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AVR446: Linear speed control of stepper motor
Features
• Linear speed control of stepper motor - Control of acceleration, deceleration, max speed and number of steps to move • Driven by one timer interrupt • Full- or half-stepping driving mode • Supports all AVR® devices with 16bit timer • Demo application for ATmega48 running on 3,68MHz, with serial interface on 19200 8/N/1.

8-bitMicrocontrollers Application Note

1 Introduction
This application note describes how to implement an exact linear speed controller for stepper motors. The stepper motor is an electromagnetic device that converts digital pulses into mechanical shaft rotation. Many advantages are achieved using this kind of motors, such as higher simplicity, since no brushes or contacts are present, low cost, highreliability, high torque at low speeds, and high accuracy of motion. Many systems with stepper motors need to control the acceleration/ deceleration when changing the speed. This application note presents a driver with a demo application, capable of controlling acceleration as well as position and speed. This linear speed controller is based on an algorithm presented in ‘Embedded Systems Programming’January 2005, ‘Generate stepper-motor speed profiles in real time’ an article by D. Austin. This algorithm allows parameterization and calculation in real time, using only simple fixed-point arithmetic operations and no data tables. Figure 1-1. Stepper motors

Rev. 8017A-AVR-06/06

2 Theory
2.1 Stepper motor
This application note covers the theory about linear speed ramp stepper motor controlas well as the realization of the controller itself. It is assumed that the reader is familiar with basic stepper motor operation, but a summary of the most relevant topics will be given. Further details about stepper motors can be found in D. W. Jones, Control of Stepper Motors. 2.1.1 Bipolar vs. Unipolar stepper motors The two common types of stepper motors are the bipolar motor and theunipolar motor. The bipolar and unipolar motors are similar, except that the unipolar has a center tap on each winding as shown in Figure 2-1. Figure 2-1. Bipolar and Unipolar stepper motors
A2 B1 B2 A1 A1 A2 B1 B2

The bipolar motor needs current to be driven in both directions through the windings, and a full bridge driver is needed as shown in Figure 2-2. The center tap on the unipolar motor allowsa simpler driving circuit, also shown in Figure 2-2, limiting the current flow to one direction. The main drawback with the unipolar motor is the limited capability to energize all windings at any time, resulting in a lower torque compared to the bipolar motor. The unipolar stepper motor can be used as a bipolar motor by disconnecting the center tap. Figure 2-2. Bipolar and Unipolar drivers withMOS transistors

1 1 1 2 2 1

2 2

2.1.2 Full vs. half stepping Stepper motors used in full-stepping mode powers one winding at a time. This way, four different settings (positions) is possible, shown in the ‘Full-stepping’ row of Table 2-1. By powering both windings simultaneous, the stepper motor is trapped between the positions obtained when full-stepping, also known as half-stepping.This gives eight positions as shown in the ‘Half-stepping’ row of Table 2-1. When powering both windings, the torque is approximately 1.4 times higher than when powering only one winding, but at the cost of twice the power consumption. The electrical cycle parts in 2

AVR446
8017A-AVR-06/06

AVR446
Table 2-1are all part of one electrical cycle. One mechanical cycle (revolution) usuallyconsists of several electrical cycles. Table 2-1. Full-stepping and half-stepping
Electric polarity Electrical cycle part Winding A Winding B Full-stepping Half-stepping 1 1 2 + + + + 2 3 4 + 3 5 6 4 7 8 + -

2.1.3 Speed properties One drawback with the stepper motor is the limited torque capabilities at high speeds, since the torque of a stepper motor will decrease with increasing speed. The torque...