Stepper Motor Controller

  
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Stepper motors are useful for relatively low-speed, intermediate-torque drive and positioning applications, particularly where accurate sub-revolution rotor position control is necessary. Motors of this type are commonly used to drive the reels on electromechanical slot machines (one-armed bandits), to position floppy disk drive heads, operate trainable
Stepper Motor Controller - schematic

camera platforms, and to power the drive wheels of small mobile robots. In times of yore, they were also used to position hard disk heads, though such applications have long ago been taken over by voice-coil type mechanisms. Stepper motors are simple and cheap to use, and you don`t need to have a fully closed-loop controller to use them accurately. Servomotors are much faster, but for guaranteeable positioning accuracy, you need to have a position encoder on the shaft to provide feedback on the actuator`s position. By contrast, as long as you don`t stray outside your system`s nominal acceleration profile (see the following), a stepper-based system can reliably maintain its position indefinitely without recalibration. There are several types of stepper motors, with varying electrical drive requirements. However, by far the most common type of motor to be found on the surplus market (or scavenged from unwanted computer equipment) is the four-pole unipolar type14, so this is the type our circuit is designed to use. Without further ado, here`s the schematic15: This project uses the ULN2803 octal high-voltage, high-current Darlington array to switch the stepper coils. This chip is readily available for around $0. 75 in small quantities, and it is a handy solution for driving moderate loads. Until recently, one could often find this chip, or its close relatives, in commercial stepper motor applications such as inkjet printers...



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