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Angle Encoders: How To Measure Angle Using Encoders(2)

May 10, 2017

Incremental vs Absolute Angular Encoders

Incremental angle encoders can only register displacement from some arbitrarily defined home position that is established at startup. This home position, usually referred to as an index, creates only one pulse throughout one complete rotation of the code disc. This enables the device processing the encoder signal to track the number of full turns of the encoder disc from the home position. However, the index pulse is not unique so the number of full rotations counted is often lost if the device is powered down and it must be rehomed on startup.

An absolute angle encoder outputs a unique digital word for each position of the code disc. Multi-turn absolute angle encoders also track the total number of full revolutions with a digital word. Thus, there is no need for homing and the information is not lost when the device is powered down. The moment an absolute angle encoder is turned on, it can report exactly where it is by reading the digital word of that particular angle.

Achieving Higher Resolution with Quadrature Angle Encoders

Interpolating the signal of a quadrature angle encoder can also deliver a significant performance boost, depending on the implementation. If the processor of the receiving device treats the leading edge and trailing edge of both the A pulse and the B pulse as individual pulses, it quadruples the effective resolution of the angle encoder. For example, this interpolation technique could enable an industry standard 1024 PPR quadrature encoder to resolve angles of just 0.088 degrees.

However, it should be noted that any errors will be multiplied by along with the resolution. These can degrade performance, which means that an attempt to increase system accuracy by creating higher resolutions through interpolation might have the unintended result of amplifying an error  and degrading system performance. This is an example of how higher resolution does not improve accuracy. Learn more about encoder accuracy vs encoder resolution here.

Quadrature angle encoders also allow the direction of that rotation to be monitored. In a quadrature encoder, the disc has two separate sets of markings (channels) that are offset so that the signal for the B channel is 90 degrees out of phase (in quadrature) with that of the A channel. This enables the receiving device to determine the direction of rotation.

Customer Question: How can I measure angle to 0.1 degree?

Answer: Assuming you are measuring the rotating shaft directly, start by determining the required resolution. If you want to measure to 0.1 degree, first, determine whether that number applies to resolution or accuracy. If resolution, you need an encoder with 10 pulses per degree. Multiplying that by 360 degrees, you get a required resolution of 3600 PPR.

Thus, a 3600PPR angle encoder can be used. Alternatively, decoding the rising and falling edges of both A and B channels of a quadrature encoder, you can quadruple the resolution of a 900PPR angle encoder to achieve the same effective resolution, although any error will also be amplified.

Unfortunately, this is only the theoretical accuracy. Evaluate any possible errors in the system, including alignment, mechanical linkages, play in the system, etc. If the system cannot physically position to 0.1 degree, you may need to lower your resolution specification.