Temperature Control System

Temperature Control System

Before going to study the temperature control system, let us see the block diagram of microprocessor based process control system. It is capable of controlling more than one physical parameter. Such systems are commonly known as data acquisition and control systems. In these system, analog signals from various sensors are converted into digital values. These digital values are read in and processed by the microcomputer. The keyboard and display in the system allow the user to enter set point values, to read the current values of process variables, and to issue commands. Relays, D/A converters, solenoid valves, and other actuators are used to control process variables under program direction.

As an example, let us study the microcomputer based temperature control system to control the temperature between 0-100° C with ON/OFF control. The Fig. 13.7.1 shows the temperature sensing and heater control circuitry using ON/OFF control. It includes

1. Sensing circuitry

2. Analog to digital converter

3. Circuit required to drive the controller.

 

1. Sensing circuitry : The sensing circuitry consists of instrumentation amplifier using transducer bridge. RTD (Resistance Temperature Dependent) is used as transducer whose resistance is changed as a function of temperature.

R_t = R₀ (1 + α Δ t)

Where R_t is the resistance of RTD at temperature t.

R₀ is the resistance of RTD at 0 °C

α is temperature coefficient of RTD (0.0039)

Δ t is difference in temperature (t - t₀ )

At 0 °C resistance of RTD is 100 Ω and at 100 °C resistance of RTD is

R100 = 100 (1 + 0.0039 (100) ) = 139 Ω

At 0 °C, bridge is balanced so output of bridge (V_ab) is 0 V and at 100 °C.

 

Now the instrumentation amplifier gain (R_f/R₁) must be chosen to get 5.12 V at 100 °C

R_f/R₁ = 5.12 V / 0.1556 = 32.89

If R₁ = 1 K the R_f  = 32.89 K. This value of R_f is not available as a standard value so it is necessary to get this value by adjusting trimpot.

2. Analog to Digital converter : To convert analog signal into digital signal 8-bit ADC 0808 is used. ADC 0808 has eight input channels, so to select desired input channel it is necessary to send 3-bit address on ADC, ADB and ADA inputs. Port lines PB₀ to PB2 are used for this purpose. Schmitt-trigger inverter circuitry generates 300 kHz clock which is required to operate ADC 0808. The zener diode and LM 308 amplifier circuitry is used to produce a V_CC and + V_REF of 5.12 V for the A/D converter. With this reference voltage the A/D converter will have 256 steps of 20 mV each.

After at least 50 ns, this address must be latched. This (SOC) is achieved by sending ALE signal using port line PB₄. After another 2.5 µs the start of conversion must be sent high and then low. Port line PB3 is used for this purpose. The end of conversion (EOC) is detected by port line PC0. The Fig. 13.7.2 show timing waveforms for the ADC 0808.

3. Circuit required to drive the controller : As shown in the Fig. 13.7.3 solid state relay is used to switch ON/OFF heating element. Relay is energised and deenergised using transistor switch which is controlled by port line PC₇. When PC₇ is high, there is a sufficient base drive to run transistor in saturation (ON) and when PC7 is low, transistor is in cut off (OFF).

Software : It includes

1. Initialization of 8255

2. A/D conversion routine

3. Software required to take control action.

Flowchart :

Control Word :

Program :

MVI A, 91H

OUT CR ; Initialize 8255

BEGIN : CALL CONVERSION ; Call conversion subroutine

CPI 80H ; compare with SETPT (80H)

JC NEXT

MVI A, 0EH ; Reset bit PC₇ to switch off heater with

OUT CR ; Bit Set/Reset Mode

JMP BEGIN

NEXT : MVI A, 0FH ; Set bit PC₇ to switch ON heater with

OUT CR ; Bit Set/Reset Mode

JMP BEGIN

Subroutine Conversion

MVI A,00H

OUT PB ; Send address to select input 0

MVI A, 08H

OUT PB ; Latch the given address by sending ALE high

MVI C, 0AH

BACK : DCR C

JNZ BACK ; Give delay greater than 2.5 µs

MVI A, 18H

OUT PB ; Make SOC high

MVI A, 08H ; Make SOC low

OUT PB A, 00H

OUT PB ; Make ALE low

AGAIN : IN PC

ANI 01

JZ AGAIN ; Wait for EOC

IN PA

RET

Review Questions

1. Describe any typical automatic process control system using 8085. Use necessary block diagrams, flow charts, algorithms and program to explain the whole system operation. AU May-04, Marks 16

2. Write a note on automatic process control. AU May-05, Marks 8