ECE 317 Laboratory #1 Force Sensitive Resistors Background Force, pressure, and position sensing are required for a wide variety of uses. In this lab, we will investigate a sensor called a force sensitive resistor (FSR). This device has properties that lie somewhere between a typical force transducer and a pressure transducer. A force transducer gives a constant output for a constant force applied to it, regardless of the area over which the force is applied. A pressure transducer gives a reading which is inversely proportional to the area of the applied force when a constant force is applied to it. The FSR has a resistance that varies roughly as the reciprocal of the square root of the area of the applied force. This relationship holds true under the condition where the force footprint is smaller than the FSR active area but larger compared to the spacing between the conducting fingers. This yields large resistance when there is no force applied to small resistance when there is force applied. Experiments In this lab, we will design a circuit that will give a voltage that varies with the force applied to the FSR. The amount of force is provided in 5 penny increments. 0 to 25 pennies in multiples of 5 pennies. The output voltage from your stack detector has some qualitative and quantitative factors. For example the surface under the FSR will affect the change in resistance for a given force applied. You can put it on the hard bench or perhaps on a softer mouse pad. You need to pick your support base. The exact position of the force i.e. center of FSR vs the edge of the FSR will affect the change in resistance for a given force. You will have to pick a position. The quantitative factor will be the force exerted by the mass of the pennies. Ultimately you need to determine how many pennies, 0, 5, 10, 15, 20 or 25, are on your detector. The FSR resistance change is non-linear for a linear force change but you should be able to use a linear circuit to determine the number of pennies on your detector. Sensor Selection We have provided some FSR sensors since you don t have the time to order the best sensor in the world for your needs. We have a few that will work for the penny stack detector. We have a square one about 1.5 on a side and a round one about 7/8 in diameter. There is a data sheet on the e-cow website for these sensors. Instead of trying to identify which exact sensor this was when it was purchased you are going to measure a relevant property. In particular the resistance of the sensor with a pile of pennies on it. Remember the resistance is sensitive to position on the sensor. Now we will pretend you are going to produce millions of these great stack detectors. Measure the repeatability of your sensor. See procedures below. Next measure 2 more similar sensors with the same number of pennies. Determine the variability of this sensor for your application. In your case express the variation as the largest deviation in resistance from the mean resistance as a percentage. 1.1
Procedure: 1.0 First you will measure how repeatable the resistance of this FSR sensor is with a given force applied to it. 1.1 Using the digital multimeter, take five measurements of the resistance of your particular sensor configuration with X number of pennies using a statistical method of your choice to determine repeatability. Note you need to be aware that the sensor resistance is very penny position sensitive. 1.2 Using a similar method, determine device-to-device variation. Test at least three different sensors. You will need to trade with some other groups. Your Sensor Someone else s Sensor 1 Someone else s Sensor 2 Trial #pennies Trial #pennies Trial #pennies 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 Average Average Average Table 1: 1.3 Now, refer to the Data Sheet for FSR on the website to get a handle on what is important for the signal conditioning circuit you need to build. Pages 18 and 23 are the most relevant. I have supplied 2 circuit diagrams in this lab manual. They are not the only ones that will work. Build 2 circuits. First the current to voltage converter circuit (Figs. 1 and 2) then voltage follower circuit (Figs. 3 and 4). Note the ceramic capacitors can be a different value, i.e. 1uF. You can modify the circuits if you want to. There are more circuits shown in the Data Sheet for FSR. For example: You could energize the FSR with the opposite supply voltage or you could design a circuit using a single supply op-amp. The single supply op-amp circuit is the most likely design you would use in a real product. Make sure you read down to section 1.4 before building anything!!!! Inverting Op-Amp sensor conditioner. The circuit in Fig. 1 is an inverting amplifier with gain. It converts the change in resistance to a change in voltage. This results in a circuit that converts a force applied to the FSR to a voltage. The circuit takes in a current from the FSR and converts it to a voltage using the standard inverting op-amp circuit. The current is determined by the 1.2
voltage across the FSR divided by the R FSR. The current flows from V O to terminal 2 (inverting input) then through the FSR to V-. Optimize the choice of R F in Fig.1 by: 1 Find the maximum positive/negative output voltage the op-am can put out. The 2 maxes will be different in magnitude. 2 Measure R FSR with a 25 penny load. R FSR-25P 3 Calculate R F so that V OUT does not clip (exceed the maximum op-amp output voltage) using Eqs 1, 2 and 3. 4 Choose a value smaller than the calculated R F since you won t be able to pick the exact value. From the standard inverting op-amp gain formula: = (1) For the circuit in Fig 1, V IN is -15V, R1 = Rf and R2 is the force sensitive resistor. So eq. 1 becomes: =15 (2) Then =15 (3) Where V OUT used in eq. 3 is the maximum +V OUT you determined in step 1 above. -15V Vin R2 FSR R1 2 3 C1 XK 1 5 8 U1A 82nF 6 LF356 Film Vout +15V -15V U1B 7 +V -V 4 C2 LF356-Power C3 Figure 1 Current to voltage (Inverting Op-Amp) converter circuit. Notice C1. This changes the frequency response of the standard inverting op-amp circuit. The cap puts a pole in the transfer function at: 1 (2 ) Hz. A pole changes this circuit configuration to an inverting low pass filter. What is the pole frequency for your choice of R1? Hz. The pole frequency is the low pass filter - 3dB frequency aka the cutoff frequency. Figure 2 shows a typical proto-board layout of this circuit. 1.3
Figure 2 Example of current to voltage converter circuit w/ an extra cap. Non-Inverting Op-Amp sensor conditioner. The circuit in Fig 3 converts the change in R FSR to a voltage change. This is done with a standard non-inverting unity gain op-amp circuit. The input to the amplifier is supplied by a resistive voltage divider composed of R FSR and R 3. V IN = 0V when R FSR =. V IN = V+ when R FSR = 0Ω. This relationship is shown in Eq.4. Equation 4 also yields V O since V O = V IN for this circuit. You must select R 1 to prevent V O from clipping. You can use R FSR for 25 pennies and Eq. 4 to calculate R 1. Warning before you build this circuit read about the next circuit shown in figure 4. = = ( )=15 (4) +15V -15V U1B 7 +V -V 4 +15V Vin R2 FSR 2 3 1 5 8 U1A 6 LF356 Vout C2 LF356-Power C3 C1 82nF Film R1 Figure 3: Non-Inverting voltage follower FSR converter. 1.4
Once again there is the capacitor C1 which puts a pole in the amplifier. This time however the pole frequency depends on C1, RFSR and R1. That means as RFSR changes so does the pole. The pole frequency is: 1 (2 ( ) ). Where represents the parallel function. What is the pole frequency for X pennies using the data from table 1 above? F XPENNIES What is the pole frequency for 25 pennies using the data from above? F 25PENNIES. From the Data Sheet for FSR you should see that R1 also influences which force region is more linear than others. I.E. lower force, medium force or high force. This suggests that putting a fixed resistor in series with the FSR may benefit the linearity of this circuit for your application. I suggest this because it could improve your ability to identify the number of pennies that are on the sensor by adding another resistor as shown in Figure 4. +15V Vin R2 FSR R3 2 3 1 5 8 U1A 6 LF356 Vout C1 82nF Film R1 Figure 4 Non-Inverting voltage follower FSR converter with added R3. Figure 5: Example of the voltage follower circuit from Fig 3. 1.5
1.4 Measure V O for 2 circuits for 0, 5, 10, 15, 20 and 25 pennies on the FSR. Plot the data over the useful range of your sensor. Remember the goal is to be able to determine how many coins are on the stack (0, 5, 10, 15, 20 or 25). You need to come up with some type of calibration curve for your sensor over this range. You would normally have a computer do some of these calculations for you but in this lab we will do it by hand. Determine which design is better for determining the number of coins on the FSR. Use that design for the next part. You only have to measure 2 circuits. Use the circuits shown in Figs 2, 3, 4 or your own versions if you like. Table 2 Circuit 1 Circuit 2 Circuit X 0 p 5 p 10 p 15 p 20 p 25 p 1.5 Once you have confidence in your circuit and data, let me know. I will cover the sensor and ask you to tell me how many pennies are put on your detector. Questions and Discussion Do you think this sensor is useful for these force type measurements? Support your opinion with the data you collected. Which circuit is more linear with regards to Vout vs #pennies on the stack? Develop an algorithm that you would use on a computer to process the raw data that is output from the sensor s voltage output circuit. A simple block diagram is acceptable. Include the range of force values that are applied to the sensor and the corresponding output values. 1.6