FEATURE Sensors
THE BENEFITS OF VIRTUAL ENCODERS M
Incremental encoders typically output
achine builders use motorised stages to achieve precise positioning and movement for numerous manufacturing
applications, including semiconductor production, additive manufacturing (3D printing) and the automotive industry. These motion systems play critical roles in tasks such as applying glue, performing inspections and conducting lithography. Motion control systems can create an a series of set points to ensure smooth, optimised movement. However, factors such as mechanical limitations, friction or compliance can cause the actual motion path to deviate from this planned trajectory, creating what is known as a ‘following error’. Motion systems must therefore correct for these deviations by continuously comparing the actual and intended positions, making adjustments as needed. This feedback loop – which can be achieved with various types of encoders – allows motion platforms to maintain high precision despite high dynamic load conditions or environmental changes.
Encoder types
An encoder is a sensor that converts mechanical movement into an electrical signal, which is then relayed to the control system to give information about parameters such as position and velocity. Traditional encoders can be primarily categorised into incremental and absolute types (see Figure 1). Incremental encoders track relative position changes, continuously feeding data to the control system. In contrast, absolute encoders provide instant position feedback, eliminating the need for a homing process. This is valuable in long-travel applications, as a machine might need to move a an incremental encoder. Absolute encoders also have advantages in terms of system reliability, reducing the need for extensive wiring and improving the safety and speed of the start-up. However, incremental time feedback, lower cost and high noise immunity, so suit applications requiring continuous position updates without the complexity or expense of absolute encoders.
14 December 2024/January 2025 | Automation
A quad B signals, which are widely compatible with motion control systems and provide good noise immunity (see Figure 2). Quadrature signals are two the system to track direction and position changes in a simple way. However, sine/ through interpolation. Sine and cosine signals are analogue waveforms generated by encoders to represent position data; sine signals provide high resolution but may be prone to signal noise, while cosine signals stabilise and validate the sine signal for more precise readings. To use sine/ cosine signals with quadrature-based control systems, an interpolator is needed to convert the analogue sine/cosine waves into a higher frequency quadrature signal, though often at a lower resolution than the full capability of the sine/cosine encoder. Absolute encoders, in contrast, generally use protocols such as BiSS-C and EnDat, which send data in serial form and on demand. This works well for precise, on-request data, but is less compatible with devices requiring continuous or real-time position updates, as absolute encoders only send position data when requested.
Synchronisation
Figure 1 (top): Incremental and absolute encoders create different position markings Figure 2 (middle): Incremental encoders can send quadrature or sine/cosine signals Figure 3 (bottom): The ACS LCM connects to the motion controller via EtherCAT, generating virtual encoder signals and trigger signals for synchronised communication between the PC, motion controller and device
In most applications, the motion of the stage must be synchronised with external devices such as lasers, cameras and glue dispensers. A camera, for example, might need to capture an image at a precise position, or a glue dispenser may need to apply adhesive at exact intervals. Synchronisation typically requires a physical trigger signal based on encoder information, which can be challenging with sine/cosine and absolute encoders. Incremental encoders with quadrature signals are therefore often preferred, because they provide continuous position data. However, these encoders can be limited by resolution, and are susceptible to signal noise in high speed applications, as the signal bandwidth may be exceeded, causing data dropouts and positional inaccuracies. Although traditional A quad B incremental encoders are widely used in motion control, they are limited in more complex set-ups, such as gantry systems with two axes that
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