Me2142_1

ME2142-1 ~ Speed/ Position Control of a DC Motor

Objectives This experiment serves the following: • • •

To become familiar with the operation of an armature-controlled DC motor. To study the transient and steady state response of a close-loop speed con control trol system. system. To study the transient response of a close-loop position control system.

Dept of Mech & Prod Engineering

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Discussion 1. Discuss the differences in open-loop and close-lop control in achieving a speed control system and the effects of loop gain and load on the outout speed. The open-loop control systems are unsophisticated systems and it is not difficult to see that they would not satisfactorily fulfil critical performance requirements. or instance! if the input voltage is set at a certain initial value! which corresponds to a certain motor speed! when a load "in this experiment! the bra#e setting$ is applied! there is not way to prevent a drop in motor speed. The only way to ma#e the system wor# is to have a means of ad%usting input voltage in response to a change to the load in order to maintain the motor speed at a desired level. The conventional electric was#ing machine is a good example of an open-loop control system because! typically! the amount of machine wash time is determined entirely by the %udgement and estimation of the human operator. &n the case of the close-loop control system! there is a lin# or feedbac# from the output to the input of the system. To obtain a more accurate system! the controlled signal y should be fed bac# and

)efence input r

'ctuating signal u

Controlled variable y Controlled (rocess

Controller

ig.1 *lements of an open-loop control system compared with the refence input! and an actuating signal "in this experiment! a voltage proportional to the negative of the output speed! is obtained through a tachogenerator$ is then fed to the controller so as to correct the error. *rror detector

oad ,otor

Controller

+

θm

-

peed Transducer

ig./ Close-loop idle-speed control system &n the figure above! the reference input voltage sets the desired idling speed. The motor speed at idle should agree with the reference value! and any difference such as the load torque is sensed by the speed transducer and the error detector. The controller will operate on the difference and provide a signal to ad%ust the input voltage to correct the error. 'll in all! the close-loop control system appears to be insentive to external


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disturbances and possible internal variations in system parameters. rom table 1! it is obvious that open-loop control systems can be applied for systems with mininal disturbances! if the cost! weight and si0e are of primary considerations. &n table /! we see that the motor was unable to achieve the desired speed without feedbac# and the increase in load causes speed deviation. Thus! there is no speed control. rom table  and 2! we see that there is a response by the control system when feedbac# is used with the response being higher with a higher gain. Therefore! if we increase the gain proportional! ta#ing into account of the compromise of instability! the system will respond more insensitively to disturbances. /. Discuss the effect of loop gain and brake scale settings on the transient response of the close-loop speed control system. rom table 3! the increase of gain and load will shorten the 345 delay time for the response of the close-loop system. Thus! in the stable region! higher gain and increased load is encouraged for faster reaction of the system towards disturbances. . Discuss the effect of loop gain and velocity feedback on the response of the closeloop position control system. The effect of loop gain and velocity feedbac# is as discussed in para / above. 6owever! we note that the system actually reacts slower compared to para /. 2. Disconnect the speed feedback signal and observe the transient response. Comment. The initial part of the transient response indicates an overshot before it settles down. This is because the pea# time for the control system decreases without feedbac#. &n a control system with speed feedbac# signal! the pea# time for the system is too large for overshot to ta#e place as the input function has already change its polarity.


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3.

Connect for positive feedback and observe the response. Comment.

)"$

+ +

C"$

8 Ts + 1

bs

ig. / (ositive eedbac# Control ystem The transfer function of the above positive feedbac# control system is given by: C(S)  "(S)

! (# - b!)s $ %

The time constant of a control system can be reduced by properly chosing the value of b. &f the value of b is set to T78! then time constant becomes 0ero. 6owever! if disturbances li#e that of this experiment causes T - b8 to become -ve instead of 0ero! the system will become unstable.


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Conclusion &n this experiment! we are introduced to some of the basic concepts of what a control system is and what it is supposed to accomplised. The basic components of a control system is described. 9y demostrating the effects of feedbac# in a rudimentary way! the question of why most control systems are close-loops is clarified. &t should be pointed out that feedbac# can ma#e or brea# a control system. This points out the challenging tas# of the design of a control system! ta#ing into account of certain performance criteria.


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Transient Response of Closed-Loop Speed Control System

Brake 5 Gain 0.05

Brake 5 Gain 0.1

Brake 10 Gain 0.05

Brake 10 Gain 0.1





Transient Response of Closed-Loop Position Control System with Speed eed!a"k

Brake 5 Gain 0.05


Brake 5 Gain 0.1

!


Transient Response with #o Speed eed!a"k Si$nal

Transient Response wfor Positi%e eed!a"k Control System


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Me2142_1

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