ME 140L Lab 7 Instruction

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page 1 ME 140 L: Mechatronics Lab Lab 7: DC Motor Characterization, Digital Encoders, and Speed Control Required Components  permanent-magnet DC motor with digital encoder  1 x N-MOSFET P24NF10 transistor  1 x 1.8 kΩ resistor  1 x 1 μF capacitor (ceramic)  1 x 1N4002 diode Laboratory Equipment Featured in this Lab  benchtop function generator  oscilloscope  benchtop digital multimeter  DC voltage supply Pre-Laboratory Readings (sections 7.1 through 7.3) 7.1 Learning Objectives  To learn how to experimentally determine specific properties of DC motors, such as the resistance of the field windings, motor constant, and torque-speed curves.  To understand the relationship between input voltage, motor current, and speed of a DC motor for “no load” conditions.  To understand the relationship between the load placed on a DC motor and the current that it draws.  To understand and apply the concept of pulse-width modulation and its application in the speed control of a DC motor. 7.2 Basic Principles of DC Motors DC motors generally contain two sets of windings, i.e., wires wrapped around core materials with high magnetic permeability. The field windings in the stator provide a magnetic field that interacts with the magnetic field generated by the armature windings on the rotor. The interaction between the poles of these magnetic fields causes the motor to spin. The figure below shows the schematic of the electrical and mechanical sides of a DC motor. i V in E A k m ω , T R A L A

page 2 ME 140 L: Mechatronics Lab Where: V in = input voltage to the motor i = motor current R A = effective resistance of the field windings L A = effective inductance of the field windings k m = motor constant E A = back EMF (voltage) of the motor ω = motor speed T = torque (load) placed on the motor Performing a KVL analysis of the electrical side of the motor, we have V L + V R + E A = V ¿ which yields the following differential equation L A di dt + R A i + E A = V ¿ For this lab, we will be investigating the motor operating in steady-state mode, i.e., after the transient phase. During the transient phase, the motor current increases, which energizes the inductor. Once the inductor is fully energized, the motor current reaches a constant value, i.e., the motor is operating in steady-state conditions. This means that the time rate of change of the motor current is equal to zero di dt = 0 which reduces the differential equation above to an algebraic equation R A i + E A = V ¿ Note that a motor is an energy transformation device, i.e., it transforms electrical energy into mechanical energy. This energy transformation is represented by the following relationships between the back emf, motor current, motor speed, and load torque. E A = k m ω i = T k m Qualitatively, this means that the back emf of the motor, E A , is proportional to the motor speed, ω , by the proportionality constant, k m (motor constant). Additionally, the current drawn by the motor, i , is proportional to the torque (load) placed on the motor by the proportionality constant, 1/ k m . Substituting the first proportional relationship, E A = k m ω , into the algebraic equation above, results in R A i + E A = V ¿ R A i + k m ω = V ¿ Dividing both sides of the equation by the motor current, i , yields V ¿ i = k m ω i + R A Thus, there is a linear relationship between V in / i and ω / i . If we obtain a dataset of V in / i and ω / i , we can graphically determine the motor constant, k m and the winding resistance, R A . 7.3 Measuring Motor Speed: Digital Rotary Encoders

page 3 ME 140 L: Mechatronics Lab A digital rotary encoder is a device used to measure the angular velocity of an object that is rotating about an axis, such as the rotor of a DC motor. A schematic of such a device is shown to the right. The encoder consists of a disk with two sets of concentric slits that allow light to pass through. As the disk rotates, the number of light pulses that reach the detector in a given period of time is proportional to the angular velocity of the disk. If we know the number of slits for a complete revolution of the disk, we can calculate the angular velocity. The encoder that you will be using in the lab has 500 slits – once the light sensor counts 500 light pulses, that means the disk has completed one revolution. In general, we have ¿ of pulses / sec ❑ × 1 revolution 500 pulses = revolutions sec For example, if the light sensor detects 16,798 pulses per second, the angular velocity of the rotating object connected to the encoder is 16798 pulses / sec ❑ × 1 revolution 500 pulses = 33.596 rev sec rev min We can use the oscilloscope to measure the number of pulses per second output by the encoder, i.e., the frequency, which is often measured in Hz. On the oscilloscope screen, these pulses appear as a square wave. The encoder outputs two channels of pulses, which are slightly out of phase with each other. If the pulse train from channel A leads the pulse train from channel B, then the encoder is detecting a clockwise rotation, as shown below. If the channel B leads channel A, then the encoder is detecting a counterclockwise rotation. light sources light sensors

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page 4 ME 140 L: Mechatronics Lab 7.4 Speed Control of DC Motors Using Pulse-Width Modulation (PWM) In section 7.2, we determined that there is a linear relationship between the input voltage, V in , and the speed of the motor, ω . Since the motor constant (slope of the linear relationship) is positive, as the motor input voltage, V in , increases, the speed of the motor, ω , increases. Varying the input voltage is easy if you have a benchtop DC power supply whose voltage can be easily adjusted. Suppose that you only have a constant voltage source available, such as a battery, which can only output a single voltage. A quick solution for varying the voltage would be to create a voltage divider using a potentiometer, but this may be impractical if the motor current is high, which would result in a large amount of power dissipation through the resistor. A more novel approach to controlling the speed of a motor is the use of “pulse-width modulation” or PWM. This essentially involves “pulsing” the motor input voltage on and off at a high frequency such that the motor maintains a constant angular velocity. By pulsing the motor on and off, and controlling the length of the “on” portion, we control the amount of electrical power input to the motor. The portion of time the input voltage is on during a given pulsing cycle is called the “duty cycle”, as illustrated in the following figure. For more information on PWM control of DC motors, see the following references: https://en.wikipedia.org/wiki/Pulse-width_modulation https://www.arduino.cc/en/Tutorial/PWM

page 5 ME 140 L: Mechatronics Lab 7.5 Laboratory Procedures and Experimental Summary Sheet WARNING: During all exercises, you should not allow the motor to “draw” more than 1 A (one amp) of current. Going above this limit may result in burnt-out circuits, etc. Exercise 1: Measuring the motor speed using a digital encoder (1) Connect the motor to the DC power supply as shown below. (2) Connect the digital encoder to a 5V DC power supply and the outputs of the encoder to channels A and B of the oscilloscope, as shown below. DC Power Supply Oscilloscope 5V DC + Vin GND + 5V GND Encoder DC Motor Channel A Channel B GND

page 6 ME 140 L: Mechatronics Lab (3) For the motor input voltages indicated in the table below, record the frequency output of the encoder (from the oscilloscope) and indicate whether the pulse train of channel A is leading or lagging the pulse train of channel B. You can calculate the motor speed post-lab. Note that the encoder we’re using in the lab has 500 pulses per revolution. DC motor input voltage (V) encoder frequency (Hz) motor speed, ω rpm channel A leading or lagging channel B? 3 4 5 6 7 8 -3 -4 -5 -6 -7 -8 (4) On the axes below, sketch both cases that show the pulse train of channel A leading the pulse train of channel B and the pulse train of channel A lagging the pulse train of channel B. A B A B

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page 7 ME 140 L: Mechatronics Lab Exercise 2: Relationship between motor input voltage, current, and speed (1) No-load test: Connect the DC motor to the input voltage source and construct an experimental setup in which you can monitor the motor input voltage and the motor speed for “no load” conditions. Note: we are intentionally withholding the schematic of the experimental setup – at this point in the course, you should try to figure this out yourself. motor input voltage, V in (volts) motor current, i , (amps) encoder frequency (Hz) motor speed, ω (rpm) 4 6 8 10 12 14 (2) Loading the motor: Using V in = 5V, and keeping it constant, touch the rotor with your finger to create a load on the motor – what do you think will change? Refer back to the equations that describe the electrical to mechanical energy transformation for the motor in Section 7.2. Record your observations using the table below. pressure exerted on rotor by finger motor input voltage, V in (volts) motor current, i , (amps) encoder frequency (Hz) motor speed, ω (rpm) light medium hard (3) Blocked-rotor test: Lastly, conduct a “blocked rotor” test by using a pliers to block the rotor such that it does not spin. Do this for input voltage values V in = 4, 6, and 8 V and measure the current drawn by the motor.

page 8 ME 140 L: Mechatronics Lab Exercise 3: PWM speed control of a DC motor The experimental setup for PWM control of a DC motor is shown below. A MOSFET ( M etal– O xide– S emiconductor F ield- E ffect T ransistor), as shown in the photo, is used as a switch to turn the 12V source voltage to the motor on and off, according to the duty cycle setting on the function generator. This transistor is represented in the schematic by a blue circle that is labeled P24NF10, which indicates the specific type of transistor. Why don’t we just connect the function generator directly to the motor? The function generator cannot provide the 12V amplitude of the on-off pulses that are needed to effectively drive the motor (the function generator can only go up to 5V.) Thus, the function generator is connected to a constant 12V source through the transistor. The function generator controls the a smaller voltage to the gate of the transistor (G terminal) which acts like a switch that turns on and off the 12V supply to the motor. A “freewheel” or “flyback” diode (1N4002G) is connected across the motor to prevent transient voltage spikes in the inductive coils (field winding inductance) as the voltage is turned on and off according to the duty cycle set by the function generator. Build the circuit as shown, using indicated the function generator settings. Once this is set up properly, set the duty cycle for the 100 Hz, 5 Vpp square wave from the function generator (your TA will show you how to do this.) For each duty cycle indicated in the table below, record the corresponding motor speed. Start with an 80% duty cycle and go down from there. + - D G S

page 9 ME 140 L: Mechatronics Lab WARNING: Ensure that the MOSFET is correctly oriented and connected within the circuit. If the MOSFET begins to heat up, something is wrong – shut off the power to the circuit and check the connections. duty cycle V G (V) voltage at transistor gate frequency from motor encoder (Hz) motor speed, ω (rpm) 80% 70% 60% 50% 40% 30% 20% 7.5 Post-Lab Activities (1) For your data from exercise 1, calculate the angular velocity (along with its sign) for each of the input voltages, assuming that the encoder attached to the motor has 500 slits per revolution. Use Excel to create a plot of the input voltage vs. the motor speed. (2) For your data from exercise 2, plot the V in / i vs. ω / i data and do a linear curve fit. From this curve fit, you can determine the motor constant, k m , and the winding resistance, R A , of the motor. Attach your Excel file and paste the graph into the PDF file that you submit on Canvas. (3) For your data from exercise 2, plot the torque-speed curves for the motor: one for each of the three voltage input values. (4) For your data from exercise 3, plot the motor speed vs. duty cycle and determine the nature of the functional relationship between the speed and the duty cycle. Attach your Excel file and paste the graph into the PDF file that you submit on Canvas.

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