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Implement stepper motor driver for gauges (Part 1)

14 May 2014  | Subbarao Lanka, Shruti Hanumanthaiah

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The world has already turned digital, and these days most information is represented in numbers. However, human nature is more "analogue" and better represented in the old-fashioned way, using pointer gauges and bar graphs. Pointer gauges can be found in many applications. Automobiles, trains, and even modern aircraft dashboards emulate analogue gauge functionality on control flat panel plasma or TFT screens. It does not look like good ol' pointer gauges will disappear in the near future.

A number of techniques can be used to control a pointer gauge and stepper motors are one widely used method. In this article, we will explore:

 • Why a stepper motor may be the best approach to drive pointer gauges
 • Basics of stepper motor operation
 • How to use a single SoC to process sensor data and drive the stepper motor
 • Common challenges faced and how to overcome them
 • Firmware architecture for optimum response time and resource management

Pointer gauges
A number of techniques can be used to control a pointer gauge. The most popular technique is to use a mechanical system, which consists of a turning coil mounted outside a two-pole permanent magnet. The applied DC current causes a magnetic force that rotates the coil and associated gauge pointer. Springs limit the coil rotation angle and the stable pointer rotation angle is in direct proportion to the coil current. Such a gauge can be equipped with an oil damper to suppress oscillations during coil angle setup and improve the system's mechanical stability with respect to vibration. This method has limitations in the operational temperature range because oil viscosity changes with temperature, causing the gauge to be unstable amid vibrations.

Other gauges use a bimetallic plate with a heater. This type consumes a lot of current during operation and readings are dependent on environmental temperature. An alternative approach uses two quadrature-located coils to set the pointer position. In this system, the pointer rotation angle is determined in relation to the coil. A mechanical damper is still required to prevent pointer flicker due to mechanical vibrations at setup time.

Stepper motor basics
An efficient way to control a pointer gauge is to use a stepper motor. A stepper motor is an electromechanical device that converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence. The motors rotation has several direct relationships to these applied input pulses. The sequence of the applied pulses is directly related to the direction of motor shafts rotation. The speed of the motor shafts rotation is directly related to the frequency of the input pulses.

Stepper motors are synchronous, dividing a full rotation into a number of steps. Unlike a DC motor that rotates continuously when a fixed DC voltage is applied to it, a step motor rotates in discrete step angles. Stepper motors, therefore, are manufactured with steps per revolution of 12, 24, 72, 144, 180, and 200, resulting in stepping angles of 30, 15, 5, 2.5, 2, and 1.8 degrees per step.

Different stepper motor steps
FULL-STEP: In full-step operation, the motor steps through the normal step angle; e.g. 200 step/revolution motors take 1.8 steps compared to 0.9 steps while in half-step operation. There are two kinds of full-step modes. Single-phase full-step excitation is where the motor is operated with only one phase energized at a time. This mode should only be used where torque and speed performance are not important such as when the motor is operated at a fixed speed and load conditions are well defined. Problems with resonance can preclude operation at some speeds. This mode requires the least amount of power from the drive power supply of any of the excitation modes. With dual-phase full-step excitation, the motor operates with two phases energized at a time. This mode provides good torque and speed performance with a minimum of resonance problems. Dual excitation provides about 30 to 40 per cent more torque than single excitation but requires twice the power from the drive power supply.

HALF-STEP: Half-step excitation is an alternate single- and dual-phase operation resulting in steps one half the normal step size. This mode provides twice the resolution. While the motor torque output varies on alternate steps, this is more than offset by the need to step through only half the angle. This offers almost complete freedom from resonance problems.

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