Lab #7 Report, Physics 122 - Temperature Measurement and Feedback Control Name:
Lab Partner:
Exercise 1: Test the LM35 temperature sensor
a)Wire up the LM35 temperature sensor, as indicated on its data sheet, and use the multimeter to measure the output voltage. What does it measure room temperature to be?
b)How much does the measured temperature increase if you hold the sensor between your fingers and how long does it take the reading to equilibrate?
c)Wire up the LM35 (using a twisted pair of wires, as this is a small signal sensitive to noise!) to a differential analog input of the DAQ card and use the measurement and automation explorer test panel to plot temperature reading vs. time. Does the reading agree with what the multimeter measured? How much is the reading varying peak-to-peak?
d)Note that the DAQ card has very high input impedance, so is drawing only a very tiny current from the LM35 output. Try placing a 1 kΩ load resistor on the LM35 output to draw more current (between the + input and - input of the DAQ analog input), to force the LM35 to output more current (but not more than its rated capacity). After this modification how much is the reading varying peak-to-peak?
Exercise 2: Measure temperature increase vs. time when a heater is turned on a) Write a Labview program that acquires one voltage reading every 0.1 sec, and plot the
b) Try using averaging to decrease the noise. Rather than acquiring one point every 0.1 sec, read 1000 samples and calculate their mean and plot these mean values on the waveform chart (one value every 0.1 sec). How much does this reduce the point-to-point noise? Include a plot in your report.
c) Bolt the sensor onto a 10Ω power resistor, which will serve as a heater. Starting with the power supply off, wire up the bench power supply to apply +5V across the resistor using alligator clips. Briefly turn on the power supply and verify that the current is what you expect (I=V/R=0.5 Amps) (the power supply has a switch for displaying voltage or current ). d) Use your program to chart temperature (converted to real units of Celsius) vs. time starting
at room temperature and rising when the power supply is turned on. What is the initial rate of temperature increase and how hot does it get after ~10 sec of heating? Keep recording after the heater is shut off. Include a plot in your lab report.
e) What is the initial rate of temperature decrease and how cool does it get after ~10 sec?
Exercise 3: Use a power MOSFET to switch on/off the heater by computer control - Show the instructor/TA
Exercise 4: Implement and characterize a "bang-bang" temperature controller
Make the temperature set point 26 degrees. Starting at room temperature, record a chart of temperature vs. time for this system and write the data to a text file (so you can open it later and re-plot it in Labview or Matlab). Turn on autoscaling for both the x-axis and y-axes. The behavior should be similar to Fig. 2 in the handout “Notes on feedback control”. Include two plots in your lab report:
(i) zoomed out to show rise time (indicate this on the graph)
(ii) zoomed in to show initial overshoot, settling time, and steady state error (indicate these on the graph) for your system as discussed in the handout.
Exercise 5: Control the heater with pulse width modulation
(c) With the reduced rate of heating use your program to chart temperature (converted to real units of Celsius) vs. time starting at room temperature and rising when the power supply is turned on. Include a plot in your lab report. Then change the pulse to have the heater on 30% of each 0.1 s interval and record another temperature vs. time plot. Plot the 10% and 30% heating plots with the same x- and y-axis limits so the two can be compared.
(i) zoomed out to show rise time (indicate this on the graph)
(ii) zoomed in to show initial overshoot, settling time, and steady state error (indicate these on the graph) for your system as discussed in the handout.
(c) Compare rise time, initial overshoot, settling time, and steady state error with the figures for the “bang-bang” controller in Exercise #4.
Prelab Homework for Lab #8 Relevant references:
"LEDs and Photodiodes notes" (posted under Electronics Notes)
"Photodiode Application notes" (posted under Data Sheets & Application notes) Datasheets for the Vishay BPW46 photodiode and 2N7000 FET
"Lock In Detection" notes (posted under Data Sheets & Application notes)
1. (a) What is the specified open circuit voltage for the Vishay BPW46 Silicon PIN photodiode?
(b) At what wavelength is this photodiode most sensitive? How sensitive is it to red light?
2. Why does modulating a signal and using Lock-in detection at a specific frequency help to detect a weak signal?
3. Simulate the principle of Lock-In detection by writing a Matlab program that does the following:
(a) Generate a 10 second long sine wave ysignal=0.1*sin(2*pi*FREF*t+0.3) (i.e., having amplitude A=0.1, frequency FREF=1000 Hz, Phase constant=0.3 radians, and sampling frequency of Fs=100,000 points per second).
(c) Show that you can determine the amplitude and phase of the small signal by using the Lock-In detection algorithm, which involves calculating the Fourier transform amplitude at the modulation reference frequency, as follows. Multiply the measured signal by 2*sin(2*pi*FREF*t) and calculate the mean of the result to determine amplitude times cos(phase constant). Multiply the measured signal by 2*cos(2*pi*FREF*t) and calculate the mean to determine amplitude times sin(phase constant). From these results have your program calculate the amplitude and phase constant. Run the program ten separate times (the random noise will be different each time) and list your results.
(d) Make the amplitude of ysignal be 0.01 (i.e., 100x smaller than the noise amplitude). Again run the program ten times and list your results.
(e) Increase the length of the signal so that it is 100 seconds long instead of 10 seconds. Does this improve the results?