Question

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Compare the different types of methods which are used for tuning valves, such as outlining the advantages and disadvantages of each. Compare at least four different methods

Answer 1

1.  Direct Synthesis Method:

This tuning method is a prototype based approach. This particular method utilises an recognized operation model in synchrony with a user identified closed-loop response profile. A positive outlook to this method is that it gives an understanding on the function of the prototype in control system design. A disadvantage to the method is that a PID controller may not be accomplished unless a suitable prototype form is used to combine the control law.

Consider a first order system given by:

As we need a desired closed loop response:

Feedback system is given by:

From the above two equations, we get:

Here lambda is a tuning parameter. Lesser the value of lambda, greater will be the closed loop response and vice versa.

2.  Ziegler-Nichols Tuning Method:

The most widespread tuning approach was suggested by Ziegler and Nichols. This tuning approach needs the finding of the maximum gain and maximum period. This can be attained by varying the controller gain till the system experiences prolonged oscillations at the maximum gain. This is attained by preserving the integral time constant at ? and the derivative time constant at 0. The PID controller parameters can be determined using standard tables. It's advantageous as it is rapid and easier to utilise than other methods and also it is a durable and substantial method for tuning. It is very limited in use as it tunes the cycle for quarter-amplitude-damping response, which exceeds and oscillatesa lot also it exits the loop with very less robustness and this leads to loop uncertainty.

3.  Relay Auto-tuning Method:

Relay-based auto-tuning is a straightforward approach to tune PID controllers that evades trial and error, and lessens the prospect of controlling the plant proximal to the stable limit. Auto-tuning is based on the principle of utilising a relay controller which is a simple on-off switch. At the beginning, the plant oscillates lacking a specific pattern everywhere about the nominal output value till a specific and reiterated output response can be established. When the needed response arrangement has been attained, the oscillation period and the amplitude of the plant response can be sustained and utilised for PID controller tuning. The maximum gain can be found as:

where,

K is the maximum gain

t is the amplitude of PID Controller output

A is the amplitude of plant response

4. Manual Tuning

Manual PID tuning is performed by fixing the reset time to its optimum value and the pace to 0 and enlarging the gain till the loop oscillates at a fixed amplitude. When the response to an error improvement happens, immediately a bigegr gain can be utlised. If response is less a comparatively smaller gain is required. Then fix the gain of the PID controller to half of that value and manipulate the reset time so it counteracts for any offset inside an adequate period. Hence, later enlarge the pace of the PID loop till transcend is lessened.