STEPPER MOTOR

A stepper motor is a brushless, synchronous electric motor that can divide a full rotation into a large number of steps. So it’s a kind of synchro with the number of poles (on both rotor and stator) increased.

Like in synchro, the speed of the motor shaft rotation is directly related to the frequency of the input pulses and the length of rotation is directly related to the number of input pulses applied.

Like synchros, they are used wherever controlled movement is required: in applications where you need to control rotation angle, speed, position and synchronisation..

There are two distinctly different ways of using stepper motors in control systems.One is the open loop mode and other is the closed loop mode.
The stepper motor is a digital device whose output in shaft angular displacement is completely determined by the number of input pulses.Consequently,there is no need for a feedback device to determine the position of motor shaft and ,therefore,of the load connected to the motor shaft.This means that an open –loop step servo system can be designed to yield the the same accuracy as that of a closed loop analog system.figure(1) shows use of stepper motor in open loop mode. Use of steppermotor in closedloop mode requires synchros as error detector.Here the motor is used like conventional servomotor.A signal from the output is fed back and is used to operate a gate controlling the pulses from a pulse generator.This is shown in the figure below:

Reference

1.http://en.wikipedia.org/wiki/Synchronous_motor

2.control systems engineering-Nagrath & Gopal

Consider the second-order system given by

The poles are given by s = –p1 and s = –p2). When we add a zero at s = –z1 to the controller, the open-loop transfer function will change to:

We can put the zero at three different positions with respect to the pole.The effect of changing the gain K on the position of closed-loop pole and type of responses.

(a) The zero s = –z1 is not present.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. Hence  time response is low

(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. Fast response is possible.

(d) The zero s = –z1 is located to the left of s = –p2 .By placing the zero to the left of bothpoles, 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 providefaster time responses.

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.

Effect of a closed loop zero on unit step response of asecond order system

C(t)is the response of the sysem witout adding zero

Cz(t)  be the response of the system with a zero at s=-z

Reference

1.control systems engineering-Nagrath

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

It has typically four parts:

1. light source(LED)

2.  rotary disc

3. stationary mask

4. sensor(photodiode)

The disc has alternate opaque and transparent sectors which are etched by means of a photographic process on to a plastic disc .As the disc rotates during half of the increment cycle the transparent sectors of rotating and statonary discs come in allignment permitting the light from LED to reach the sensor thereby generating an electrical pulse .For fine resolution encoders ,multi-slit mask is often used to maximize the reception of shutter light.The output of the encoder is fed to a counter which counts the number of pulses:the count being a measure of angle through which the encoder shaft has rotate.By sampling the counter at regular intervals by means of clock pulses it is possible to compute the speed of the encoder shaft

It consists of two tracks and two sensors whose outputs are called channels A and B. 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. The code disk pattern and output signals A and B are illustrated in Fig.. By counting the number of pulses and knowing the resolution of the disk, the angular motion can be measured. The A and B channels are used to determine the direction of rotation by assessing which channels “leads” the other.

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.

The values of the poles and the zeros of a system determine whether the system is stable, and how well the system performs.

.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

Pole –zero plot

In the plot axes represent the real and imaginary parts of the complex variable s.Such plots are known as pole-zero plots. It is usual to mark a zero location by a circle (◦) and a pole location a cross (×). The location of the poles and zeros provide qualitative insights into the response characteristics of a system

The location of the poles in the s-plane therefore define the n components in the homogeneous response as described below:

 1. A real pole pi = −σ in the left-half of the s-plane defines an exponentially decaying component Ce^−σt, in the homogeneous response. The rate of the decay is determined by the pole location; poles far from the origin in the left-half plane correspond to components that decay rapidly, while poles near the origin correspond to slowly decaying components.

 2. A pole at the origin pi = 0 defines a component that is constant in amplitude and defined by the initial conditions.

 3. A real pole in the right-half plane corresponds to an exponentially increasing component Ce^σin the homogeneous response; thus defining the system to be unstable.

4. A complex conjugate pole pair σ+ jω in the left-half of the s-plane combine to generate a response component that is a decaying sinusoid of the form Ae^−σt sin (ωt + φ) where A and φ are determined by the initial conditions. The rate of decay is specified by σ; the frequency of oscillation is determined by ω.

 5. An imaginary pole pair, that is a pole pair lying on the imaginary axis, +jω generates an oscillatory component with a constant amplitude determined by the initial conditions.

6. A complex pole pair in the right half plane generates an exponentially increasing component.

Effects of poles and zeros

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. An output value of infinity should raise an alarm bell for people who are familiar with BIBO stability. Real parts correspond to exponentials, and imaginary parts correspond to sinusoidal values.

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 I 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.

Servomechanism

A 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 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.

 The defining characteristic of a servomechanism is that the controlled output of a mechanism is automatically compared with the controlling input. The difference between the settings or positions of the output and the input is called the error signal, which acts to bring the output to its desired value. Servomechanisms may be mechanical, electrical, hydraulic, or optical. The process of sending the error signal back for comparison with the input is called feedback, and the whole process of the input, output, error signal, and feedback is called a closed 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).

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.

 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) 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.

(4) control of an output from a remotely located input, without the use of mechanical linkages (remote control, shaft repeater).

Pneumatic servomechanisms

A servomechanism in which power is supplied and transmission of signals is carried out through the medium of compressed air.

Pneumatic servomechanism have the advantages of low cost,high power to weight ratio,ease of maintanensce,cleanliness and a readily- available and cheap power source. However it have the disadvantage  are high,nonlinear friction forces ,deadband  due to stiction and dead time due to the compressibility of air.

Servomotor

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.

A 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. Typical servos give a rotary (angular) output. Linear types are common as well, using a screw thread or a linear motor to give linear motion.

Industrial servomotor
The grey/green cylinder is the brush-type DC motor. The black section at the bottom contains the planetary reduction gear, and the black object atop the motor is the optical rotary encoder for position feedback. This is the steering actuator of a large robot vehicles.

RC servo

Small R/C servo mechanism
1. electric motor
2. position feedback potentiomete
3. reduction gear
4. actuator arm

RC servos are hobbyist remote control devices servos typically employed in radio-controlled models, where they are used to provide actuation for various mechanical systems such as the steering of a car, the flaps on a plane, or the rudder of a boat.

 Applications

Servomechanisms were first used in in military fire-control and marine navigation equipment. 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 machine tools and aircraft engines, robots, remote manipulators and teleoperators, telescopes, antennas, space vehicles, satellite tracking antennas ,remote control airplanes,anti -aircraft gun control systems,mechanical knee and arm prostheses, and tape, disk, and film drives. . 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.

•    Communication satellites

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.

•  . Another example of a servomechanism is the automatic control system by which a THERMOSTAT, (q.v.) in one of the rooms of a house controls the heat output of the heating furnace.
•  Space vehicles

 Unmanned spacecraft are automatically turned to point their cameras, radio antennae, and solar     panels in the desired directions by servomechanisms. The input in that case is the sensing of the direction of the sun and stars, and the output is the control of small jets that turn and orient the spacecraft

•   Automobiles

The power steering system in an automobile is an example of a servomechanism. The direction of the front wheels is controlled by the angle of the steering wheel. Should the motion of the car turn the front wheels away from the desired direction, the servomechanism, consisting of a mechanical and hydraulic system, automatically brings the wheels back to the desired direction.

Cincinnati Milacron T3 Industrial Robot

In 1973 the first commercially available minicomputer-controlled industrial robot is developed by Richard Hohn for Cincinnati Milacron Corporation. The robot is called the T3, The Tomorrow Tool.

Cincinnati Milacron, T3 robot was adapted and programmed to do drilling operations and the circulation of materials in airplane components under the sponsoring of Air Force ICAM (Integrated Computer Aided Manufacturing)

The  T3  Robot  is  a  six-axis  servo-controlled  articulated  arm  type  manipulator  powered  by hydraulic  actuators.  It is  manufactured  by  the Cincinnati  Milacron  Company  located  in  Cincinna Ohio.  All  of the actuators,  except  the  one  on the elbow  joint,  are  rotational  actuators.  The actuator  on  the elbow  is  a  linear  piston  type actuator.  All  of  the  joints,  including  the  elbow are  rotational  and  give  the  T3 six  degrees  of freedom.  The  actuators  support  and  move  the  mass of  the  T3 and  provide  a  rated  lifting  capacity  of 100 pounds

Cincinnati Milacron built large industrial robots primarily for the welding industry.The robot division has been published by Asea Brown Boweri (ABB),which continues to offer the product line. Cincinnati Milacron was one of the first companies to change from hydraulic to electric robots. Milacron pioneered the first computerized numerical control(CNC) robot, with improved wrists and the tool center  point (TCP0 concepts. The first hydraulic machine, the T3 was introduced in 1978 and closely resembled the General Electric Manmate , ITTArm, and other predecessors. Constructed of cast aluminium,it is available in two models of six-axes revolute joined arms. The largest,the T3-776,uses ballscrew electric  drives to power the shoulder and elbow pitch.The ball screws replaced the hydraulic cylinders originally used on the T3 robots.The elbow  is a classic example of the intermediate drive elbow. The same technique only upside down, appear in the shoulder. Shoulder yaw is provided by the standard bull gear on a base mounted motor drive. End users have discovered that ballscrews are not sufficiently reliable and are pressuring for alternatives. The eventual disappearance of the ballscrew in industrial robots seems inevitable .base mounted motor drive. End users have discovered that ballscrews are not sufficiently reliable and are pressuring for alternatives. The eventual disappearance of the ballscrew in industrial robots seems inevitable 

The  T3  Robot  is  a  six-axis  servo-controlled  articulated  arm  type  manipulator  powered  by hydraulic  actuators.  It is  manufactured  by  the Cincinnati  Milacron  Company  located  in  CincinnaOhio.  All  of the actuators,  except  the  one  on the elbow  joint,  are  rotational  actuators.  The actuator  on  the elbow  is  a  linear  piston  type actuator.  All  of  the  joints,  including  the  elbow are  rotational  and  give  the  T3 six  degrees  of freedom.  The  actuators  support  and  move  the  mass of  the  T3 and  provide  a  rated  lifting  capacity  of 100 pounds.