FEATURE Sensors & Sensing Systems
Optical encoder feedback system for miniature motor driven applications
By Jonathan Colao, Applications Engineer, Analog Devices M
otor rotation information such as position, speed and direction must be accurate in order to produce
precise drivers and controllers across a wide variety of emerging applications, such as pick-and-place machines that mount microscopic components in the limited PCB area. Recently, motor controls have been miniaturised, enabling new applications in surgical robotics for healthcare and in drones for aerospace and defence. The smaller motor controllers also enable new applications in industrial and commercial installations. The challenge for designers is to meet the high-accuracy requirement of the position feedback sensor in high-speed applications, while at the same time adding all components into the limited PCB space in tiny enclosures, such as robotic arms, for example.
Motor control Motor control loops shown in Figure 1 are mainly made up of a motor, a controller and a position-feedback interface. The motor turns a rotating shaft that causes the arms of a machine to move accordingly. The motor controller instructs the motor when to apply force, stop or continue rotating. The position interface in the loop provides rotational speed and position information to the controller. This data is central to the proper operation of a
Figure 1: Closed- loop motor control feedback system
pick-and-place machine for the assembly of a tiny surface-mount PCB. All these applications require accurate position- measurement information about the rotating object.
The position-sensor resolution must be very high – enough to accurately detect the motor shaft position, correctly pick up a tiny component and place it accurately on a board. Also, higher motor rotational speeds lead to higher loop bandwidth and lower latency requirements.
Position-feedback system In a lower-end application, an incremental sensor along with a comparator may suffi ce for position sensing, whereas a
higher-end application will require more complex signal chains. These feedback systems comprise the position sensor, followed by analogue front-end signal conditioning, the ADC and its driver before data gets into the digital domain. One of the most precise position sensors is the optical encoder, typically composed of an LED light source, a marked disc attached to the motor shaft and a photodetector. The disc features a masked pattern of opaque and transparent areas that obscure the light or allow it to pass through. The photodetectors sense the resulting light, and the on/off light signals are converted to electric signals. As the disc turns, the photodetectors, in conjunction with the patterns of the disc, produce small sine and cosine signals, at mV or µV levels. This system is typical for an absolute position optical encoder. The signals are fed to an analogue signal conditioning circuit, usually consisting of a discrete amplifi er or an analogue PGA, to push the signal to the 1Vp-p
range Figure 2. Position-feedback system 16 November 2021 | Automation
– commonly to fi t an ADC input voltage range for maximum dynamic range. Each of the amplifi ed sine and cosine signals are then acquired by a simultaneous sampling ADC driver amplifi er.
automationmagazine.co.uk
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