Page 50 - Instrumentation and Measurement

4.2.2 Dead Time

In any control system with feedback the system cannot respond instantly to any change and thus there
are delays while the system takes time to accommodate the change. Such delays are referred to as
dead time or lags. For example, in the control of the temperature in a room by means of a central
heating system, if a window is suddenly opened and the temperature drops or the thermostat is
suddenly set to a new value, a lag will occur before the control system responds, switches on the
heater and gets the temperature back to its set value. A source of dead time in a control system is the
response time of the measurement sensor.

Transfer delays are a common event with control systems where flow is concerned, e.g. water flowing
along a pipe from point A where the control valve is to point B where the rate of flow is required and
monitored. Any change made at some point A will take some time before its affects are apparent at
a point B, the time delay depending on the distance between A and B and the rate of flow between
them. The term distance-velocity lag is sometimes used to describe such delays. An example of a
transfer delay is where a hopper is loading material onto a conveyor belt moving with a velocity v.
The rate at which the material leaves the hopper is controlled by a valve with feedback from a weight
sensor. If the weight of deposited material per unit length of belt is monitored a distance L from the
hopper discharge point, then there will be a time delay of L/v in the control system.

4.2.3 Capacitance

In the level control of the level of water in a tank, an important attribute of the system is its
capacitance. If we have water leaving the tank and the control signal used to determine the rate of
flow of water into the tank, then the greater the surface area of the water in the tank the longer it will
take the controlled inflow of water to respond and restore a drop in level. We talk of the system
having capacitance and the greater the capacitance the longer it takes to react to changes. If the
capacitance was decreased then the system would react quicker to make the changes necessary to
restore the required level. As an illustration of the effect of capacitance consider a domestic heating
system controlled by a thermostat. The larger the space being heated the longer it will take the
controller to respond and restore a drop in temperature. Again, we talk of the capacitance of the
system.

4.3 On-Off Control

With on-off control, the controller is essentially a switch which is activated by the error signal and
supplies just an on-off correcting signal (Figure 4.1). The controller output has just two possible
values, equivalent to on and off. For this reason, the controller is sometimes termed a two-step
controller.

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