Pulse width Modulation - Introduction to Electronics | ECE 110, Lab Reports of Electrical and Electronics Engineering

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Laboratory #9 – pulse width modulation (revisited) Grade Pre-lab: /0 Lab: /12 Total: /12 Objectives * Learn how PWM module works * Understand how to use PWM in car design * Learn how to use logic analyzer * Learn about sequential circuits * Learn how to program Car Board * Learn how to use Car Board Lecture References Orange Book * 9.19-9.21- Binary Number System * 9.21-9.22- Synchronous Counter * 11.1-11.2- Storing Information * 1.2-11.11- Flip-flop’s (FF’s) * 11.14-11.16- Counters Transparencies * Burnet Handouts 15,16,17- Sequential Circuits (Flip-flop’s and Counters) * Haken Set 3- pg. 2-16 Mallard * Homework 11- Digital Numbers * Homework 12- Comparators * Homework 13- Flip-flops * Homework 14- Counters EXPERIMENT #9 – PWM pulse width modulation At this point you should have a basic, workable design – a circuit that will navigate the track through most turns, will detect the presence of a split, and will stop on a white piece of paper. From now until the design challenge you should focus on improvements to your circuit so that it navigates every turn, and every split. Your design must be robust enough that it performs well even though the vehicles are all different, even though the table used for the challenge track has a slightly different reflectivity, even though ... You get the idea. During this lab you will experiment with a sequential circuit that can be using to control the speed of the motors. In lab 3 you experimented a little with pulse width modulation (PWM) to drive the motors. Instead of relying variable resistor to vary the motor speed we will supply the motor maximum voltage/current for a limited period of time – a period set by the duty cycle of the signal. The longer the motor is provided with current during each period of the square wave, the faster the motor runs. A block diagram of the pulse width modulator is shown in the figure below. It consists of an oscillator (74LS624) – this provides the clock pulse that runs the counter, a counter (74LS169) – this circuit simply counts from 0 to 15 over and over, and the digital comparator (74LS85) - the heart of the device (you will figure out what it does during this lab). The counter supplies one set of inputs to the comparator and the user supplies the other set - it is these user supplied inputs (labeled the A inputs) that will be used to determine the duty cycle of the resulting square wave. The PWM accepts inputs from sensors directly OR from logic that uses the sensor data in a more sophisticated algorithm. The output of the PWM comes from the digital comparator on the PWM module – either pin 5 (A>B) or pin 7 (A<B). As you experiment with the circuits you will see the difference between the two signals. Whichever output you choose, it can be applied directly to the CA, or CAB (as in the figure below) module or it can combined with more logic. pulse (square wave with 50% duty cycle) is used to change the state of the counter so that it continuously cycles through 0-15. Carefully squeeze the 14-pin test clip and clip it to the oscillator (74LS624) - the chip on the right as viewed from above. The pins on the logic probe are numbered just like the chip. Find the ground pin on the oscillator chip and connect the logic probe with the black label to it - this provides a ground reference for the other signals. Connect the probe with the brown label to pin 6. 1. Turn on the logic analyzer and the vehicle and you should see the clock on the screen of the logic analyzer. Draw the clock and estimate its period. Is this period short/long enough to drive the motors and respond quickly enough to changes in sensor output? What would determine the period for this application? Attach the logic analyzer to the counter – The counter, as the name suggests is to simple count from 0 to 15 over and over and over ... With the power off on the vehicle, connect the 16-pin test clip to the counter (74LS169A) and connect the black wire to ground (pin 8). Connect the logic probes as specified in the table below. 2. Once the probes are connected you should see the clock as supplied by the oscillator and the four outputs from the counter. Sketch these timing diagrams in your notebook. Do they look like the ones you drew for prelab? 3. Each of the 4 signals coming from the counter is a periodic square wave. How do the periods of each of these signals compare with the period of the clock pulse?</font> To specify the duty cycle of the square wave output to the PWM, 4 signals need to be supplied. They represent a number from 0-15 that is compared to the output of the counter which determines how many of the 16 pulses the output should be high - as we shall see. A simple way to specify these four signals is to use one of our other modules called the MUX module because is has a multiplexer chip that lets these four signals originate from the switch on the module so that we can set the number by hand OR from sensors. Place this module on the protoboard near the PWM and hook up the power and ground. Connect pins 4, 7, 9, and 12 from the MUX module to pins 10, 12, 13, and 15 of the PWM. Look closely at the red switch on the MUX module - the swithes are labeled 1, 2, 3, 4, and 5. Switch 5 controls the select pin on the MUX (74LS257 chip) and should be UP to manually set the duty cycle using the switch. Switch 5 in the down position allows an alternate set of signals to drive the PWM's duty cycle. REMEMBER: power and ground must be connected first for this module to work. Controlling the duty cycle/introducing the MUX module - Set switch 1 and 2 UP and 3 and 4 DOWN. Monitor both the clock pulse from the PWM (pin 6) and the resulting square wave output from the PWM (pin 5) with channel 1 and 2 of the oscilloscope. 4. Looking at the schematic of the PWM, or looking at the module itself, you can see that the socket coming out of the PWM module is connected directly to one of the chips on the module. Which one? Explain the function of the chip. 5. Draw the clock pulse and the signal from pin 5 of the PWM module. Explain the shape of the pulse from the PWM. 6. Now monitor the signal from pin 6 and pin 7 of the PWM. Draw these signals and explain their shape. 7. Put the switch into different positions and describe how the signal from pin 5 of the PWM is related to the switch settings. 8. To use the PWM in your design you need to choose which output you will use – either the A>B output from pin 5, or the A<B output from pin 7.