Motors, Batteries and Pulse Width Modulation - Laboratory 3 | ECE 110, Lab Reports of Electrical and Electronics Engineering

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Laboratory #3 - Motors, Batteries, & Pulse Width Modulation Grade Pre-lab: /2 Lab: /10 Total: /12 Objectives * Learn how to use multimeter to measure current * Characterize a non-ideal source (battery) * Understand different methods of controlling a motor’s speed - Understanding Pulse Width Modulaton (PWM) - Understanding variable resistor method * Learn to take into consideration power efficiency and conservation of energy * Burn up a resistor for fun * Building and probing more complex circuits Lecture References Orange Book * 0.17-0.20- Measuring Voltage and Current * 0.22-0.26- Time Averaging * 0.25- Time averaged power * 0.16-0.17- Series and Parallel connections * 1.1-1.13- KCL and KVL * 2.4-2.6- Resistor power ratings * 2.9-2.15- Voltage and Current division Transparencies * Burnet Handouts 3- Conservation of Energy * Brunet Handouts 4, 5- Kirchhoff Law’s * Haken Set 1- pg 2-28 Mallard * Homework 4- Kirchhoff’s Laws and Node Voltages * Homework 5- Series and Parallel connections * Homework 6- Current-Voltage (IV) Characteristics EXPERIMENT #3 PRELAB After lab 6 you will be involved with designing logic to drive your vehicle - mostly ignoring how your logic signals actually drive the motors. You will assume that when your logic dictates that one of the motors should be on it will be. But, as some of you will find, this does not always happen as you plan it even when your logic works perfectly. Depending on your final design, controlling the speed of your car may be very important. Some designs work better at very slow speeds, others at faster speeds and the variable resistor on the back of the car is often a very poor method of determining the speed especially when one design navigates some obstacles best at a slow speed and some at a much faster speed. The vehicles are quite old as are the motors, they are just toy motors. You will also find that many of the vehicles have motors that are not matched. One of the motors always seems to have more torque than the other at slower speeds. Your design must work DESPITE these differences. Therefore, understanding how the motors react to a range of voltages is necessary. There are two methods that we will use to drive the motors on the vehicle. One method is to directly modify the current flowing through the motor using a variable resistor and the other is to use a method called 'pulse width modulation' where the current flowing through the motor is pulsed on and off at maximum current. The rate at which the current is turned on and off determines the speed of the motor. Which one to choose? The variable resistor is very simple to build but it has some serious drawbacks that the pulse width modulator addresses. The variable resistor circuit demands much more power. Remember from lab 2 that even when the motor is running very slowly, or not at all, power is still being dissipated in the variable resistor because the variable resistor is used to present a variable load to the battery allowing control over the current flowing through the motor. The pulse width modulator does not have this drawback since all of the current flowing from the source is delivered directly to the motor. The overhead in energy is slight because the circuitry used to modulate the motor by turning the current on and off operates at very, very small currents. As attractive as reducing power consumption is to the designer (batteries have a finite amount of energy stored) there is another important difference between these two methods of controlling the speed of the motors - the pulse width modulator drives the motor at maximum torque, while the variable resistor circuit drives the motor with a variable torque depending on the speed. In terms that are meaningful to your design this really says that you can make the motors run very slowly and have them still move the heavy vehicle body using the PWM. Not so with the variable resistor. Graph showing how a hypothetical motor responds to different voltages. For voltages less than 1V the motor does not rotate the wheels. For voltages above 1V the speed of the motor varies linearly with voltage. EXPERIMENT #3 - your car's muscle Vehicle's Power Supply All of the vehicle's power is provided by a battery inserted into the body of the car. The output of the battery passes through a voltage regulator which changes the voltage directly supplied by the battery so that two voltages are available - ~10V directly from the battery and ~5V from the voltage regulator used primarily to run the TTL logic. Below is a figure of the car chassis WITHOUT the protoboard showing where you can tap into these two voltages. Before starting to characterize the motors, let's characterize the battery since these are non-ideal voltage sources. Remember the characteristics of a non-ideal source? A non-ideal voltage source delivers power to a circuit connected to it but the voltage across the battery terminals changes when the load connected to the battery is changed. You will characterize the non-ideal behavior of the batteries that you will use in the cars. These batteries are new Nickel Metal Hydride batteries and should hold a charge for the entire period even when they are running the cars. But they do need to be charged occasionally so is important that you learn how to charge them now. It should become a habit that when you enter the lab from now on you check the charge of your batteries and charge any that need charging. Your TAs will demonstrate how to charge them using the chargers at each lab station. 1. Take one of the batteries and measure the voltage across the terminals. Record this value. If the voltage that you recorded is less than 10V let your TA know so that he/she can charge the battery for the next session. Now insert the battery into the car and measure the voltage across both sets of red and black connectors. Record the voltages, let's call them Vmotor - the voltage measured across terminals that supply the higher voltage to drive the motors and V logic - the voltage used to drive the TTL circuits. Vmotor should be >10V and Vlogic should be around 5V. Vmotor should be close to the voltage that you just measured with the battery out of the car. Is it? Remove the battery from the vehicle - we wish to characterize the battery alone. Using cables with banana plugs at both ends, connect the battery to the variable resistor in the test box. Make certain BEFORE you connect the variable resistor that the knob is turned ALL THE WAY to the left - the OPEN side. During this part last semester many, many variable resistors were burnt because is is easy to turn the resistance down to nearly zero and the least robust component is the variable resistor. To guard against this you will be inserting a 'fuse' into the circuit. The 'fuse' will be a 1/4W 10Ω resistor. Put the 10Ω resistor into a test box - keep the test box open. Insert the test box in series with the battery and variable resistor. If you turn the variable resistor down too far you will see smoke and smell the resistor burning - now the resistor is the weakest component rather than the more expensive variable resistor. Try not to let this happen...yet. 2. Draw your circuit containing the battery, the variable resistor, and the “fuse”. 3. Vary the resistance using the knob, but DO NOT turn the knob all the way to short. Fill in the table below. Recall that you must disconnect the resistor from the circuit to accurately measure the resistance. This must be accomplished by unplugging the box from the circuit. What happens to the battery voltage Vmotor as the resistance changes? An ideal voltage source should provide the same voltage NO MATTER WHAT load is connected to it. The I-V characteristics of an ideal voltage source as given in your lecture shows a vertical line indicating that no matter what load is placed across the terminals the amount of current demanded is provided (see figure). No voltage source is perfect not even our power supplies as you would find if (or should I say when) you try to put a short across the terminals. There is always some internal resistance - the I-V curve of most batteries and supplies looks more like the figure to the right (ignoring the extremes where all realistic devices must fail). A simple model that describes the behavior of a non-ideal source consists of an ideal voltage source connected to a resistor - often called the internal resistance. Our batteries are definitely non- ideal sources and so can be described by this model. Mathematically, the model can be written as Vb=Vbideal+RinternalI, where Vb is the voltage across the real battery, Vbideal is the voltage across a model, ideal source, and I is the current. We have been assuming that all of our voltage sources are ideal – like the power supply, when using the battery this is no longer true. Now the voltage at the battery terminals is a function of the current flowing through the device. 4. Redraw the circuit that you used to obtain the data in your table but replace the battery in your circuit diagram with a model of a non-ideal voltage source - an ideal voltage source connected, in series, to a resistor that represents the internal resistance of the battery. 5. What is the internal resistance of your battery? You can calculate it from the data that you just gathered. 6. What is the voltage of the ideal voltage source? This can be found very simply...if you get stuck ask your TA. 8. Monitor the voltage across the motor. Monitor the current flowing through the motor. Fill in the table below for 4-8 different settings of the variable resistor knob on the back of the car, starting with the lowest resistance. (Remember to turn ON the vehicle's power in the back using the switch.) The battery voltage must be computed since once the battery is inside the car we can no longer probe the battery directly. The jacks on the top of the car hold the voltage from the battery connected to the INTERNAL variable resistor NOT the battery alone (see figure on previous page). But you know how the voltage across the battery varies with current from the previous part so you should be able to compute both Vb and the setting for the variable resistor but you need not calculate this resistance. Use the table below to tabulate your values – we are calling the variable resistor setting 1, 2, 3, 4, ... 9. As the resistance of the variable resistance increases, what happens to the power dissipated in the motor? 10. As the resistance increases, what happens to the power delivered by the battery? 11. Account for the difference in power between that dissipated in the motor and that delivered by the non-ideal battery. Where does the power not dissipated in the motor go? Now connect both motors to the jacks that deliver the motor voltage. Now both motors are connected to the battery and variable resistor as shown in the schematic below. Notice that there is only one variable resistor connected to BOTH motors. We will be looking at how the system behaves when both motors are running. 12. Now that you have hooked up both motors disconnect one for a moment and turn the power on to the car. Now reconnect the other motor. How does the motor speed of both motors compare to the speed of only one running? Explain the difference (and there should be a difference). Hint: try hooking both motors together to the power supply set to deliver 5V instead of the battery - now disconnect one of the motors, reconnect it. Do the motors behave the same when they are connected to the battery and the motor? Does the current flowing increase or decrease? Does the voltage across the motor increase or decrease? 13. Now we will be making some measurements to validate your premise about why the two motors behave differently from a single motor. 14. With only one motor connected measure the voltage across the motor Vm and the current flowing through the motor Im. Record the values. 15. With both motors running measure the voltage across BOTH motors (i.e. this voltage is equivalent to the voltage across the battery), the current supplied by the battery Ib , and find the current flowing through each motor separately (Im1 current through motor 1, Im2 current through motor 2). 16. Is the voltage across the single motor the same as the voltage across both motors when they are running? Explain why the voltages are or are not the same. Is this consistent with your explanation for why the motors run more slowly when they are both connected? Explain. 17. Do the three currents Ib, Im1, and Im2 where Ib is the current supplied by the battery, and Im1, and Im2 are the currents flowing through the two motors, obey KCL? 18. Are the currents flowing through the two motors the same? 19. Are the currents flowing through the two motors the same as the current flowing through the motor when only one was connected to the battery? 20. Now to enjoy the fruits of your labors you can take the car and put it on one of the tables in the middle of the lab and let it run. a. Did you car track a straight line? b. Did the motors turn as fast as when the car was on the bench? Explain.