Temperature Control System

Temperature Control System

The National LM35 is a temperature sensor. It is a temperature-sensitive voltage source. Its output voltage increases by 10 mV for each °C increase in its temperature. The Fig. 17.3.1 shows the circuit connection for temperature sensor LM35. The voltage output from this circuit is connected to a negative reference voltage, V_s as shown, the sensor will give a meaningful output for a temperature range of - 55 °C to 150 °C. The output is adjusted voltage to 0 V for 0 °C. The output voltage can be amplified to give the voltage range you need for a particular application. The output voltage from LM35 can be applied to ADC and we can get digital equivalent of analog voltage corresponding to current temperature.

The AD590 is another commonly used temperature sensor. It is temperature sensitive current source. It produces a current of 1 μA/°K. This current can be converter to voltage source by current to voltage converter. The advantage of current source sensor is that voltage drop in long connecting wires do not have any effect on the output value.

The output of amplifier shown in Fig. 17.3.2 can be applied to ADC to get the digital equivalent of current temperature.

Signal Conditioning

The primary objective of industrial process control is to control physical parameters such as temperature, pressure, flow rate, level, force, light intensity, and so on. The process control system is designed to maintain these parameters near some desired specific value. As these parameters can change either spontaneously or because of external influences, we must constantly provide corrective action to keep these parameters constant or within the specified range.

To control the process parameter, we must know the value of that parameter and hence it is necessary to measure that parameter. In general, a measurement refers to the transduction of the process parameter into some corresponding analog of the parameter, such as a pneumatic pressure, an electric voltage, or current. A transducer is a device that performs the initial measurement and energy conversion of a process parameter into analogous electrical or pneumatic information. Many times further transformation or signal enhancement may be required to complete the measurement function. Such processing is known as signal conditioning.

 

Example 17.3.1 Design a system for temperature measurement using 89C51 microcontroller along with suitable sensor and signal conditioning. Display the temperature on LCD. If the temperature rises above 100°C, signal it using a buzzer. Draw the complete block diagram and flowchart. Also write algorithm and program for the system. Discuss the hardware and software requirements in detail.

Solution: The National LM35 is a temperature sensor. It is a temperature-sensitive voltage source. Its output voltage increases by 10 mV for each °C increase in its temperature. The Fig. 17.3.3 shows the circuit connection for temperature sensor LM35. The voltage output from this circuit is connected to a negative reference voltage, V, as shown, the sensor will give a meaningful output for a temperature range of - 55 °C to 150 °C. The output is adjusted to 0 V for 0 °C. The output voltage can be amplified to give the voltage range you need for a particular application. The output voltage from LM35 can be applied to ADC and we can get digital equivalent of analog voltage corresponding to current temperature.

Since the ADC0808 has 8-bit resolution with a maximum of 256 (2⁸ steps) and the LM 35 produces 10 mV for every degree of temperature change, we can condition Vin of the ADC0808 to produce a V_out of 2560 mV (2.56 V) for full-scale output. In order to produce such output we have to set V_ref 2.56. this makes Vout of the ADC 0808 correspond directly to the temperature as monitored by LM 35.

Algorithm

Step 1: Initialize ports

Step 2: Initialize LCD

Step 3: Start A/D conversion

Step 4: Wait for EOC (End of Conversion)

Step 5 : Read digital output of ADC

Step 6: Calculate temperature

Step 7: If temperature is greater than 100 °C make buzzer ON

; otherwise make buzzer OFF

Step 8: Display temperature on LCD

Step 9 : Repeat steps 3 through 8

Main Program

MOV 81H, #30H ; Initialize stack

MOV P3, #0FFH ; Configure port 3 as input

MOV P2, #0FFH ; Configure port 2 as input

START: ACALL R_ADC ; Read data from ADC

ACALL BAC ; Convert binary to ASCII

ACALL COMT ; If temperature is greater than 100 °C make buzzer ON

; otherwise make buzzer OFF

ACALL LCD ; Display it on LCD

SJMP START ; Repeat

Analog to Digital Conversion Routine

R_ADC : CLR P1.3 ; Make SOC low

CLR P1.4 ; Make ALE low

MOV A, #00H ; Set address for channel 0

MOV P1,A ; Get the channel number and set its address

SET P1.4 ; Send ALE

NOP ; Pulse

CLR P1.4

SET P1.3 ; Send start of

NOP ; Conversion

CLR P1.3 ; Pulse

WAIT : JB P3.0, WAIT ; Wait for ECO signal

WAIT : JNB P3.0,WAIT

MOV A,P2 ; Get digital data

MOV R5, A ; Binary input

RET ; Return

Binary to ASCII Conversion Routine

BAC : MOV A, R5 ; Get binary data

MOV R0, #40 ; Load RAM address

MOV B, #10 ; B = 10 (0AH)

DIV AB ; Divide by 10

MOV@R0, B ; Save the next digit

INC R0 ; Increment RAM address

MOV B, #10 ; B = 10 (0AH)

DIV AB ; Divide by 10 again

MOV @R0, B ; Save the next digit

INC R0 ; Increment RAM address

MOV @R0, A ; Save the last digit

MOV R0, #40 ; Load RAM address for decimal

MOV R1, #50 ; Load RAM address store ASCII data

MOV R2, #3 ; Initialize counter

BACK: MOV A, @R0 ; Get decimal digit

ORL A, #30H ; Make it ASCII

MOV @R1, A ; Save it

INC R0 ; Increment RAM address for decimal data

INC R1 ; Increment RAM address for ASCII data

DJNZ R2, BACK ; Repeat until the last digit

RET

Compare Temperature Routine :

COMT : MOV A, R5 ; Get binary data

CJNZ A, #101, NEXT ; compare it with 101

NEXT : JC BOFF ;

SETB P3.7 ; Make Buffer ON

AJMP LAST

BOFF : CLR P3.7 ; if Yes, Make Buzzer OFF

LAST : RET

LCD Routine :

LCD : MOV A,#3CH ; [Send command code to set font = 5×10 dots,

LCALL COMMAND ; DL = 8-bits and N = 2 lines].

MOV A,#0EH ; [Send command code to set display

LCALL COMMAND ; and cursor ON]

MOV A,#01H ; [Send command code to

LCALL COMMAND ; clear LCD]

MOV A,#86H ; [Send command to set DD RAM

LCALL COMMAND ; address to the seventh location]

MOV R2,3 ; Initialize counter

MOV R1,#50 ; Load RAM address of ASCII data

B1 : MOV A,@R1 ; Get ASCII digit

LCALL DISPLAY ; Display Digits

INC R1 ; Increment RAM address

DJNZ R2, B1 ; repeat until the last digit

MOV A,#”

LCALL DISPLAY ; Display space

MOV A,#'K'

LCALL DISPLAY ; Display letter K

MOV A,#'g'

LCALL DISPLAY ; Display letter g

RET

COMMAND Routine:

LCALL READY ; Check whether LCD is ready?

MOV P1, A ; Issue command code

CLR P1.5 ; Make RS = 0 to issue command

CLR P1.6 ; Make R/ppppp = 0 to enable writing

SETB P1.7 ; Make E = 1

CLR P1.7 ; Make E = 0

RET ; Return

READY Routine :

CLR P1.7 ; Disable display

CLR P1.5 ; Make RS = 0 to access command register

MOV P0,#0FFH ; Configure P0 as an input port

SETB P1.6 ; Make R/ppppppp = 1 to enable reading

READ : SETB P1.7 ; Make E = 1

JB P0.7,READ ; Check DB7 bit. If it is 1, LCD is busy

; hence check if until it is 0

CLR P1.7 ; Make E = 0 to disable display

RET ; Return

DISPLAY Routine :

LCALL READY ; Check whether LCD is ready?

MOV P0; A ; Issue data

SETB P1.5. ; Make RS = 1 to issue data

CLR P1.6 ; Make R/ppppppp = 0 to enable writing

SETB P1.7 ; Make E = 1

CLR P1.7 ; Make E = 0

RET ; Return

Review Questions

1. Explain the interfacing of sensor with microcontroller.

2. What is signal conditioning?