2.2 Robot Drives

2.2.4 Stepper Motor Drives

The stepper motors used in our design contain four input wires, one to each coil, plus a common voltage input. The stepping sequence of the motor can be controlled differently by assigning the input pulse on different combinations to the inputs A, B, C, and D indicated on figure 2.8 below.

Figure 2.8   stepper motor terminals

The possible stepping sequences are wave-drive, two-phase, and half-step. The wave-drive format activates one coil at a time in an A-B-C-D (or D-C-B-A, if reversing rotation direction sequence). The wave-drive mode consumes the least amount of power and has high positional accuracy.

Two-phase drive activates two adjacent coils during each sequence (AB-BC-CD-DA). This sequence mode offers an improved torque while decreases the rotation speed of the stepper motor.

 Half-step activation alternates between the one-coil and two-coil modes (A-AB-B-BC-C-CD-D-DA), providing an eight-step sequence for smoother rotation.

2.2.4.1 UCN5804 Stepper Motor Driver

The stepper motors use a 9-12 Volts to support the movement of the camera. We use the integrated circuit stepper motor driver UCN5804B which can drive up to 50 volt 1.5 an output, and the inputs are compatible with TTL circuits. This driver provides four Darlington transistors at the output connecting to the stepper motor.

The following is the circuitry for each stepper motor with the UCN 5804 Integrated Circuit stepper motor driver.

Figure 2.9   UCN 5804 stepper motor driver

From the figure above, pin 9 and 10 determines the stepping sequence of the stepper motor. Since we decided to use two phase stepping sequence, and according to the truth table below, both pin 9 and 10 are at logic 0.

Table 2.2 Logic control for the stepper sequence

2.2.4.2 Circuit Connections

Figure 2.10 UCN5804 circuit connections

Figure 2.11 UCN5804 circuit and 555 IC board

As displayed in the figure 2.10 above, pin 9 and 10 are also used for ON/OFF switch of the stepper motor. This is because when both pin 9 and 10 are high, the step inhibit function is performed by the UCN5804 chip to stop the motor from running. Since our project only requires both pins to be low (half-step) or both high (step inhibit) at the same time, only one ON/OFF switch needs to be connected to both pins. The a, b, c, d pins in the figure are connections to the input pins of the stepper motor. Each input signals will control motor in the way described by the stepper motor section previously discussed. The diodes, LEDS, are put in so one can visually observe the stepping sequence and for debugging purposes. The direction (pin 14) is used to rotate the stepper motor clockwise or counterclockwise.