Using Absolute Encoders with Your Application
Every absolute rotary encoder is used to determine the speed or position of something – the difference is in how that encoder determines that movement. The “how” defines what type of encoder works in your application.
Absolute encoders work in situations where accuracy for both speed and position, fail tolerance, and interoperability matters more than system simplicity.
Accuracy Even in Complex Systems: Absolute Encoders
An absolute rotary encoder determines its position using a static reference point. The method is slightly different depending on whether the absolute rotary encoder is optical or magnetic, but the principle is the same either way. There are two discs, both with concentric rings with offset markers. One disc is fixed to the central shaft; the other moves freely. As the disc turns, the markers along the track of absolute encoders change position on the fixed disc. Each configuration along the disc of an absolute rotary encoder represents a unique binary code. Looking at the binary code within the absolute rotary encoder determines the absolute position of the object. For optical absolute encoders, the marker is an opening which lets through light. For magnetic absolute encoders, the markers are a magnetic sensor array that passes over a magnet and detects the position of the magnetic poles.
By having an integrated reference, an absolute rotary encoder is intrinsically able to deliver higher quality feedback:
- Higher overall resolution and orientation
- Better start up performance because of low homing (or initial position) time
- Accurate motion detection along multiple axes
- Multiple output protocols for better electronics integration
- Better recovery from system or power failures
Another key feature of absolute encoders is the different output options. Encoders can’t just collect feedback data; they have to send it somewhere in a language that the larger system can understand. Absolute encoders use binary coding, which is translatable into many different protocols. If you have multiple components using the same communications bus (such as multiple electronics systems on a fire truck), then it is critical that the absolute rotary encoder can communicate with the bus.
The only cost of absolute encoders is increased system complexity.
Absolute Rotary Encoder Alternatives
If overall system simplicity matters more than performance, then there are alternatives to absolute encoders.
Resolvers are electro-mechanical precursors to encoders, based on technology going back to World War II. An electrical current creates a magnetic field along a central winding. There are two windings that are perpendicular to each other. One winding is fixed in place, and the other moves as the object moves. The changes in the strength and location of the two interacting magnetic fields allow the resolver to determine the motion of the object.
The simplicity of the resolver design makes it reliable in even extreme conditions, from cold and hot temperature ranges to radiation exposure, and even mechanical interference from vibration and shock. However, the forgiving nature of resolvers for both origin and application assembly comes at the expense of their ability to work in complex application designs because it cannot produce data with enough accuracy. Unlike absolute encoders, resolvers only output analog data, which can require specialized electronics to connect with.
An incremental encoder determines relative position, looking only at the differences between measurements. The encoder engine sends out pulses in channels (called quadratures) and the offsets in these pulses indicate motion.
Incremental encoders provide excellent speed and distance feedback and, since there are few sensors involved, the systems are both simple and inexpensive. However, incremental encoders are susceptible to environmental factors like vibration (something that is mitigated as sensor technology improves), and they can lose resolution at high speeds due to output frequency limitations. They are also limited by only providing change information, so the encoder requires a reference device to calculate motion.
Absolute Situations: When You Need Absolute Encoders
The absolute rotary encoder itself understands the positioning information – it doesn’t need to rely on outside electronics to provide a baseline index for the encoder position. Absolute encoders enable applications which rely on non-linear positioning to work without additional external components.
In real life, absolute encoders allow more precision work from applications:
- Determining multi-axis orientation for CNC machines used in parts manufacturing
- Automatically determining the height of scissor beds used in hospitals
- Accurately positioning multiple stabilizers for large vehicles like cranes or aerial lifts
- Moving automatic doors or bays without a limiting switch
- Continuing robotic movement cleanly even after a power failure
Especially when compared to resolvers and incremental encoders, the obvious strength of absolute encoders is how their positioning accuracy affects the overall application performance.