3 considerations for encoder selection

To ensure an exact volume of insulin gets delivered with each pump of a medical infusion device, or that a robotic arm moves to a precise line on the assembly point at the right time, an electric motor must be combined with an encoder. A rotary or shaft encoder is an electro-mechanical device that provides information on the position, count, speed, and direction of a motor, and is connected to an application with a controlling device, such as a programmable logic controller (PLC). The PLC uses the encoder’s feedback to ensure precise, accurate motor control.

Given the critical role that this feedback provides for accurate motion control, encoders must be properly selected.

Encoder technologies

The two main encoder types are incremental and absolute. Incremental encoders identify real-time feedback and track precise motion relating to changes in position and direction, rather than referencing a specific point. This is achieved by providing feedback on the relative movement between positions with continuous high and low feedback pulses. Absolute encoders show exact position; however, increased complexity makes them more expensive and means that incremental encoders are more cost effective for most applications. Adding an incremental encoder interface, such as an application-specific integration circuit (ASIC), can also add exact position reference capability.

Encoder sensors usually operate on optical or magnetic principles. Optical encoders pass infrared light emitted from an LED through a metal code wheel comprised of clear and opaque segments, creating distinct light signals received by optoelectronic sensors. This technology enables optical encoders to create highly accurate, precise positioning. In addition, the measurement of an optical encoder, such as Portescap’s E9, is unaffected by potential magnetic interference.

A magnetic encoder contains a magnetized disc with several poles surrounding the circumference. When the disc rotates, sensors detect magnetic field change, such as those measured by Hall effect devices that monitor the change in voltage. Magnetic encoders, such as Portescap MR2, are suitable for demanding applications which could include the potential for impact or ingress. The MR2 magnetic encoder, for example, is insensitive to temperature and has low sensitivity to unwanted external fields.

How they work

As the encoder rotates, it generates two square wave outputs, A and B, which are normally 90° out of phase with one another. By measuring the phase shift of the A and B outputs, the encoder’s direction can be determined. To measure its distance of travel or speed, the encoder’s resolution must also be considered. Resolution is the number of points of measurement within one 360° revolution of the shaft, also known as the duty cycle or period. Generally, the greater the number of points – lines per revolution (LPR) or pulses per revolution (PPR) – the greater the measuring accuracy. For example, Portescap’s M-Sense magnetic encoder has up to 1,024LPR.

The A and B outputs switch between high and low. The two bits of information thereby create 4x the counts for each line or pulse, also known as quadrature decoding. Quadrature decoding can increase resolution up to 4x, for example, turning the Portescap MR2 encoder’s 512 lines into 2,048 counts or angular steps. In addition to the two A and B output channels, a third Z channel is sometimes included which can be used to determine the reference position.

Where to use encoders

Understanding how encoders provide feedback for motor control, we can see how their use is critical across various applications. For the insulin pump example, a drug delivery system requires a precise amount of medication dispensed at a specified rate, and the encoder confirms that the exact dose is delivered. This example also shows how greater numbers of lines for increased encoder resolution can ensure precision to the most exact rate of flow.

Manufacturers can use robotic grippers, for example, to handle relatively delicate components. It’s key to ensure that the right amount of pressure and speed are used to correctly handle components to avoid damage. With an encoder, the robotic gripper’s function is optimized by the motion control of its motor’s speed and position, specific to each component it handles.

Similarly, pick-and-place applications used in the assembly of electronic equipment require high-speed motion control to quickly and repeatedly detect the size and weight of printed circuit board (PCB) components, placing them with precision. Encoders enable this high-speed, high accuracy control to ensure productivity and quality of manufacture.

Portescap
https://www.portescap.com

About the author: Chris Schaefer is a Portescap applications engineer. He can be reached at 610.235.5446 or Chris.schaefer@portescap.com.