How the Control Loop is Completed
In this experiment the controller is given instructions by a computer.
On a chemical plant this could be a large computer located in a control
room, handling many control loops at the same time. Alternatively it
could be, as in this case, a simple cheap computer dedicated to
controlling one small part of a plant or piece of equipment.
Computers are electronic devices required to be attached to measuring
instruments which produce electrical signals. Similarly the electrical
output from the computer must be converted to a suitable form which can
affect the process.
The electrical signals from any measuring device are analog
signals i.e. they vary continuously with the measurement. They have to
be changed into digital signals which are either on or
off, represented in the computer by the presence or absence of a
voltage. The device which does this is called an analog-to-digital
converter or ADC. This receives a continuous electrical signal
and produces a number inside the process control computer.
Similarly, the digital signal from the computer usually has to be
returned as a continously varying analog signal. This is the inverse
of the above and is performed by a digital-to-analog converter or
DAC.
In the system studied here there are a number of stages required to
convert the level in the tank to a change in flowrate.
- First the level in the tank, a value between 0-255mm, is measured.
- The measuring device converts this number into a voltage signal
between 0-1.8 V. It should be noted that this is not a linear
relationship.
- The ADC takes this voltage signal, converts it into a digital signal
and feeds it to the computer.
- The computer then evaluates a digital signal corresponding to the
results from the control algorithm.
- The DAC takes this signal and converts it back to a voltage.
- This voltage signal, in the range 0-5 V, is then converted to a
current, in the range 4-20 mA.
- This drives a current-to-pressure converter in which a signal in the
range 4-20 mA is converted into an air pressure in the range 3-15 psi.
- Finally it is this air pressure, acting on the diaphragm of the
control valve, which sets the valve actuator position and hence the
outflow from the tank.
The above procedure can be shown more clearly in a diagram.
Please move Back to previous page