Goal:

In this exercise, you will lear how to design a LabVIEW VI ( virtual instrument) to control the temperature of a heating element (low value resistor). The temperature desired is by the user. The VI will use the DAQ board installed in the PC to :

• measure the voltage generated  by a external temperature sensor and
• output a voltage to control a relay to turn ON/OFF a heating element that is attached to the temperature sensor.

External Components:

• LM34 is a precision Fahrenheit temperature sensor.  This is 3-pin device that outputs ( Refer to the arrangement above and the Fig 1 on Page 2 of the data sheet ) a DC voltage proportional to the measured temperature. The output of the sensor is  calibrated ( 10 mV / deg F).
• The relay    is similar to the ones you have used in EE205 Lab 11, Fig 7 . This relay will control the current (use +5 V supply) to the resistor that is used as a heating element. The relay cannot be connected to the output terminals of the DAQ board directly as the outputs of the board are not capable of sourcing enough current to energize the relay coil. A 741 op-amp used as a buffer ( voltage follower )  will work very well in switching the relay.
• We need approximately 2 Watts of power to be dissipated from the heating of the resistor . Calculate the value of the resistance from the known  values of voltage applied and the power dissipation desired (2 W).
• Position the resistor and the temperature sensor very close to each other. This will allow efficient thermal contact between the temperature sensor and the resistor.

First, please read and compelete the LabVIEW tutorial for data input / output

The VI you will design will be similar to the LabVIEW exercise of measuring and generating DC voltages. Following are few helpful hints :

• Use a WHILE loop instead of FOR loop as the measurements need to be taken continuously.
• The measurements need be made every 1 second. Set the DELAY in the loop appropriately.
• The measured voltage from the analog channel is compared with a SET TEMPERATURE that is entered by the user in the front panel. The comparison will determine whether the control voltage (+5 volt) that is generated by the analog output will turn ON or OFF the relay.
• A BOOLEAN indicator on the front panel of  the VI should show whether the the heater is ON or OFF (relay is energized or not)
• The temperature  should be displayed in  deg F  on the front panel as shown in the figure below ( Fig 2).
• You will have to scale this voltage to display its temperature in deg. F on the front panel.
• Hint : Use a COMPARE function between output of the analog in data and the SET TEMPERATURE value. The output of the COMPARE function is a boolean value.
• You may leave "i" terminal of the WHILE LOOP disconnected.

Below is shown a typical Front panel of the VI. Your could be different but it should show all the scales, displays, controls with correct color as shown below.

Design specification:
Temperature control range: 80 deg F to 95 deg  F

Prelab:

You must have the circuit diagram of the temperature control system ready when you come to the lab. The diagram should include the pin numbers of all the components and the value of resistors. Also calculate the resistance value of the heating element.

• Design the opamp circuit of a unity gain buffer. This circuit is used to drive the relay.
• Draw the circuit with the relay and opamp pin numbers.
• Calculate the value of the 2W resistor. It will be powered with a 5V supply (applied through the relay).

LM 34
LM741
Relay Schematic (5 V Relay 201A0500)
One 1/2 Watt resistor ( value to be determined ).

• Connect the temperature sensor Vout pin to the terminal board(AICH0) of the DAQ card. Test at room temperature (approx 70 deg F) and connect the DMM to make sure that the output is correct. If you touch the sensor with your hand, you should be able to output voltage of the temperature sensor.
• Connect the buffer circuit you designed in pre-lab to the terminal board (DAC0). The op-amp is powered by 12 volts supply ( +12 Vcc and -12 Vcc). Do not turn on the power to the op-amp yet.
• Connect all grounds together. (power supply, op amp circuit and DAQ board ground points)

• Run your VI  in LabVIEW.  Confirm the output voltage at the Analog voltage OUT terminals. Show the set up of your circuit  to the instructor.
• The voltage being measured by the DAQ board should be scaled by the VI to display correct temperature on the front panel.
• Set the temperature to be controlled at 85 deg F. The delay in executing the loop (the rate at which the data is read) should be set at 1000 msec.
• Run the VI and observe the control voltage at the Analog Out. Confirm the operation of the relay. If you observe the temperature displayed in the front panel of VI increasing, your VI is working.  The plot on the front panel will increment every second .

• Set the temperature on the front panel to 85 deg F and observe the measured temperature on the plot.  Print the front panel.
• Set the temperature in the front panel to 70 deg F and observe the cooling rate. Compare this response time with the heating rate.
• Repeat this for temperature set at 95 deg. F.

• Discuss the setup of your system.  This should also include the circuit diagram.
• Include the VI front panel printouts  ( for 85 deg F and 95 deg F) and the diagram.
• How long did it take for the heating element to heat the resistor from room temperature to 95 deg F?

written by George Hunka
last modified by Siddharth M. Deliwala on 2/8/04