OSCILLOSCOPES, MULTIMETERS, & STRAIN GAGES

Transcription

1 Community College of Allegheny County Unit 1 Page 1 OSCILLOSCOPES, MULTIMETERS, & STRAIN GAGES The Overweight Sub That Cost Billions: After Spain invested $2.7 billion in a program for diesel-electric submarines, in 2012, it was discovered that the first one weighing 2,200-tons was 70-ton overweight and would probably sink if it went out to sea. The error occurred because a decimal point was put in the wrong place. "Apparently, somebody in the calculations made a mistake in the very beginning and nobody paid attention to review the calculations," Revised: Dan Wolf, 1/7/2018

2 Community College of Allegheny County Unit 1 Page 2 OBJECTIVES: Measurement Concepts: Oscilloscope Measurements Digital Meter Measurements Analog Meter Measurements Signal Generator Operation Voltage Measurements: Peak-to-peak (PP), Peak (P), RMS Time Measurements: Period and Frequency Waveforms: Sine, Triangular, Square, Sawtooth DELIVERABLES THAT YOU MUST SUBMIT 1. Graph and Data for Tables 1, 2, and 3 2. Table for Experiment 5 3. Practice Problems On-Line Reading Material: 1. Read sections: a) Introduction b) Basics of O-Scopes c) Oscilloscope Lexicon d) Anatomy of an O-Scope e) Using and Oscilloscope 2. EQUIPMENT REQUIRED: 1. Signal Generator 2. Oscilloscope 3. Analog Volt Meter 4. Digital Volt Meter 5. Variable voltage power supply Ω resister Ω resister Ω resisters 1% tolerance (Qty=3) Ω 10-turn potentiometer 10. Strain Gauge type BF350-3AA 350 (mounted on metal bar), Qty=2

3 Community College of Allegheny County Unit 1 Page 3 INTRODUCTION: The object of this experiment is to learn how to use the oscilloscope by measuring the periods and amplitudes of various waveforms (shown in Figure 2). The oscilloscope is an electronic instrument widely used in making electronic measurements. The most noteworthy attribute of an (ideal) oscilloscope is that it does not affect the quantity being measured. An example of an AC signal is shown in Figure 2. The voltage is on the vertical (y) axis and the time is on the horizontal (x) axis. Notice that if we plot a DC (or constant) voltage on this figure, it would be a horizontal line. There are two main quantities that characterize any periodic AC signal. The first is the peak-to-peak voltage (Vpp), which is defined as the voltage difference between the time-varying signal s highest and lowest voltage. Thus, Vpp is defined as: Vpp = 2 Vpeak The second is the frequency of the time-varying signal (F), defined by: F = Frequency = 1 T = Period where F is the frequency in hertz (Hz) and T is the period in seconds (as shown in Figure 2). The voltage RMS value is the effective value of a varying (AC) voltage. It is the equivalent steady DC (constant) value which gives the same effect. For example, a lamp connected to a 6V RMS AC supply will shine with the same brightness when connected to a steady 6V DC supply. RMS Voltage is defined as: Vrms = Vpeak Something Important to Remember: An oscilloscope will show you Vpp and Vp however an analog or digital multimeter will normally show you Vrms. This means that the maximum voltage in the circuit will be higher than the value shown on the multimeter (120Vac in your house is actually 170Vpeak but shown as 120V on the multimeter).

4 Community College of Allegheny County Unit 1 Page 4 Figure 1 - Generated Waveforms SINE WAVE TRIANGLE WAVE SQUARE WAVE SAWTOOTH SAWTOOTH PULSE Figure 2 Sine Wave Fundamentals Vpeak = Vo Vpp = 2 * Vpeak Frequency = 1 / Period Vrms =.707Vpeak

5 Community College of Allegheny County Unit 1 Page 5 Experiment #1 Sine Wave: 1. Set a signal generator to a 60Hz sine-wave output with 10V peak-to-peak. 2. Display the voltage on an oscilloscope. a) Measure and record the amplitude, period, and frequency of the signal. b) Measure and record the voltage with an analog meter. c) Measure and record the voltage with a digital meter. d) Adjust the signal generator until you have an exact 60Hz, 10V peak-to-peak signal. 3. Using the signal generator, change the frequency and voltage of the signal without looking at the oscilloscope or meter. You now have an unknown waveform. a) Measure and record the amplitude, period, and frequency of the signal. b) Measure and record the voltage with an analog meter. c) Measure and record the voltage with a digital meter. d) Compare your measurements with the dials on the signal generator. Are they (reasonably) close to each other? e) Sketch this waveform and complete Table 1 Experiment #2 Square Wave: 1. Set a signal generator to a 100Hz square-wave output with 5V peak-to-peak. 2. Display the voltage on an oscilloscope. a) Measure and record the amplitude, period, and frequency of the signal. b) Measure and record the voltage with analog meter. c) Measure and record the voltage with a digital meter. d) Adjust the signal generator until you have an exact 100Hz, 5V peak-to-peak signal. 3. Using the signal generator, change the frequency and voltage of the signal without looking at the oscilloscope or meter. You now have an unknown waveform. a) Measure and record the amplitude, period, and frequency of the signal. b) Measure and record the voltage with an analog meter. c) Measure and record the voltage with a digital meter. d) Compare your measurements with the dials on the signal generator. Are they (reasonably) close to each other? e) Sketch this waveform and complete Table 2

6 Community College of Allegheny County Unit 1 Page 6 Experiment #3 Triangular Wave: 1. Set a signal generator to a 1000Hz triangular-wave output with 6V peak-to-peak. 2. Display the voltage on an oscilloscope. a) Measure and record the amplitude, period, and frequency of the signal. b) Measure and record the voltage with an analog meter. c) Measure and record the voltage with a digital meter. d) Adjust the signal generator until you have an exact 60Hz, 6V peak-to-peak signal. 3. Using the signal generator, change the frequency and voltage of the signal without looking at the oscilloscope or meter. You now have an unknown waveform. a) Measure and record the amplitude, period, and frequency of the signal. b) Measure and record the voltage with an analog meter. c) Measure and record the voltage with a digital meter. d) Compare your measurements with the dials on the signal generator. Are they (reasonably) close to each other? e) Sketch this waveform and complete Table 3 Experiment #4 Voltage Divider: 1. Connect the circuit shown in Figure 3 where R1=100 ohm and R2=1000 ohm and V equal to +5V or +12V (your choice). 2. Measure the voltage across R1 and R2. Note that the ratio of voltage between R1 and R2 should equal the ratio of resistance values for R1 and R2. Thus: R1 R2 = 100ohm 1000ohm = VR1 = 1: 10 VR2 So: R1 = 100 ohm VR1 = R2 = 1000 ohm VR2 = 3. Note that if R1 = R2 then VR1 must equal VR2. Modify the circuit so R1 = R2 then measure VR1 and VR1 and record here: R1 = ohm VR1 = R2 = ohm VR2 =

7 Community College of Allegheny County Unit 1 Page 7 Experiment #5 Bridge Circuit: 1. Connect the circuit shown in Figure 4 where: V = 10Vdc Ra = Rb = Rc = 350 ohm (ideally these are 1% tolerance) Rx = 1000 ohm ten-turn potentiometer Use both an oscilloscope and voltmeter to measure the center point. You will notice that the oscilloscope signal is not easy to read due to low level noise. 2. If you adjust the potentiometer to exactly 350ohms, the voltage measured at the center will be 0 Volts. 3. Carefully turn the potentiometer a small amount and observe that the voltage measured changes with a change of Rx. 4. Replace Rx with one of the strain gages and adjust the potentiometer until you get zero volts at the center point. 5. Apply pressure on the strain gage and observe that the voltage at the center point changes. Record your observations below. Balanced circuit with no pressure applied. Light Pressure applied Heavy Pressure Applied Voltage Measured (One Strain Gauge) Voltage Measured (Two Strain Gauges) 6. The output voltage of the bridge, VO, will be equal to: Rx Vo = [ Rx + Rc Rb Ra + Rb ] Vin From this equation, it is apparent that when Ra/Rb = Rc/Rx, the voltage output VO will be zero. Under these conditions, the bridge is said to be balanced. Any change in resistance in any arm of the bridge will result in a nonzero output voltage.

8 Community College of Allegheny County Unit 1 Page 8 7. Replace the potentiometer with the second strain gage and replace the Rb resister with the potentiometer. Now adjust the potentiometer until you get zero volts at the center point. Re-test and complete the last column of the Table. This circuit is a half-bridge strain gauge circuit and should have twice the sensitivity as the quarter-bridge strain gauge circuit. Figure 3 Figure 4

9 Community College of Allegheny County Unit 1 Page 9 Signal Generator Frequency Voltage Vp and Vpp and Vrms Table 1 SINE WAVE Measured Period Frequency Meter Voltage 60Hz 10Vpp

10 Community College of Allegheny County Unit 1 Page 10 Signal Generator Table 2 SQUARE WAVE Measured Frequency Voltage Vp and Vpp Period Frequency Meter Voltage 100Hz 5Vpp

11 Community College of Allegheny County Unit 1 Page 11 Table 3 TRIANGULAR WAVE Signal Generator Measured Frequency Voltage Vp and Vpp Period Frequency Meter Voltage 1000Hz 6Vpp

12 Community College of Allegheny County Unit 1 Page 12 PRACTICE PROBLEMS: These do not have to be turned in but you should take a look at them and make sure you understand the concepts. Ask the instructor to explain anything that you are not comfortable with. 1. With regards to Figure#3, if V = 10V, R1 = 5000 ohms, R2 = 2000 ohms, how much current will flow in R1? How much current will flow in R2? 2. With regards to Figure#3, if V = 10V, R1 = 8000 ohms and the current in R1 is 1mA, what is the value of R2? 3. With regards to Figure#4, if Vin = 10 Volts, Ra = Rb = Rc = 350 ohms and Rx = 352 ohms, what voltage will be at Vo? 4. With regards to Figure#4, if Vin = 5 Volts, Ra = Rb = Rc = 350 ohms and Rx = 352 ohms, what voltage will be at Vo? 5. Looking at questions 3# and #4, we can see that reducing Vin affects the value at Vo. So, if want to see a larger value at Vo, we can increase Vin. But if we increase Vin, more current wil flow through the Ra/Rb and the Rc/Rx nodes. This is ok as long as the amount of current flow doesn t exceed the capacity of the resisters and strain gauge. How much current will flow through Rc in question #3? If Rc is a quarter watt resister, will the current be acceptable? Note that the power equation is: P=I 2 R. 6. There is one inherent problem with interfacing switches to embedded microprocessors

13 Community College of Allegheny County Unit 1 Page 13 switch debounce. Anytime a mechanical switch closes (or opens), the contacts do not make clean contact immediately, they bounce a quantity of times before making final closure see Figure 5. An embedded microprocessor is fast enough to detect each of these bounces as a valid switch closure. With this in mind, each of the bounces could be incorrectly acted upon. The solution is to implement either debounce hardware (see Figure 6) or a debounce delay function in software. Debounce delay is when the up detects the first switch closure, waits for a period and then rechecks the switch. If the switch is still closed the switch closure is assumed to be complete and the new state is accepted. The length of the debounce delay period is dependant on the construction and condition of the switch. The amount of switch debounce is primarily determined by the switch type and its age. In general, switch bounce will occur for a period between 5 and 60mS. Note: Solid state switches are switches with no moving parts and are very popular. They are more expensive than mechanical switches and require additional support circuitry but they do not suffer from contact wear and do not experience the bounce problem. This means the switch debounce task is not required so the software is smaller and faster. The decision whether to use mechanical or solid-state switches really gets down to a tradeoff of hardware versus software costs. If the product quantities are low (i.e. space shuttle), the extra hardware cost for solid-state switches will be less than the software costs (development, size, and risks). If the product volume will be in large quantities (i.e. clock radios), the one-time software costs will be less than the cost of the more-expensive solid-state switches. This decision is important and should always be considered by both software and hardware engineers. Use the internet to identify a software algorithm, source code (any language), or detailed explanation for switch debounce. Print it out and turm it in as part of this lab.

14 Community College of Allegheny County Unit 1 Page 14 Figure #5 Switch Debounce Switch Bounce Non-Debounced Switch +5 Volt 0 Volt Switch is Pressed Switch is Released Coin Acknowleged Debounced Switch Coin Initially Detected

15 Community College of Allegheny County Unit 1 Page 15 Figure 6 Electronic Debounce Circuits +V R1 RC Debounce Circuit R2 C1 +V To CPU Digital Debounce Circuit