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# ( Hardware Design )
# Bipolar Stepper Motor Control Circuit
A major hardware design task in this project was to drive the bi-polar motors. These motors have no center taps on their windings. Therefore, to reverse the direction of the magnetic field produced by a coil, we need to reverse the current through the winding.
Fig HW-1: Applied Motion Products, Inc. Scotts Valley 5017-935 bipolar stepper motor
Each motor has four wires: blue, red, white, and yellow. A test with a multimeter showed blue and red wires both belong to a coil, while yellow and white are of another. After a survey of stepper motor control materials, we decided to use the following circuit [1]:
Fig HW-2: A Half H-Bridge Stepper Motor Control Circuit
The circuit above is a half H-bridge. It takes two of these circuits to control one motor. Point A and B are connected to one coil of the motor, say blue and red wires. Point X and Y are digital signals from the MCU, either at 5V or 0V. X xor Y is always 1, meaning their logic values are complementary of one another at all times. Next we will walk through two steps of the 4-step switching sequence to analyze the behavior of the circuit:
  1. First X = 1: the base-emitter junction of Q2 is forward-biased. Current flows from the collector to the emitter via the base. - At the same time Y = 0: Q4 is off and current flowing from +V through the resistor to the base of Q3 has nowhere to go. This current therefore raises the voltage at the base of Q3, turning it on.
    >> So for XY = 10, current flows from +V down Q3 to the coil from B to A and to ground via Q2.
  2. Similarly, for XY = 01, current flows from +V down Q1 to the coil from A to B and to ground via Q4.
The next two steps operate on the other coil in a similar manner. Table HW-3 [3] below summarizes the 4-step switching sequence for counterclockwise rotation:
Step Blue Red White Yellow
1 0V 5V 5V 0V
2 5V 0V 5V 0V
3 5V 0V 0V 5V
4 0V 5V 0V 5V
Fig HW-3: Switching sequence for counterclockwise rotation
Diodes D1, D3, D4, and D6 provide protection for the circuit against inductive kicks caused by abrupt change in current when switching one step to another, given by the equation V = -L(dI/dt). For an abrupt change in current (transistor switching), dI/dt is very large, producing a large voltage across AB. When VA or VB is larger than +V, current is shorted to the positive terminal via diodes D1 or D4. When VA or VB is more negative than ground, current is shorted to ground via diode D3 or D6.

For this circuit, we use 1N4001 for all the diodes and NPN TIP31 for all the transistors.
# Light Sensing Circuit
The next module is the light sensing circuit. This circuit consists of two photocells mounted on either side of the vehicle. The photocell produces a variable resistance based on light intensity. We connect it in series with a fixed resistor between 5V and ground to make a voltage divider circuit as follows:
Fig HW-4: Light sensing circuit for one sensor. r = 330 ohms. Photocell = 500 ohms (light) ~ 1500 ohms (dark).
The photocell gives a low resistance under a bright light and a high resistance in the dark.

Ain = 5V(r/(R+r)) R is small (light), then Ain is large and vice versa.

Ain is connected to the MCU's A/D inputs to convert this voltage to a number between 0 and 255. If this number is larger than some threshold value, appropriate control signals are sent to the H-bridge circuits, actuating the motors.
# Implementation of Circuits
We implemented the circuits for the following module as follows:

Mega32: We built a prototype board for mobility. We learned to solder printed circuit boards for the first time. We followed the guidelines given on [2]. We did not include MAX233 and RS232 connector because no serial communication was needed. In addition to the power connector, we also soldered a 9V clip so the board can be powered by battery.

Stepper Motors: For the H-bridge circuit (Fig HW-2), we first wired it up on white boards. We tested it and verified that it was functional. However since the circuit had 19 I/O and power pins, we decided to transfer the circuit to a large solder board to avoid wires coming loose when the car was moving or being handled. Being new at soldering, we made a few mistakes of wires or pins being not fully intact where they were supposed to make contact. We learned to debug circuits this way. This H-bridge circuit also has two 9V clips attached in parallel. So the motors can be powered by a 9V battery, or two of them for added power.

Light Sensors: Two light sensing circuits (Fig HW-4) were wired up on a white board. These circuits are simple and can be visually checked for loose connections. Having them on a white board, the sensors can easily be moved closer together or farther apart depending on the light source and the environment.

4/22/2005 - Cornell University ECE 476 Final Design Project "LightRover"
Thientu Ho
David Shu