This edited article come from encoder manufacturer Leine & Linde
It’s sometimes the small modules such as rotary encoders in production equipment that make it necessary to shutdown a plant for their replacement in case of faults. This results in enormous costs, often way out of proportion to their size. In the windpower industry, revenue loss from downtime can average $50,000/week, part replacement can vary from $20,000 to $300,000, and if a crane must be mobilized, expect a $250,000 charge.
Such charges can be avoided by using rotary encoders capable of self diagnosis that indicates internal problems so maintenance can be planned as early as possible.
Rotary encoders are key components of modern machinery and plant controls, so it makes sense to equip them with a diagnostic system that continuously monitors the internal functions of the rotary encoder and provide a basis for initiating maintenance measures in good time. Rotary encoders used in production plants are often subjected to high stresses from shock and vibrations as well as high temperatures. The following typical component specifications reflect these stresses: Vibration-resistance up to 100 m/s2, shock resistance up to 1,000 m/s2, axial load 100N and radial loads to 300N. Under these general conditions a positioning accuracy of 0.1 mm must still be ensured.
Why rotary encoders fail
It is no wonder that rotary encoders break down in spite of their apparent durability. Encoders can fail for several reasons. For instance:
- Worn-out ball bearings due to poor installation: The connection of the rotary encoder to the motor shaft is made either with a coupling (typical for shaft encoders) or by plugging onto the shaft (typical for hollow shaft encoders). A torque support keeps the encoder from spinning. If the specified tolerances are not complied with, imbalance results, which causes premature wear to the ball bearings. The result: the increment-disc wobbles. Individual areas lose contrast, which in turn looses several pulses. This means the rotary encoder still functions, but the entire drive unit becomes irregular as the frequency inverter attempts to compensate for these fluctuations.
- A “loose contact” produces an imbalanced drive unit which puts excess strain on soldered joints and (terminal) contacts. Hence, bad contacting causes sporadic faults.
- Dirt in the rotary encoder clinging to the increment disc: The encoder then detects two increment lines as one and produces one pulse too few. This can happen with equipment in dusty regions common for windpower equipment, and especially when the nacelle cover is opened. In such cases it is advisable to use encoder versions with external plug connections.
- Moisture in the connector or in the housing: Cables too thin let moisture penetrate into the rotary encoder through the cable gland and cause sporadic malfunctions. This is especially possible in offshore windpower applications.
- Overheating: Rotary encoders are often installed behind a fan so the exhaust air from the motor passes over the encoder. If a motor runs hot, as it could as bearing fails, the hot exhaust air from the motor can cause the failure of the rotary encoder.
A fault with many installations is also the fact that monitoring the rotary encoder is only implemented in the frequency inverter. However, between the inverter and rotary encoder there are enough cables and terminals to cause problems. For example, a crushed cable can lead to a signal interruption, which is mistakenly interpreted as an encoder fault by the inverter. Then an O&M crew would swap out a component without fixing the problem. A lot of these problems can be avoided if the encoder could tell why it is malfunctioning.
One such encoder from the author’s company has a built in early warning system called Advanced Diagnostic System (ADS). This automatic self-diagnosis works like this:
The rotary encoder internally monitors the completeness of the pulses and the correct pulse sequence. Even a single counter difference from the programmed division is registered by the system and reported via a potential-free switching output. This can, for example, be evaluated and displayed by an overriding system control system. For this an additional wire is necessary. In parallel to the switching output, the fault is indicated by a flashing LED on the rear of the housing. In case of upgrading, this allows using ADS without additional wiring. Often, rotary encoders are installed in visible position, so a sporadic visual check by the user is sufficient. The flashing LED can be easily seen at a distance of 10 m.
As part of ADS, the manufacturer stores the time of the fault and the corresponding error code in the rotary encoder so users have the opportunity to read out and analyze the error code via an RS-232 interface (once the encoder has been removed). For statistical purposes, the number of operating hours, speed of rotation and current temperature are also measured in the encoder. The highest and lowest temperature is also stored. In addition, there is a differentiation between individually occurring errors and continuous problems.
Leine & Linde
The Sweden-based encoder manufacturer distributes in the U.S. through HEIDENHAIN Corp., Schaumburg, Illinois.
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