In our previous post of this series, we learned that the selection of a feedback device is critical for precise motion applications, and that where it's located is important as well. Today's post covers some additional information regarding the difference between absolute and incremental feedback and why should I care, as well as a few other considerations.
A train is zipping by you (you didn't see the lead car) - and you are counting cars as they go by - 1, 2, 3. You know how many cars may have passed by, but you probably do not know what position each car was in. You only know incrementally, that 12 cars passed by. Now, you stop and are looking at the cars go by, you can see each car number - you see that cars 12 through 24 passed by. You know if this train had just stopped at the crossing, you are now at car 24 - absolutely. If the train stopped in the prior example, you only know that 12 cars passed, so you are not absolutely sure of what position car this is in front of you.
Feedback sensors report either absolute or relative incremental position. A single turn absolute device has the capability to report its position within one electrical cycle when the system is powered up. By contrast, the incremental position sensor typically provides output pulses for each increment of motion, but without reference to a particular location within the device's range of motion.
This data, in combination with periodic marker pulses, a machine home switch and a counter, allow load position to be known. However, should the electronic feedback circuits lose power the system loses track of its location. For some critical applications using incremental encoders, the controller can connect to an uninterruptible power supply to maintain position information. Alternatively, a multi-turn absolute encoder will provide the same function without the need to keep power applied.
Environment is another consideration in selecting the correct feedback device. Applications in high vibration or other rugged environments need a device that can tolerate this environment Thinking about the different technologies available, some devices are more suited (like a resolver) to a rugged environment, while others (glass encoder scale) might not. Some devices which harbor electronics onboard the device are not accustomed to extreme heat environments - Just something else to consider when choosing a device.
Wait - one more thing to consider before we dive into the different devices - Geometry. Yes, that class you had back in grade school is front and center now! Since many applications have either a rotary element, a linear element or both, this can play a role in the selection of a device as well. There exist feedback devices that are designed specifically for each of these cases, and are offered in a variety of mounting configurations to meet most any needs. In the end though, the basic principle of feedback device operation will apply to all these geometrical options.
That's a lot to consider! The good news for the machine builder is that most motion control manufactures take all of this into account as part of a complete system solution. Oh, and one more thing about the feedback devices today - technologies of many feedback devices today allow the motor manufacturer to "smarten" up the device by storing pertinent motor data within the feedback device itself. This in turn allows the motor drive combination to become extremely simple to set up.
In future posts - we'll start checking into each of these devices: Hall-effect, Resolver, rotary encoder, linear encoder, and sine encoder.