Reference no: EM133129264

Industrial Process Control

Objectives
The main objective of experiment is to investigate the best controller type for controlling a heating system. Study the difference between ON/OF and continuous control in terms of the ability to stability the system by eliminating offsets/deviations.

Experimental Procedure

Experiment 1: Two Step (On-Off) Control
This is one of the most common forms of closed loop feedback control and simply involves the electrical power being switched on when the measured temperature falls below the set value and switched off when it rises above it (see Figure 1).

Connect the deviation signal to channel B on the Oscilloscope and the power supply signal to channel A. Ensure that other connections and settings are as in Figure 1. Check that the X-Y link on the PC326 is connected. Set the oscilloscope ground (zero voltage) signal baseline to the central x-axis on the scope display grid (y = 0) for channel B and to 2 cm below the centre line (y-2) for channel A. Ask the demonstrator to show you how to do this.

With the controller set to two-step control on the PCS327 the deviation signal should approximate a sine wave oscillating about the zero deviation line.

Use the Erase button on the scope to reset the trace.

Two-step control is one of the most common forms of closed loop control and simply involves power to the electrical heater being switched on when the measured temperature falls below the set value and switched off when it rises above. When overlap is introduced, the controller output signal causes the power applied to the heater to alternate between maximum and minimum levels as the controlled condition falls below a lower limit or rises above an upper limit (see diagram). The limits heater power dial allows the power applied to the heater during the on periods to be set anywhere between 5 and 100%.

The measured temperature continues to increase, after the power has been switched off, until it reaches a maximum value and then it starts to decrease as the airs cools. Once below the set value, the power is switched back on but due to the process time lags the temperature continues to decrease until the heat input reaches the thermocouple.

The shape of the deviation curve may be altered by decreasing the rate of heat input or by changing the time delays in the system. The rate of heat input can be altered by varying the heater power knob and so varying the maximum electrical power; the overall time delay of the process may be altered by varying the distance-velocity lag.

Investigate the influence of varying (a) the heater power supply (i.e. fuel input rate), and
(b) the setting of the air inlet baffle (i.e. load to the furnace). Observe and record the oscilloscope traces accurately, highlighting changes in frequency and amplitude observed and showing that it is possible to get a net positive or a net negative deviation. Comment on the ability of the controller to control the process.

Experiment 2: Continuous (or Proportional) Control

In proportional control, the output from the controller (i.e. power supplied to the heater) can be continuously varied to be approximately in balance with the process input requirement. This is accomplished by relating the controller output, V, to the deviation or error, E:

V = KE + M (1)
where

M is the constant power input at the set value i.e. zero deviation
E is the deviation or error between the measured and set value
K is the proportional constant (usually called gain or sensitivity)

The value of deviation which will cause the controller to operate over its full output range is usually expressed as a percentage of the range of values which the measuring instrument is designed to read. This percentage is termed the proportional bandwidth

given by:

Gain = 100%
Proportonal Band in %

Thus a small or narrow proportional band width represents a high gain or sensitivity and
hence only a small change in deviation is required to produce a large change in controller output. The above relationships are further illustrated graphically (see enclosed diagrams) for a reverse acting controller. It is called reverse acting because the output decreases with increasing temperature as in our case.

Now begin to decrease the bandwidth and observe that the steady state deviation voltage (from the oscilloscope trace) is also decreased, until finally instability occurs. It is evident that in order to try to reduce the deviation to zero, the gain or sensitivity must be increased to such a value that the system becomes completely unstable. A compromise gain level (%bandwidth) must therefore be used which maintains a stable system but results in a steady state deviation signal.
The sustained deviation, dependent on the proportional bandwidth, which occurs during steady state running is known as "offset". The application of integral controller action can eliminate offset.

Experiment 3: Continuous Control - Step Change
In this set of tests the internal set value disturbance switch is used to introduce a step change to the set value and the response of the system temperature deviation observed.

Draw traces of the system response (integral action OFF) for bandwidths of 40-50% and 100- 120%. Allow enough time for the system to stabilise between readings.

Experiment 4: Integral and Derivative Controller Actions
For the same settings as above, investigate the influence of integral action (between 0.5-1.0 on the dial) on the response of the system at a bandwidth of 40-50%. Also show that integral action can eliminate the offset but introduces.

Investigate the effect of introducing derivative controller action (between 0.1 - 0.3 on the dial) for a given integral setting as above and show that the extra oscillations induced by integral action following a step change can now be damped out.

Record traces of the system response.

HINT:
Discuss the difference between controllers in terms of oscillations/amplitude, response and settling time (how quick or slow the response it to reach the set point), any deviations/offsets.

Discuss the influence of the different P-bands on P-control, the different integral times on PI-control, and the different derivative times on PID-control.

You should summarise the results of all the controllers in a table (s) including deviations, settling times and oscillations.

Attachment:- Industrial Process Control.rar