Download Experiment 4_Measurement and Equipment Control Slides

Experiment 4: Measurement and
Equipment Control
Freshman Lab
GENG 250
Spring 2014, Prepared by Ali Abu Odeh
1
Introduction
Control systems are used to command or
regulate a process. The process may be
anything from the acceleration of a car to the
output of a factory. In this experiment, the
speed of a DC electric motor is controlled in
real time. The speed is controlled using an
open loop control system and two different
closed loop control schemes.
Spring 2014
2
Background Information
Control systems are classified into two types:
open loop and closed loop. An open loop system
controls a process independent of the output. In
other words, the input affects the output, but the
conditions of the output have no effect on the
system.
For example, adjusting the desired speed changes
the power supply voltage, which changes the
speed of the DC motor as seen in figure 1.
Spring 2014
3
Figure 1: Open loop control of a DC motor.
The higher the voltage, the faster the motor
spins. However, if the motor speed suddenly
changes, the controller does not have the ability
to self-correct the error. In this case a correction
must come from the human operator.
Spring 2014
4
Closed loop systems are characterized by
feedback. In control systems using feedback,
a transducer is used to measure the variable
being controlled (in this case motor speed).
Spring 2014
5
This measurement is then sent back to the
controller to be used as part of the input.
Feedback allows the control system to compare
the desired value to the measured value and
make corrections to the output. Closed loop
control has the ability to respond to unexpected
changes in the variable being controlled without
the help of a human operator.
Spring 2014
6
Figure 2 displays the block diagram of a closed
loop control system for controlling the speed of
a DC motor. The user inputs the desired speed.
The feedback is the measured motor speed,
which is sensed by an opto-switch transducer.
The transducer converts the motor speed
(Revolutions Per Second or RPS) to a voltage
waveform with a corresponding frequency. The
error signal, e(t), is defined as the difference
between the desired motor speed and
measured speed.
Spring 2014
7
The control scheme uses the error signal in
order to compute the control signal, c(t), which
adjusts the power supply, and thus, the speed
of the motor.
Figure 2: Closed loop control of a DC motor.
Spring 2014
8
There are several types of closed loop control
schemes. The appropriate scheme depends on
the type of process to be controlled. In order to
determine the best scheme, the response time,
overshoot and stability are analyzed. The
response time is defined as how fast the system
responds to a change in the desired input or
output load. Overshoot is defined as the
maximum error in a closed loop system that is
responding to a change in the desired input or
load.
Spring 2014
9
Open-Loop Control Scheme
There are many types of open-loop control
schemes. Examples include direct command of
the control signal, as in the engine speed of a
car, or a timed system, like in most water
sprinklers for plants and gardens. As displayed
in figure 1, the open-loop control scheme does
not use any measurements of the output to
control the system. The control signal is
described mathematically by the following
equation: c(t) = co
Spring 2014
10
Closed-Loop On-Off Control Scheme
The control signal of this scheme has two
possible values. The sign of the error signal
(positive or negative) is used to determine which
value to use. The most common application of
an on/off control scheme is an air conditioner. If
the room temperature is more than the desired
temperature, the controller turns on the air
conditioner (applies 240VAC).
Spring 2014
11
If the room temperature is cooler than the
desired temperature, the air conditioner is
turned off (0VAC applied). While on/off control
works well for air conditioning, you will observe
that it does not work well for motor speed
control. The control signal is described
mathematically by the following equation:
 c1
c(t )  
c2
for e(t )  0
for e(t )  0
Spring 2014
12
Closed-Loop Proportional Control Scheme
Another type of control scheme computes the
control signal as a proportion of the error
signal. In other words, the control signal, c(t),
equals the error signal, e(t), multiplied by a
constant of proportionality, KP, called the
proportional gain.
c(t) = KP e(t) + co
Spring 2014
13
Thus, the larger the error, the larger the response
of the controller. While the proportional control is
superior to on/off control for many applications,
care must be taken in the choice of the value, KP.
A value too small causes the controller to
perform virtually the same as the open loop
control system. A value too large causes the
system to become unstable. The best KP value is
between these two limits and chosen to give the
smallest error when the motor is loaded while
still being stable as shown in figure 3.
Spring 2014
14
Figure 3: Proportional, closed-loop control with large KP
value (unstable) and appropriate KP value (stable).
Spring 2014
15
Procedures
The speed of a DC motor is to be controlled.
The motor is used as an actuator in this
experiment. The motor speed changes
according to the voltage of the power supply.
LabVIEW VIs are written to control the power
supply, and thus, the motor speed.
Feedback is provided via an opto-switch
transducer. This transducer produces a number
of pulses for each motor revolution.
Spring 2014
16
The oscilloscope measure driver is used to
measure the frequency of the pulses, which is
then used to determine the motor speed in
revolutions per second.
Figure 4. DC Motor Disk and Equipment Connections for Measuring and
Controlling the Motor Speed.
Spring 2014
17