MEASUREMENT & MONITORING
ENCODERS
PORTESCAP
Why they ’ re used and how to make your selection
Encoders provide feedback for accurate motor control relating to speed and positioning . Chris Schaefer , Applications Engineer at Portescap , looks at the technologies involved and explains how to choose the encoder for your application .
To ensure that an exacting volume of insulin is delivered with each pump of a medical infusion device , or that a robotic arm used in manufacturing assembly moves to a precise point at the right time , an electric motor has to be combined with an encoder . A rotary or shaft encoder is an electro-mechanical device which 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 programable logic controller ( PLC ). The PLC uses the encoder ’ s information , commonly known as ‘ feedback ’, to ensure precise accuracy of motor control .
ENCODER TECHNOLOGIES The two main types of encoder are known as 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 . They achieve this by providing feedback on the relative movement between positions with continuous high and low feedback pulses . Absolute encoders show exact position however their 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 .
An encoder ’ s sensor usually operates on an optical or magnetic principle . Optical encoders pass infrared light emitted from an LED through a metal code wheel , comprising clear and opaque segments , which create distinct light signals received by optoelectronic sensors . This technology means that optical encoders are able to create highly accurate , precise positioning . In addition to its high accuracy , the measurement of an optical encoder , such as Portescap ’ s E9 , is unaffected by potential magnetic interference .
Meanwhile , a magnetic encoder comprises a magnetised disc with a number of poles surrounding the circumference . When the disc rotates , sensors detect the change in magnetic field , such as those measured by Hall effect devices , which monitor the change in voltage . Magnetic encoders , like the Portescap MR2 , are ideal for use in 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 AN ENCODER WORKS As the encoder rotates , it generates two square wave outputs , A and B , which are normally 90 degrees 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 taken into account . Resolution is the number of points of measurement within one 360 degree revolution of the shaft , also known as the duty cycle or period . Generally , the greater the number of points , which are termed 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 1,024 lines per revolution in a compact design .
Each output , A and B , switches between high and low . The two bits of information thereby create four times the counts for each line or pulse and this is known as quadrature decoding . Thereby , quadrature decoding can increase resolution by up to four times , for example turning the Portescap MR2 encoder ’ s 512 lines into 2048 counts or angular steps . In addition to the two A and B output channels , a third channel , Z , is sometimes included which can be used to determine the reference position .
WHERE ENCODERS ARE USED Understanding how encoders provide feedback for motor control , we can see how their use is crucial across various applications . Taking our original example of insulin administration , a drug delivery system requires a precise amount of medication dispensed at a specified rate and the encoder is used to confirm that the exacting dose is delivered . This example also shows how the greater number of lines for increased encoder resolution can help ensure precision to the most exacting rate of flow .
A robotic gripper can be used for example in manufacturing to handle relatively delicate components . It ’ s key to ensure that the right amount pressure and speed is used to correctly handle the component to avoid damaging it . Thanks to an encoder , the robotic gripper ’ s function is optimised 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 PCB components , placing them with precision . Encoders enable this high speed , high accuracy control to ensure productivity and quality of manufacture .
For further information , please visit www . portescap . com
70 PECM Issue 49