Air Pressure Control Theory 566f1r

The purpose of PID control is to calculate and minimize errors between the actual value (PV) and the desired value (SP) of a process. PID consist of three separate parameters which can be defined base on the current rate of change. P represents current errors, I is the accumulation of past errors while D indicates future errors. The response of a controller can be divided into three, which are responsiveness to errors, degree of overshoots and degree of oscillations. PID control enhances overall temperature regulation. Combining, the three parameters of current data, past data, and the rate at which data is changing by setting a case-specific algorithm allows for a stable control of pressure. By compensating for temperature error between the process variable and the setpoint, a steady pressure can be maintained. The open-loop control and closed-loop control are often not clearly distinguished. In simple , closed-loop control represents an automated process in which no manpower is needed to stabilize the process. Open-loop control on the other hand refers to a manual control method in which disturbances is needed to be manually dealt with using the controller. PV will be measured and fed into a control device where the MV can be adjusted by hand. Open-loop control can only counteract against disturbances, for which it has been designed for. In an open loop system, a process can be categorized into two categories; self-regulating process or a non-self regulating proces.

Figure 1 Self-regulating process

Pressure control process is a self-regulating process. The process moves to a new steady position upon introduction to an open loop test as shown in Figure 1. In an open loop test, parameters of response rate (RR), dead time (T d) and time constant (TC) shall be analysed. These parameters are the essence to the optimum controller setting. In this experiment, a reformulated tangential method was used to calculate the three parameters stated. The formula is as given: Response Rate, RR=

tan θ a ∙ Δ MV b

Dead time, Td(time) = Td (length) x b Time constant, TC (time) = TC (length) x b Table 1- Tuning Rules by Ziegler-Nichols: Mode

P

I

P

100RR⋅Td

PI

111.1RR⋅Td

3.33Td

PID

83.3RR⋅Td

2Td

D

0.5Td

The process parameters is then substituted into the tuning rules based on the mode of control selected. The calculated optimum P,I and D values shall gives an optimal control of the process towards the desired setpoint.