The motors that are used in automatic control systems are called Servomotors. When the objective of the system is to control the position of an object then the system is called servo mechanism. The servomechanism is a feedback control system in which the output is mechanical position.
The feedback system samples the output to produce a feedback signal which is proportional to current output. Usually the feedback system consists of sensors and associated circuit or devices. Transducers, etc., are used as feedback systems.
PANTECH SOLUTION has designed the trainer module to perform DC position control. This module consists of a high performance Permanent Magnet DC Motor, often with an integral tacho generator and a chopper based power circuit.
On successful completion of this manual you should be able to:
Figure1 . Block Diagram
The block diagram consists of
A comparator is a circuit which compares two signals and determines which one is greater. The result of this comparison is indicated by the output voltage. In our module a comparator compares the carrier signal with position feedback loop signal or speed feedback Signal. The output of comparator is given as input to the delay circuit. Delay circuit is used to avoid the short circuit problems when the two MOSFETs operate at the same time.
Delay circuit output is given to the driver circuit. The driver circuit amplifies this signal and converts it into the desired output level. The driver circuit output is in the form of PWM pulses. These pulses are given to the gate of the power circuit MOSFET devices.
The power circuit consists of MOSFET based Four quadrant bipolar chopper circuit. The PWM pulses are obtained from the firing circuit. The output of power circuit is connected to motor load.
Motor setup consists of PMDC motor, Position sensor and Tacho generator. The position sensor sense the motor position and the tacho generator sense the motor speed. This position and speed values are given to position feedback loop and speed feedback loop for motor control.
Position Feedback Loop consists of error detector, P and I gain control knobs. The output position and the input position of the motor is given to error detector provided in the position feed back loop. The error detector compares these two signals and produces the error signal. The error signal will be a weak signal and so it has to be amplified and integrated or differentiated to produce a control signal using P and I controller. The output of the position feedback loop is given to the comparator.
Speed Feedback Loop
Speed Feedback Loop consists of error detector and P gain control knobs. The output speed and the input speed of the motor is given to error detector provided on the speed feedback loop. The error detector compares these two signals and produces the error signal. The error signal will be a weak signal and so it has to be amplified and differentiated to produce a control signal using P controller. The output of the speed feedback loop is given to the comparator.
Front Panel Description
DC Position Control system Trainer consists of the following sections.
In a system when the output quantity is controlled by varying the input quantity, the system is called control system. The output quantity is called controlled variable or response the input quantity is called command signal or excitation.
The basic components of an automatic control system are error detector, amplifier and controller, power actuator, system and sensor (or) feedback system. The block diagram of an automatic control system is shown below
Figure 3. Block Diagram of Automatic Control System
The error amplifier compares the reference input signal V with feedback signal. The output is a voltage proportional to the difference between the two signals.
This constitutes the output sensor and associated amplifier. The feedback signal V is the voltage proportional to the output variable of the system.
In Position Control System the reference input will be an input signal proportional to desired output. The feedback signal is a signal proportional to current output of the system. The error detector compares the reference input and feedback signal and if there is a difference it produces an error signal. The error signal is used to correct the output if there is a deviation from the desired value.
The controller process the error signal and gives an output voltage signal V known as the control voltage. This suggests the necessary corrective measures required in the actuating signal V to be applied to the system.
Figure 4. Block Diagram of P Controller
The integral controller is a device that produces a control signal, u(t) which is proportional to integral of the input error signal, e(t).
Figure 5. Block Diagram of I Controller
The Proportional - Plus - Integral Controller produces an output signal consisting of two terms-one proportional to error signal and the other proportional to the integral of error signal
In the above figure, the angular position of the output shaft is intended to follow the reference voltage (2 ) but it should be clear that if the motor drives a toothed belt linear outputs can also be obtained. The potentiometer is mounted on the output shaft. The voltage from this potentiometer must be a linear function of angle, and must not vary with temperature, otherwise the accuracy of the system will be in doubt.
The feedback voltage (representing the actual angle of the shaft) is subtracted from the reference voltage (representing the desired position) and the resulting position error signal is amplified and used to drive the motor so as to rotate the output shaft in the desired direction. When the output shaft reaches the target position, the position error becomes zero, no voltage is applied to the motor, and the output shaft remains at rest. Any attempt to physically move the output shaft from its target position immediately creates a position error and a restoring torque is applied by the motor.
The dynamic performance of a simple scheme described above is very unsatisfactory as it stands In order to achieve a fast response and to minimize position errors caused by static friction, the gain of the amplifier needs to be high, but this in turn leads to a high oscillatory response which is usually unacceptable. For some fixed - load applications, matters can be improved by adding a compensating network at the input to the amplifier, but the best solution is to use 'tacho' (speed)feedback (shown as dotted in the above Figure) in addition to the main position. Tacho feedback has no effect on the static behavior, but has the effect of increasing the damping of the transient response. The gain of the amplifier can therefore be made high in order to give a fast response, and the degree of tacho feedback can then be adjusted to provide the required damping. Many servo motors have an integral tacho generator for this purpose.
The example provided above deals with an analog scheme in the interest of simplicity, but digital position control schemes are now gradually taking precedence, especially when brushless motors are used. Complete 'Controllers on a card' are available as off-the-shelf items, and these offer case of interface to other systems as well as providing improved flexibility in shaping the dynamic response.
In many practical cases the desired performance characteristics of control system are specified in terms of time domain quantities. Frequently, the performance characteristics of a control system is specified in terms of the transient response to a unit-step input since it is easy to generate and is sufficiently drastic. In specifying the transient characteristics of a control system to a unit-step input, it is common to specify the following.
The delay time is the time required for the response to reach half the final value, for the very first time.
The rise time is the time required for the response to rise from 10% to 90%, 5% to 95%, or 0% to 100% of its final value. For under damped second order systems, the 0% to 100% rise time is normally used. For over damped systems, the 10% to 90% rise time is commonly used.
The peak time is the time required for the response to reach the first peak of the overshoot.
The maximum overshoot is the maximum peak value of the response curve measured from unity. If the final steady - state value of the response differs from unity, then it is common to use the maximum percent overshoot. The amount of the maximum (Percent) overshoot directly indicates the relative stability of the system.
The settling time is the time required for the response curve to reach and stay with in a range about the final value of size specified by absolute percentage of the final value (usually 2% or 5%). The settling time is related to the largest time constant of the control system.
Static DC to DC converters, called as choppers, achieve a similar function transformers but in DC. Choppers are widely used for traction motor control in electric automobiles, trolley cars, marine hoists, fork lift trucks, and mine haulers. They provide smooth acceleration control High efficiency, and fast dynamic response. Choppers are also used in DC voltage regulators.
In four quadrant operation the output current as well as output voltage can take positive or negative values. The circuit diagram for MOSFET based chopper is shown below.
In the first quadrant the power flows from the source to the load and is assumed to be (+)ve. In the second quadrant the voltage is still positive but the current is negative. The power is there- fore negative. In this case power flows from the load to the source and this can happen if the load is inductive or back emf source such as a DC motor. In the third quadrant both the voltage and current are negative, but the power is positive, and the power flows from the source to the load. In the fourth quadrant voltage is negative but the current is positive. The power is therefore negative.
The four quadrant chopper is widely used in reversible DC motor drives. The reversible DC motor drive system requires, power flow in either direction, in order to achieve fast dynamic response. By employing four quadrant chopper it is possible to implement regeneration and dynamic braking by means of which fast dynamic response is achieved
For bipolar voltage switching the devices (T and T ) are switched ON/OFFsimultaneously.14When this pair of switches are OFF the other pair of switches T3 and T2 are switched ON and vice-versa. The gating signals are generated by comparing a switching frequency triangular Wave form V with the control voltage V . The pulse generated, when V > V are used to turntric ON T and T . The complementary pulses are used to turn ON T and T . The waveforms shows14 the generation of the gating signals.
The duty cycle ratio for the switch pair T and T is14
and duty cycle ratio for the switch pair T and T is32
and the average output DC voltage is given by
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