The internal control function of a furnace using an analog loop is shown in
Fig. 13.14. The temperature of the furnace is transmitted to the computer in the
PLC as an analog signal, where it is converted to a digital signal and recorded
in the computer memory. The voltage reference signal Vref is converted to an
analog signal and the two signals are subtracted as shown in an analog error
differencing circuit. The amplified difference signal is then fed via a control circuit
to the furnace heater, making the control loop analog.