Lab 3: Design of a stepper motor controller
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1. UNIVERSITY OF Dept of Electrical and Computer Engineering SASKATCHEWAN CME 331 Microprocessor Embedded System Khan A Wahid Last updated October 6 2014 Lab 3 Design of a stepper motor controller Safety In this lab voltages used are less than 15 volts and this is not normally dangerous to humans However you should assemble or modify a circuit when power is disconnected and do not touch a live circuit if you have a cut or break in the skin Learning outcome This lab provides an opportunity to work with an ARM CortexM4F microcontroller MCU using the Stellaris LM4F120 LaunchPad that has a Cortex LM4F120H5QR microprocessor Upon completion of this lab the students will be able to a Learn basic operation of a stepper motor and design a controller circuit to drive the motor b Write C code using Keil uVision to drive the stepper motor in two different directions at different speeds and calculate and display motor speed in rpm on the 7 segment display c Learn how to use the system timer SysTick and header file in your design Required hardware Materials Equipment Diodes 1N4001 4 Stellaris LM4F120 LaunchPad MOSFET IRL2703 4 Stepper motor RB Soy 03 Resistors 330 ohm 4 12 V supply 7 segment display board Checkpoints Throughout this manual wherever you find a Sron sign it means that there is something that you need to a show to your instructor and or b observe measur2. convert a value to decimal for display purpose such as divide the number by 10 and take the quotient and remainder or convert the number to BCD binary coded decimal Use any technique of your choice Some instructions will be given in the lab 14 Obtain an optical tachometer and measure the motor speed Take at least three readings Compare the readings with your calculation Show your design to the instructor Part 5 Answer the following questions in your lab notebook Sro 15 From the lab experiments comment on the three drive modes and compare the motor performance full step vs half step vs wave drive Which mode will offer a smoother operation Which mode will give you the highest torque Is there any other way you could drive the stepper motor Give one example of each driver mode 16 Optional Which rpm reading is more accurate optical tachometer vs software Calculate the average error in the reading of the motor speed 17 Explain in your notebook the operation of the driver circuit given in Fig 2 Why are diodes used in backward What else could have been used instead of the FET transistor References 1 CME331 class website https www engr usask ca classes CME 33 1 2 LM4F120H5QR Datasheet Aug 29 2012 http www ti com lit gpn lm4fl 20h5qr 3 Stellaris LM4F120 LaunchPad User Manual SPMU289A Revised December 2012 http www ti com tool sw ek Ilm4f120x1 4 Lab dev board schematics https
3. e and explain in your lab notebook Lab description In this experiment we will use a 200 steps revolution stepper motor With this rating the motor will provide a smooth spinning and each new set of step pulses will cause the motor to rotate by 1 8 You will use software generated step pulses to drive the motor All three modes of operation wave drive full step and half step drive will be tested You will later modify your code to control the speed and direction of the motor Two switches will be used to start stop the motor and control the spinning direction CW CCW The speed in rpm in decimal will be displayed on the 7 segment board Stepper motor fundamentals Stepper motors are used when precise control of movement is needed They are used in printers and scanners to move paper or scan heads in disk drives to move read write head in high precision robotics to move robot s arms or legs etc In these applications precise control of CME 331 Laboratory 2 positioning is more important than motor speed rpm or high torque It is very easy for a microcontroller to control both position and speed of a stepper motor using a simple open loop digital interface Open loop control means there is no need of any feedback circuit which eliminates the need for any expensive feedback and sensing devices The position of the motor can simply be tracked by using the number of input step pulses A stepper motor can be of two types b
4. i polar and unipolar A bipolar motor has two coils and typically current flows through both coils at all times The drive shaft rotates a precise number of degrees as current is applied to each coil in sequence On the other hand the unipolar motor provides for bi directional currents by using a center tap dividing each coil into two parts generally 5 or 6 wires Since half of the electro magnets are energized at one time it will have less torque than an equivalent bi polar stepper motor However unipolar motors are easier to interface with a microcontroller We are using a unipolar stepper motor in our lab Review lecture notes posted on the course website for details Lab procedure The lab has several parts as described below Part 1 Drive the motor in wave drive mode 1 You will need to generate a series of step pulses to drive the stepper motor In your assignment 2 you have written C code to generate these pulses using the system timer or SysTick These four step waveforms are to be outputted through pins PDO PD3 as shown in Fig 1 a Modify your code so that the width of each pulse is 3msec 3 msec gt PDO 6 LEADS BLE PDI PD2 Sar C R yi RED THIT BLU a b Fig 1 a Software generated step pulses wave drive b stepper motor winding diagram 5 2 Obtain from the support office the development board and the stepper motor The winding diagram of the stepper motor is given in Fig 1 b Carefully n
5. ocessor and make it permanently unable to be debugged or re programmed 3 9 Sron Modify your C code to unlock SW2 and implement the on off and CW CCW features to the motor controller While modifying you code it highly recommended drawing a flow graph or flow chart that shows the flow of your algorithm or steps It will help you better debug the code Show the operation to the instructor CME 331 Laboratory 5 Part 3 Drive the motor in full step and half step modes 10 Modify your C code to generate the full step pulses to drive the motor In this case two phases are always on and the motor will run at full rated torque Compile and download your code and observe the motor performance smoothness torque speed etc 11 Modify your C code to generate the half step pulses to drive the motor This drive mode alternates between two phases on and a single phase on Compile and download your code and observe the motor performance 12 D Show both drive modes to your instructor Part 4 Optional Calculate and display motor speed in rpm on 7 segment display 13 Choose any drive mode Write software to calculate the motor speed in revolution per minute or rpm To achieve it you need to know how long it takes to perform one complete revolution see Step 6 Use the 7 segment display to display the rpm in decimal While displaying the rpm on 7 segment you will need to convert it into decimal There are many ways you can
6. ote the six different color wires of the motor The microcontroller does not have sufficient output current capability to drive each coil of the stepper motor As a result we have to use a driver circuit consisting of four FET transistors IRF2703 to drive each coil Four current limiting resistors are added A snubber circuit using four diodes 1N4001 is used to protect the transistors For more information review the lecture notes CME 331 Laboratory 3 The motor driver circuit is already built in the development board Consult the schematics of the development posted on the course website The connection diagram of the lab and the driver circuit is given in Fig 2 12V O Yellow ZN 1N4001 LM4F120 LaunchPad CW CCW IRL2703 start stop PD3 PD2 PD1 PDO Fig 2 Connection diagram 3 Now connect the stepper motor with the motor interface on the development board Turn on the 12V power supply Download your code to the LaunchPad and the motor should spin counter clockwise CCW The experimental setup is shown in Fig 3 CAUTION You should take extra caution when handling a motor and 12V supply Always turn on the 12V power supply to the motor last Limit the current to 1 2A Static charges may damage the MOSFET so while you work on your code and the LaunchPad is idle or off you should turn off the 12V supply Check the temperature of the MOSFET and the motor regularly Slight heat is genera
7. rse website It contains all GPIO definitions and some important control registers that you may need in this course and other labs As a result from now on you can use this file in all your labs In that case you do not need to include any port related definitions in your main C code simply include the header file in your code Unlocking SW2 PFO From earlier labs you have learnt how to configure Port F to use SW1 However SW2 is by default locked internally inside the microcontroller as it is a non maskable interrupt NMI p 213 section 5 2 3 1 2 A special 2 step procedure is required to unlock it As NMI pin the Alternate Function Select Pull Up Resistor Pull Down Resistor and Digital Enable are all locked for PFO until a value of 0x4C4F434B is written to the Port F GPIO Lock Register GPIO_PORTF_LOCK p 637 After Port F is unlocked bit 0 of the Port F GPIO Commit Register GPIO_PORTF_CR p 638 must be set to 1 to allow access to PFO s four control registers In LM4F this protection is currently implemented only on the NMI and JTAG SWD pins PD7 PFO and PC 3 0 all other bits pins in the GPIO_PORTX_CR registers are hardwired to 1 e no change is possible ensuring that it is always possible to commit new values to the GPIOAFSEL GPIOPUR GPIOPDR or GPIODEN register Requiring this procedure makes it unlikely to accidentally re configure the JTAG and NMI pins as GPIO which can lock the debugger out of the pr
8. ted in them but excessive heat is an indication of abnormal operation or miss handling In such case turn off the power supply and ask for help Fig 3 Experimental setup CME 331 Laboratory 4 4 Now modify you C code to vary the speed of the motor It may be achieved by varying the width of the pulses Observe the operation of the motor at variable speeds 5 Modify your C code so that the motor changes the direction clock wise Observe the motor spin in CW direction 6 Now modify your C code to spin the motor by exactly 360 i e one complete revolution You will need it later in the lab when you calculate the rpm Show the operation of the motor to your instructor and demonstrate how motor speed and direction can be varied Part 2 Add header file and switches to control the motor 7 In this part you will add two switches one will turn on and off the motor i e works as push on and push off switch the other will control the spin direction i e push CW and push CCW You will use the two on board switches SW1 and SW2 However these push button switches are single pole single throw so you will need to use software techniques to accomplish the task Review the lecture notes for more details on this topic 8 Since you will use Port F now and ports A and B later in part 3 it is a good time to start using a header file that contains all port definitions Download the header file cme331_LM4F120 h from the cou
9. www engr usask ca classes CME 33 1 WebNotes_2014 Drawing_dev_board jpg 5 http www robotshop com ProductInfo aspx pc RB Soy 03 CME 331 Laboratory Appendix A LaunchPad interface Fig I Header pins on the LaunchPad EK LM4F120XL EK TM4C123GXL O 5V VBUS LM4F120 or IN SINGLE Virtual Serial Port TM4C123 PACKAGE BLUF PEE UART USB device only ii R1 0E LOCKED Fig II Connection diagram of on board LEDs and switches in the LaunchPad END
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