where two motors need to be synchronized during starting. Once started, and the rotors are shorted, the synchronizing torque is absent. The higher the resistance during starting, the higher the synchronizing torque for a pair of motors. If the starting resistors are removed, but the rotors still paralleled, there is no starting torque. However there is a substantial synchronizing torque. This is a selsyn, which is an abbreviation for “self synchronous”.

STEPPER MOTOR

A stepper motor is a “digital” version of the electric motor. The rotor moves in discrete steps as commanded, rather than rotating continuously like a conventional motor. When stopped but energized, a stepper (short for stepper motor) holds its load steady with a holding torque. Wide spread acceptance of the stepper motor within the last two decades was driven by the ascendancy of digital electronics. Modern solid state driver electronics was a key to its success. And, microprocessors readily interface to stepper motor driver circuits.

INCREMENTAL ENCODERS

Are they useful to us in any way?

•Incremental encoders are position feedback devices that provides incremental counts. Thus, incremental encoders provide relative position, where the feedback signal is always referenced to a start or home position. For incremental encoders, each mechanical position is uniquely defined. The current position sensed is only incremental from the last position sensed. Incremental encoders are also non-contacting optical, rotary, quadrature output device. •These incremental encoders are also called optical encoders or optical incremental encoders because they utilizes optical technology. Optical incremental encoders are highly sort after as position feedback devices due to its durability and ability to achieve high resolution. Avago’s optical incremental encoders are exceptionally recognized for its reliability and accuracy.

Here K_dt represents the gain of the device and the truncated value y(kT) is subsequently differentiated.

For instance, if the input to the PG represents the position/displacement in radians, the unit of K_dt is (pulse/rev). Hence the units of y(kT) are pulses and after differentiation , we obtain the velocity signal y’(kT) in (pulses /T). [2]

References:

1. http://engknowledge.com/
2. G.A.Perdikaris, Computer controlled systems: Theory and Applications, Kluwer Academic Publishers, 14, 1996

3.wikipedia and control engg text

What do the poles and zeros contribute to in the control system ?
Poles and Zeros of a transfer function are the frequencies for which the value of the transfer function becomes infinity or zero respectively. 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 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. Systems that satisfy this relationship are called proper. We will elaborate on this below.

Let's say we have a transfer function defined as a ratio of two polynomials:

Where N(s) and D(s) are simple polynomials. 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. Because of our restriction above, that a transfer function must not have more zeros then poles, we can state that the polynomial order of D(s) must be greater then or equal to the polynomial order of N(s).

•Example •Consider the transfer function:H(S)=(s+2)/(s2+0.25) •We define N(s) and D(s) to be the numerator and denominator polynomials, as such: •N(s) = s + 2 •D(s) = s2 + 0.25 •We set N(s) to zero, and solve for s: •So we have a zero at s → -2. Now, we set D(s) to zero, and solve for s to obtain the poles of the equation: •And simplifying this gives us poles at: -i/2 , +i/2. Remember, s is a complex variable, and it can therefore take imaginary and complex values.

REFERENCES:wikipedia and control engg. main text

:http://en.wikibooks.org/

Effect of Adding a Zero

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+z₁) / ((s+p₁)(s+p₂)) , z₁>0

Effect of adding a zero to a second-order system root locus.

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)

(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. Thus K for the system can be overdamped, critically damped or underdamped.

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

Here, the system can have only real poles. Hence only one value for K to make the system overdamped exists. 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 jω-axis) lying further from the jω-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, we can now change the damping ratio and the natural frequency . 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. [1]

References:

1. http://www.palgrave.com/

CINCINATI MILACRON T3 ARM

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 is controlled using a Hierarchical Control System.A Hierarchical control system is partitioned vertically into levels of control. The basic comand 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 subgoals. These subgoals are in turn transmitted to the next lower control decision level as sequences of less complex but more frequent commands. In general,the decisions and corresponding decompositions at each level take into account: (a) conrmands from the level above, (b) processed sensory feedback information appropriate to that control decision level, and (c) status reports from decision control modules at the next lower control level.

The figure shown above depicts the schematic block diagram of the integrated control structure as configured on the Cincinnati Milacron T3 Robot. 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. That is, these levels 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 shown in figure is part of the T3 Robot equipment as purchased from Cincinnati Milacron. This controller is subordinate to the T3 Level of the RCS and communicates through a special interface.

SERVO MECHANISM AND ITS APPLICATION

Servomechanism is an automatic device for the control of a large power output by means of a small power input or for maintaining correct operating conditions in a mechanism. It is a type of feedback control system. The constant speed control system of a DC motor is a servomechanism that monitors any variations in the motor's speed so that it can quickly and automatically return the speed to its correct value. Servomechanisms are also used for the control systems of guided missiles, aircraft, and manufacturing machinery


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.

The common type of servo provides position control. Servos are commonly electrical or partially electronic in nature, using an electric motor as the primary means of creating mechanical force. Other types of servos use hydraulics, pneumatics, or magnetic principles. Usually, servos operate on the principle of negative feedback, where the control input is compared to the actual position of the mechanical system as measured by some sort of transducer at the output. Any difference between the actual and wanted values (an "error signal") is amplified and used to drive the system in the direction necessary to reduce or eliminate the error. An entire science known as control theory has been developed on this type of system.