Control System Lab Experiment No.-5 HVACT ModellingName- Rahul Kumar Meena(B12089)Group No.-8Group Member- Reeta Meena Seela Aiswarya

Objective- Modelling of HVACT on-off control and finding it's transfer function.

Theory- A simple feedback mechanism of on-off control can be described as follows:

where e=r-yis the control error which is the difference between the reference signal, r, and output of the system y. The control law implies that maximum corrective action is always is used which explains the name of on-off control.

A system with on-off control will always oscillate, in many cases the amplitude of theoscillations is so small that they can be tolerated. QNET heating and ventilation trainer (HVACT) is the system consists of a plexiglass duct, with a heater in one end and a blower in the other end. There is a thermistor sensor placed inside the duct to measure the temperature of the chamber and another thermistor sensor outside the chamber to measure the room temperature.

On-off control is one of the simplest control strategies. The heater is switched on when the temperature is lower than the desired value, and the heater is switched off when the temperature is higher than the desired value. To avoid rapid switches it is common to introduce a hysteresis in the relay switch.

The on-off control input and the measured temperature output from the experiment have an interesting property that makes it possible to find a simple model for the process. The temperature response is a ramp due a voltage step therefore the temperature is the integral of the voltage. In our experiment the process can be modeled by the simple transfer function P(s).

WhereP(s)=(Kv)/s And the parameter Kv is the slope of the ramp.

Hysteresis helps to avoid unintentional switches.

Fig.- on-off control variation

block diagram of feedback of HVACT:

Observation and Result-Effect of offset in the Signal generator on Temperature:-

1)Offset=1 C

2) Offset = 1.5 C

3) Offset =2 C

Result:- By increasing the offset in the signal generator the reference temperature increases. If we keep all other settings unchanged and just increase offset, we can never get the actual temperature equal to the reference temperature.

Vary the heater voltage relay amplitude Vh_amp and observe the heater voltage effect on temperature variation-

1) Vh_amp = 2.5V

2) Vh_amp = 5V

Result:- By increasing heater voltage we get more damped response or we can say that we get actual temperature approximately equal to the reference temperature within the same time range.

Observing the effect of changing relay mean-

1) Vh_off = 2.5V

2) Vh_off = 1V

Result:- By increasing the relay mean the overshoot and undershoot values increases (w.r.t. reference temperature).

Observing the effect of changing relay width-

1) DTh = 0.5 oC

2) DTh = 1 oC

Result:- By increasing the relay width the overshoot and undershoot increases w.r.t. reference temperature.

For the given signal generator set and control parameter set, find the parameter Kv for our on- off control system which has transfer function P(s) = (Kv)/s-