Sensors / Steppers / Motors

Optical or capacitive encoder feedback?

How to decide which device is best for a step motor system

By Eric Rice, Applied Motion Products

Step motor systems utilize encoders to provide feedback to the motor drive or controller and improve overall performance. Typically installed to the rear shaft of the motor, encoder feedback functionality can dramatically improve system performance in regard to accuracy, smoothness, torque and positioning (Figure 1). But which encoder works best with a step motor?

Optical encoders have played a role in various industrial applications for a long time. They are accurate, reliable and user-friendly. They offer a wide range of resolutions. Optical encoders operate by passing light, generated by LEDs, through a slotted disc mounted to the motor shaft to an array of optical sensors. As the shaft turns, the slots in the disc alternatively pass or block the light emitted by the LEDs. This pulsing of light as the shaft turns creates a series of digital pulses that provide position and speed information back to the motor controller (Figure 2).

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While offering similar benefits, capacitive encoders utilize newer technology in providing the same position and speed information as optical encoders (Figure 3). The different operating principle of capacitive encoders lies in generating the digital feedback pulses. An ac field transmitter emits a signal that is modulated by a metal pattern on a disc attached to the motor shaft. As the shaft turns, the sinusoidal metal pattern on the disc creates a signal modulation due to varying capacitive reactance that is repetitive and predictable. A field receiver on the other side of the disc receives this modulated signal and converts it into digital pulses for use by the motor controller.

Both optical and capacitive encoders are excellent choices for the most common motion control applications. However, capacitive encoders offer benefits in harsh environments. Optical encoders are susceptible to signal degradation or loss if exposed to dust, oil or similar contaminants. Capacitive encoders are generally immune to these environmental contaminants.

As encoders are offered in different configurations, whenever possible use encoders with differential signals. Differential signals are inherently more noise immune than single-ended signals. The usage of single-ended signals should be limited to installations where ambient electrical noise is well controlled and single-ended signals are acceptable. Encoders with an index channel that creates one pulse per revolution are useful for accurate referencing of the motor axis to a home position.

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