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EXPERIMENT No. 2 DRIVING STEPPER MOTOR
1. Objectives The student is expected to achieve the following objectives:
Understand how to deal with a PIC microcontroller. Write a simple code to drive a seven segment. Knowing the different kind of stepper m otors. To be familiar with stepper motor and test it. Controlling the direction, speed, and the position of stepper motor.
2. Apparatus
Seven segment display SC52-11EWA. PIC16F877A. Stepper motor kit. Multimeter. DC power supply.
Figure 1:Stepper motor kit
3. Prelab
Read the PIC16F877A data sheets. Read about the working principle of the stepper motor. Read the ULN2003 data sheets.
4.Introduction Seven Segment Display: LED displays are nothing more than sets of Light Emitting Diodes. The difference is that they have different shapes in order to display specific information. So driving LED displays is the same as regular LED. LED. Seven-segment displays may use liquid liquid crystal display (LCD), arrays of light-emitting diodes (LEDs), and other light-generating or controlling techniques such as cold cathode gas discharge, vacuum fluorescent, incandescent filaments, and others. Figure 2: 7-segment display
In a simple LED package, each LED is typically connected with one terminal to its own pin on the outside of the package and the other LED terminal connected in common with all other LEDs in the device and brought out to a shared pin. This shared pin will then make up all 1
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of the cathodes (negative terminals) OR all of the anodes (positive terminals) of the LEDs in the device; and so will be either a "Common Cathode" or "Common Anode" device depending how it is constructed. Hence a 7 segment plus DP package will only require nine pins to be present and connected. Numbers to 7-segment-code: A single byte can encode the full state of a 7-segment-display. The most popular bit encodings are gfedcba and abcdefg - both usually assume 0 is off and 1 is on. Table1: the hexadecimal encodings for displaying the digits 0 to 9:
Digit 0 1 2 3 4 5 6 7 8 9
gfedcba 0x3F
0x06 0x5B 0x4F 0x66 0x6D 0x7D 0x07 0x7F 0x6F
abcdefg 0x7E 0x30 0x6D 0x79 0x33 0x5B 0x5F 0x70 0x7F 0x7B
a on off on on off on on on on on
b on on on on on off off on on on
c on on off on on on on on on on
d on off on on off on on off on on
e on off on off off off on off on off
f on off off off on on on off on on
g off off on on on on on off on on
Stepper motor The stepper motor is synchronous motor, driven by a DC power supply. This motor can rotate a specific number of degrees for every electric pulse received by its control unit, so it can be used to control systems such as CNC machines where the position of the machine can be controlled precisely by the stepper motor. Moreover, it does not need any form of feedback. There are four types of stepper motors: Unipolar, Bipolar, Bifilar, and multiphase. The first two are discussed here in more detail: 1- Unipolar stepper motor: It consists of four separate electromagnets. To run the motor current has to be passed in each coil in a certain sequence. The current is passed in each coil in only one direction and thus it is called ‘Unipolar’.
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Figure 3: Circuit symbol for unipolar stepper motor There are 6 terminal wires available from the motor. Two of them are usually connected to the supply voltage (sometimes they are combined into just one wire and hence in that case only 5 terminal will be available rather than 6). The other 4 wires are the terminal of the four electromagnets. 2- Bipolar stepper motor: In a bipolar motor the current can be passed in each coil in two directions and hence the name. The bipolar motor is more powerful than the unipolar because it is possible to reverse the magnetization in each of the electromagnets. We can use an H-bridge (e.g., constructed of 4 transistors) to enable the current to flow in both directions in each coil.
Figure 4: Circuit symbol for bnipolar stepper motor Full stepping, half stepping and micro-stepping By applying full voltage to one phase of the motor, full stepping is achieved. The motor will advance by the stepping angle. By applying equal voltage to adjacent phases, half stepping can be achieved. By applying fractions of voltage to adjacent phases, micro-stepping is achieved. Micro-stepping can be applied in very small steps down to around 125th of the full step.
5. Procedure Part I:
Drive of a 7-segment display: 3
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Using PIC16F877A, 7-egment, and any other components needed build a 7-segment driving circuit, and write an assembly code to drive it to count from 1-9. Part III:
Identification of the stepper motor:
1) Try to rotate the motor’s shaft by your hand. What do you notice? What is the difference between it and other motors? 2) Depending on the wires number, what’s the type of this motor? 3) Using the multi-meter measure the resistance between the wires and identify the coil wires from the supply wires, and sketch the internal diagram for this stepper motor. Part II:
Driving the stepper motor:
Using Pic16F877A, ULN2003, and any other components needed build a stepper motor driving circuit, and write an assembly code to drive the stepper motor with fixed speed and fixed direction.
Figure 4: Driving the stepper motor
Part III:
Controlling the direction of the stepper motor:
Edit the program you have written in part II to change the direction of rotation (cw & ccw) using one or two push buttons. 4
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Part IV:
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Controlling the speed & direction of the stepper motor:
Edit the program you have written in part III to control the speed and direction, it’s required to have minimum three speeds (use push buttons to change the speed) and display the speed(1,2,3) on the 7segment. Part V:
Controlling the position of the stepper motor:
1) Find the resolution of the stepper motor (deg/step) by counting the steps nee ded to rotate the motor 360 degrees. [Hint: connect LED to one of the phases and count the pulses]. 2) Write an assembly code to rotate the motor to reach 90 degrees. (You can use push buttons).
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