SOFTWARE OBJECTS

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Software objects are defined based on the sequence of operation. Some of those software objects include schedule, optimum start, calendar and control loop.

PID CONTROL LOOPS

Proportional-Integral-Derivative (PID) control is the most common control algorithm used in industry and has been universally accepted in industrial control. The popularity of PID controllers can be attributed partly to their robust performance in a wide range of operating conditions and partly to their functional simplicity, which allows engineers to operate them in a simple, straightforward manner.

PROPORTIONAL RESPONSE

The proportional component depends only on the difference between the set point and the process variable. This difference is referred to as the Error term. The proportional gain (K_c) determines the ratio of output response to the error signal. In general, increasing the proportional gain will increase the speed of the control system response. However, if the proportional gain is too large, the process variable will begin to oscillate. If K_c is increased further, the oscillations will become larger and the system will become unstable and may even oscillate out of control.

INTEGRAL RESPONSE

The integral component sums the error term over time. The result is that even a small error term will cause the integral component to increase slowly. The integral response will continually increase over time unless the error is zero, so the effect is to drive the Steady-State error to zero. Steady-State error is the final difference between the process variable and set point. A phenomenon called integral windup results when integral action saturates a controller without the controller driving the error signal toward zero.

DERIVATIVE RESPONSE

The derivative component causes the output to decrease if the process variable is increasing rapidly. The derivative response is proportional to the rate of change of the process variable. Most practical control systems use very small derivative time (T_d), because the Derivative Response is highly sensitive to noise in the process variable signal. If the sensor feedback signal is noisy or if the control loop rate is too slow, the derivative response can make the control system unstable.

Software objects

Sequence of Operation Line No.	Object Required	Reason
2,3,34,37,58,61,83,85,111,114, 138,140,167, 169, 180, 181	schedule	To start the equipments according to the weekly schedule
12, 14, 17, 69, 92, 121, 146, 170	Loading schedule	according to the cooling load
6, 38, 62, 87, 116	Running hour schedule	To start the equipments according to the running hours
5, 35, 36, 59, 60, 84, 112, 113	Inter lock	Starting sequence of the chiller plant without making any destructions to the chiller
13, 60, 69	Pre cooling	To reduce the cooling load, before starting chillers
7, 39, 40, 63, 64, 88, 89, 117, 118, 142, 143, 182	Calendar	Holiday Calendar
	Control Loop	Modulation of differential Pressure controller valve
16, 24, 151	Control Loop	Modulation of cooling coil valve
15, 17	Control Loop	To verify the Dampers has closed