Considerations for Optical Encoders Integrated to Ready-to-Use Motors

Machine designers need to reduce product time to market to gain a competitive advantage, which has increased demand for ready-to-adapt, value-added motor solutions. Design and development hours may be reduced with time-saving options such as motors that are easily modified with customized feedback devices. Designer’s demand for precise sensing and monitoring of motor speed and position requires innovative feedback designs that can be quickly integrated with high-performance motors. Optical encoders are used to provide accurate, high-resolution feedback signals that reflect the motor direction of motion, position, and velocity. Encoder designers must consider, in addition to the performance of the encoder, the specific motor application requirements and available integration options.

Basics of Optical Encoders

Optical encoders can be configured as either a transmissive or reflective type. This blog examines the more common transmissive style.

An optical encoder consists of four major elements – a light source, light sensor, optical disc, and signal conditioning circuitry. The light sensor is a phototransistor that is sensitive to the infrared light emitted from the light source. The optical disc (or code wheel) is a thin disc made of an opaque material with slits at specific intervals. The disc is located between the light source and sensor. The encoder assembly is integrated with the motor through an optical disc attached to the motor shaft.  As the motor shaft rotates, light passes through the disc slits and is alternately blocked by the solid spaces between the slits, which creates an output from the encoder in the form of electrical pulses. The encoder output signals are characterized by amplitude, duty cycle, and phase difference.  Factors that can affect these parameters and overall encoder performance include materials and coatings, geometrics of the code wheel, and sensor specifications.

Figure 1: Rendering of an optical encoder

Materials and Coatings

Encoder output signal quality is determined by the degree of light blockage from the source to the sensor. Since the infrared light used in optical encoders can pass through certain materials more easily than visible light, the selection of the proper material and coatings is imperative.  The encoder disc material must completely block the light from the sensor to ensure proper signal composition. In the cases where plastic 3D printed code wheels are used (such as in prototyping), the plastics and resins may not have sufficient opaqueness to block signals and thus may require additional coatings to provide optimum performance. An alternative is to utilize metal 3D printing or a fine metal disc and wire cut slits.

Shape and Geometry of the Code Wheel

The encoder code wheel can be cup shaped or disc shaped depending on space, packaging, and assembly constraints. The code wheel shape and geometry can also be determined by the orientation of the sensor and direction of the sensing line. Each shape has its own advantages and limitations based on packaging and performance criteria.

Figure 2: Shape and geometry of the code wheel

The geometry of the code wheel, sensor, and their relative orientations play a crucial role in encoder performance. For a given rotation speed, duty cycle, and phase, the optical diameter of the code wheel, slot placement, slot width, and sensor geometry all determine optimal performance. Other important factors include the distance between the emitter and detectors (gap width), the location of the code wheel between the emitter and detectors, and the aperture width and spacing between detectors (channel center to center distance) for dual channel encoders.

Sensor Specifications

Sensor parameters, such as the emitter wavelength, operating temperatures, and moisture sensitivity level, play an important role in the output of the encoder as a system. Output is governed by switching characteristics (pulse duration) and various timing elements (delay, rise, fall, storage, and turn-off time). These parameters guide the sensor selection and encoder performance requirements of the application.

Final Integration

Once the basic encoder characteristics are determined, the last step in the design process is to consider the best integration methods to match the encoder to the motor. To mount the encoder disc assembly to the motor shaft, typical ready-to-use motors will have an extended rear shaft and basic mounting holes to attach a housed encoder with the appropriate couplings. Other options integrate the encoder assembly within the motor housing with a disc mounted directly on the shaft that allows the motor housing to contain and protect the encoder assembly.

Thinking of utilizing an optical encoder in your application? Reach out to Portescap’s engineers here; we’d love to collaborate with you on your next project!