Exercise 1: Touch and Position Sensing EXERCISE OBJECTIVE When you have completed this exercise, you will be able to describe and demonstrate the use of a capacitance sensor as a touch sensor and a position transducer. DISCUSSION This is a schematic of the circuitry in the CAPACITANCE SENSOR circuit block. oscillator. FACET by Lab-Volt 207
Transducer Fundamentals The sensing capacitor (C S ) repeatedly charges through R2 and R3 and then discharges through R3. S, R2, and R3. 1.46 fo = (R2 + 2R3) C s If the capacitance decreases, output frequency will a. increase. b. decrease. c. not change. If C S output frequency is a function of the position of the movable plate. 208 FACET by Lab-Volt
The fact that the human body has an inherent capacitance also allows you to use the circuit as a touch sensor. When you physically touch the capacitor plate, your body capacitance adds to that of the sensor, WARNING: involved here makes this procedure safe, capacitors should never be touched otherwise. By design, capacitors store electric charge and touching them may cause serious injury. with an oscilloscope or frequency counter. plate. A sensor used in this way is a proximity detector. In practice, the proximity detector often replaces mechanical switches since there are no moving parts to wear out. FACET by Lab-Volt 209
Transducer Fundamentals PROCEDURE In this PROCEDURE, you will: demonstrate position sensing by sliding the movable plate of the capacitance sensor. You will verify your results by observing the oscillator output frequency with an oscilloscope. Insert a two-post connector in the BLOCK ENABLE position of the CAPACITANCE SENSOR circuit block. Connect oscilloscope CH 1 to the OUT jack, and connect the probe ground to the GND jack. What signal appears on the scope? a. positive dc level b. negative dc level c. square wave 210 FACET by Lab-Volt
The movable plate of the capacitance sensor slides freely up and down between a set of guide rails. A scale printed on the left guide rail allows you to read the position of the top edge of the movable plate. The scale ranges from 0 to 3 cm in 0.5 cm increments. FACET by Lab-Volt 211
Transducer Fundamentals Position the movable plate so that its top edge lines up with the 0-cm mark. The plate is in its a. highest position. b. lowest position. c. middle position. remove it. Repeat several times. What happens to the square wave when you touch the plate? a. Amplitude increases. b. Amplitude decreases. c. Frequency increases. d. Frequency decreases. a. position sensor. b. touch sensor. c. proximity detector. the scope. a. before you touch the plate b. after you touch the plate c. at the instant you touch the plate 212 FACET by Lab-Volt
capacitance sensor as a a. position sensor. b. touch sensor. c. proximity detector. Use the vertical position control to position the low level of the square wave on the center graticule line as shown. Horizontal: 0.5 output frequency. f(0) = khz (Recall Value 1) Move the plate up so that its top edge lines up with the 3-cm mark. Measure the frequency of the square wave. f(3) = khz (Recall Value 2) FACET by Lab-Volt 213
Transducer Fundamentals shown. f O at 0 cm = khz (Step 8, Recall Value 1) f O at 3 cm = khz (Step 10, Recall Value 2) Calculate the difference in frequency (f) by subtracting the frequency at 3 cm from the frequency at 0 cm. f = khz (Recall Value 3) You can calculate a conversion factor (CF) to determine the frequency change per cm of displacement by dividing the total frequency difference (f) by the total displacement (d T ). T CF = khz (Step 11, Recall Value 3 CF = Recall Value 4) You can use the conversion factor to determine the frequency at any position, P, (in cm) as follows. f P = f(0) (CF x P) Calculate the frequency at 0.5 cm. f P = (Step 8, Recall Value 1) [ (Step 11, Recall Value 4) x 0.5] khz f(0.5) = khz (Recall Value 5) Move the plate so that the top edge is at 0.5 cm. 214 FACET by Lab-Volt
Measure the frequency on the scope. f(0.5) = khz (Recall Value 6) Is your measured value about the same as your calculated value of (Step 12, Recall Value 5)? a. yes b. no You can increase the sensitivity of the measurement by increasing the frequency range over the same total displacement. f o 1.46 = (R2 + 2R3) C s Place CM 15 in the ON position to change the value of R2 from 10 k to 5 k. This between the highest and lowest plate positions. Move the plate to the 0-cm position and measure the frequency. f(0) = khz (Recall Value 7) Move the plate to the 3-cm position and measure the frequency. f(3) = khz (Recall Value 8) FACET by Lab-Volt 215
Transducer Fundamentals With the CM active, the total frequency difference over the 3-cm displacement is f = f(0.0) f(3) f = (Step 15, Recall Value 7) (Step 16, Recall Value 8)] khz f = khz Calculate the conversion factor. T CF = Recall Value 9) The table shows that by increasing the range of frequencies and measuring over the same displacement range, you also increase the sensitivity of the measurement. frequency range (0 3 cm) conversion factor Normal khz (Step 11, Recall Value 3) (Step 11, Recall Value 4) With CM khz (Step 15, Recall Value 7) (Step 16, Recall Value 8) (Step 15, Recall Value 7) (Step 16, Recall Value 8 Remove the BLOCK ENABLE two-post connector from the CAPACITANCE SENSOR circuit block. Make sure all CMs are cleared (turned off) before proceeding to the next section. CONCLUSION You can use a capacitor in an RC oscillator circuit as a simple sensing device (capacitance sensor). The output frequency of the oscillator is a function of the capacitance of the sensing capacitor. The capacitance sensor operates as a touch sensor by detecting body capacitance added to the sensing capacitance. The capacitance sensor operates as a proximity detector by sensing an object close to, but not necessarily touching, the sensing capacitor. You can use a capacitance sensor with a movable plate to measure the position or displacement of an object. 216 FACET by Lab-Volt
REVIEW QUESTIONS 1. A capacitance sensor requires a movable plate to operate as a a. touch sensor. b. proximity detector. c. position sensor. d. All of the above 2. Which component is the sensing device in this capacitance sensor circuit? a. C C b. C S c. R2 d. R3 3. a. frequency that is a function of input capacitance. b. capacitance that is a function of input frequency. c. voltage that is a function of input capacitance. d. 4. As the movable plate of the variable capacitor moves from point A to point B, you can determine the a. point A. b. point B. c. points A and B and calculating the difference. d. None of the above 5. The displacement of the movable plate of a variable capacitor is a function of the a. b. surface area of the movable plate. c. overlapping area between the two plates. d. total surface area of both plates. FACET by Lab-Volt 217