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Implement stepper motor driver for gauges (Part 1)

14 May 2014  | Subbarao Lanka, Shruti Hanumanthaiah

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MICRO-STEP: In micro-step mode, a motor's natural step angle can be divided into much smaller angles. For example, a standard 1.8 degree motor has 200 steps/revolution. If the motor is micro-stepped with a 'divide-by-10', then each micro-step would move the motor 0.18 degrees and there would be 2,000 steps/revolution. Typically, micro-step modes range from divide-by-10 to divide-by-256 (51,200 steps/rev for a 1.8 degree motor). The micro-steps are produced through proportional current in the two windings according to sine and cosine functions. This mode is only used where smoother motion or more resolution is required.

Advantages of using stepper motors
 • The motor has full torque at standstill (if the windings are energized).
 • Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 – 5% of a step and this error is noncumulative from one step to the next.
 • Excellent responsiveness to starting/stopping/reversing.
 • The motor's response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.
 • It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.
 • A wide range of rotational speeds can be realised since speed is proportional to the frequency of the input pulses.

Stepper motor control system
Stepper motor control primarily consists of a sensor, SoC, and stepper motor. The sensor output is given to a SoC, which then drives the stepper motor based on the processed sensor output.

Vehicle dashboards have a speedometer indicating the speed of vehicle and a fuel meter showing the fuel level. There are dedicated sensors for each of these – Vehicle Speed Sensor (VSS), Wheel Speed Sensor, fuel sensor, tyre pressure monitoring sensor, etc. These sensors give out electrical signals based on a voltage, current, or frequency that is proportional to the level of the parameter, which the sensor is measuring.

The SoC is responsible for three main functions:
 • Measure the input data from sensor
 • Process the measured data
 • Drive the stepper motor

Measuring the input data from sensor
Voltage Input: Voltage input measurements require just an A/D converter in the SoC.

Challenges: Noisy A/D converter output

Solution: The noise in the ADC output can be removed using software-based filters like IIR, average filter etc.

ADC offset error
Solution: Ground the input of the A/D converter and fetch the digital value. This digital value is used as the offset which is subtracted from the A/D converter output value for each corresponding input voltage.

Mismatch between sensor output and A/D converter input range. This reduces the resolution of input measurement.

Solution: Modern day SoCs come with programmable gain amplifiers where the gain of the amplifiers can be adjusted to scale the sensor output voltage range to A/D converter input range.

Current input
This is similar to voltage input measurements. Connect a resistor of the appropriate value to the input of the A/D converter. The input current is converted to a voltage and is measured using the A/D converter.

Frequency input
There are two classic methods for measuring frequency.

Counting Cycles for a Fixed Time

Figure 1 shows the topology for counting the number of cycles in a fixed period.

Figure 1: Counting cycles for a fixed time.

The counter's value is latched at sample rate fsample and incremented every cycle of the input signal finput. The double latch enables the old count value to be subtracted from the new, resulting in a new accumulated value for each sample period.

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