Analog-to-Digital Converters for Motor Control Applications-EEWORLD

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Analog-to-Digital Converters for Motor Control Applications

Designers must address challenges such as current and voltage monitoring, optical encoder feedback, and resolver-to-digital conversion

In motor control applications, designers must address challenges such as current and voltage monitoring, optical encoder feedback, and resolver-to-digital conversion. These processes are important for applications that require precise control of motor speed and mechanical motion, such as industrial assembly line robots and automotive power steering.

The converters used in these applications must be fast, synchronously sampled, monotonic, free of pipeline delays, small in size, and low in power (see Figure 1). Some applications require high-voltage isolation and safe operation, while others must interface with resolver-type position sensors. Successive Approximation
AD Converters

Optical encoders can provide low-cost, high-precision position sensing for servo control applications, such as industrial robots and lathes that require axis feedback to the controller for precise positioning of mechanical motion. These applications require synchronous sampling and fast loop settling times, and successive approximation AD converters are an ideal choice. Figure 2 shows a successive approximation AD converter, the AD7266, which is a 12-bit, 2Msps, 2-channel synchronous sampling analog-to-digital converter.

∑-△ AD Converters

The speed and position of the motor can be determined by monitoring the current in each phase of the motor. ∑-△ A/D converters can monitor current very accurately, making them ideal for motor control applications. In many industrial applications such as cranes, pumps, and blowers, motors operate in high temperatures or hazardous environments. In these applications, intrinsic safety is very important and current isolation is necessary.

The integration of isolation circuits and digital-to-analog conversion circuits can achieve high data rates, low power consumption, and current isolation in a single package. ∑-△ modulators provide a serial bit stream data format - its time average is proportional to the input signal - making them easy to isolate. Figure 3 is an example of an isolated ∑-△ modulator.

Resolver As shown in Figure 4a, a resolver is a type of rotary transformer that usually has a primary winding on the rotor and two secondary windings on the stator. The variable reluctance resolver of Figure 4b is a special type that has no windings on the rotor.

Although both the primary and secondary windings are on the stator, with this design, the rotor will produce sinusoidal ripples on the secondary winding as the angular coordinates change. In either form, the resolver output voltage (S3-S1, S2-S4) has the following form:

S3-S1 = EO sinΩt sinθ
S2-S4 = EO sinΩt cosθ

Where θ is the shaft angle, sinΩt is the rotor excitation frequency, and Eo is the rotor excitation amplitude.

In automotive applications such as power steering systems and industrial applications with large temperature changes, such as glass manufacturing, reliable resolver-to-digital conversion is required to ensure stability in harsh environments and accurate positioning of mechanical motion.

Reference address：Analog-to-Digital Converters for Motor Control Applications

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