By Daniel Rellstab | Product Manager at SMC Switzerland
JULY 2024
When discussing electric actuators we are indeed talking about control, precise motion control. The role of an encoder is crucial as it memorises the starting position or datum of the electric actuator and provides location feedback to the drive, thus closing the control loop. Without an encoder, the motor would start moving but without knowing where it is and where to go. So, which encoder type should you choose for such an important motion control function: incremental or absolute?
Watch-like precision
The latest battery-less absolute encoders use a combination of three different geared disks, like a watch with second, minute and hour counters. This design makes it possible to make many rotations and consistently obtain new images. Previously, this type of encoder comprised a single absolute value disk supported by a battery to count each revolution.
Continuing with the watch metaphor, an incremental encoder is like a wind-up clock. Upon depletion of the stored energy, you need to reset the correct time and wind it up again.
Choosing between absolute and incremental encoders for an electric actuator depends on your specific application.
Step by step approach
An incremental encoder is the most economical option. After performing the reference run users can rely on precise position control, but only as long as there is power. In the event of power failure, it’s necessary to complete a reference run again. In some critical applications there’s no margin for such a possibility, especially considering the time required for establishing the homing routine and returning to the datum position.
SMC´s step motor controller – JXC#1 Series
On a positive note, an incremental encoder can rotate almost endlessly in the same direction, making it ideal for index tables that must turn continuously. They are also a good match for miniature drives where space is at a premium.
Driving home the benefits
The notable advantage of an absolute encoder is that it mitigates the need to return to a datum or home position in the event of power loss. In such instances, the controller knows the location of the electric actuator and can continue positioning immediately. No time is wasted performing referencing or homing routines to restart each axis. The axes simply resume from the point where they stopped at the time power was lost.
And there are several further benefits. For instance, you can avoid any possible collisions that would otherwise result in costly repairs and downtime. Without the need for homing, users of absolute encoders also avoid any potential errors that can occur during this process, negating the need for further referencing.
Charging towards progress
The disadvantage of a conventional absolute encoder is its battery. Stored energy depletes and battery changes require careful planning to avoid unexpected stoppages. These changes have time and cost implications. We must also consider the environmental ramifications of battery disposal.
SMC´s Just easy electric actuator, integrated controller, slider type – EQFS#H Series
A battery-less absolute encoder provides a clear way forward in the vast majority of electric actuator applications. Yes, some disadvantages are evident in a few specific use cases. For instance, there’s a limit to the number of revolutions achievable, making them unsuitable in applications where an index table must rotate indefinitely in the same direction. Battery-less encoders are also sensitive to external magnetic fields created by other machine components located nearby. This unwanted influence can affect positioning feedback. However, there’s a simple solution that involves providing a small distance of separation, assuming space allows.
Be absolutely clear
The relationship between cost and the benefits available from incremental and absolute encoders point towards the latter as the optimal solution for the majority of electric actuator applications. Sure enough, there are some minor disadvantages to absolute encoders, which include a small cost differential, slightly larger dimensions and the potential for magnetic field interference (easy to fix). However, benefits such as location memory in the event of a power outage (without the need of a battery) far outweigh the minor drawbacks.
For this or any other requirement involving electric actuation, contact our team of experienced engineering experts at SMC.
