The emphasis on reliability, flexibility, and high-tech applications has encoder manufacturers responding with products that are meeting the challenges.
Manufacturers are always looking for ways of increasing the reliability of their processes. In systems where motion control is central, encoders play an important role and are often times the weak link in a system malfunction, particularly in production environments.
Production plants use rotary encoders that are often subjected to high mechanical stresses such as shock and vibration as well as high temperatures. The cost of machinery downtime in manufacturing is often all out of proportion to the expense of a simple defective component, such as an encoder. So it makes sense to equip them with a diagnostic system that continuously monitors the device’s functioning and provides a basis for initiating maintenance measures in good time. Such a system should be able to quickly and clearly detect worn-out ball bearings due to poor installation, dust contamination, moisture issues, and overheating, to name a few.
Leine & Linde offers an integrated early warning system called ADS (or Advanced Diagnostic System) in its 800 Series of rotary encoders. Specifically, the automatic self-diagnosis system works by internally monitoring the completeness of the rotary encoder pulses and the correct pulse sequence. Even a single counter difference is registered by the system and reported. This error is relayed as a flashing LED on the rear of the encoder housing. The time of the fault and the corresponding error code are stored in the encoder so that the user has the opportunity to read out and analyze the error code through an RS-32 interface once the encoder has been removed. A report can also be displayed directly at the machine if additional wiring is added to an overriding system control.
Apart from reliability, flexibility is another important quality of motion control systems and components. With wireless applications proliferating and adding flexibility to traditional hard-wired systems and processes, it’s no surprise that some encoders have gone wireless as well. An example of that is the SwiftComm Wireless Interface for SSI absolute encoders from BEI Industrial Encoders, which eliminates the expense and maintenance of long cable runs. Using SSI communications protocol at the transmitter and receiver end, the wireless receiver plugs directly into the SSI port in the controller, just like a conventional wired system. The interface works with any SSI position device: single or mulit-turn encoders, linear scales, magnetostrictive devices, or any other SSI sensor.
The SwiftComm is a robust and secure wireless interface with the built-in reliability needed for real-time industrial control. Able to interface with any SSI absolute or incremental sensor, SwiftComm lets position and speed feedback data be sent seamlessly to control systems without expensive cabling. The SwiftComm system includes the transmitter-receiver pair, which communicates using a point-to-point frequency-hopping 2.4 GHz RF protocol.
SwiftComm simplifies encoder installations in difficult applications like cranes, rotating tables or mobile applications. Because of its flexible input/output electronics, it can interface with many different industrial sensors and control systems. Its proprietary radio protocols include a broad security code range, data encryption, handshaking, interference recovery, and error checking. The interface also comes with NEMA 4 weatherproof enclosures, panel mounting options, antenna choices and a wide-range of dc power inputs.
Quadrature to USB
USB communications have been standard computer interfaces for years, and even encoders can take advantage of the ease and flexibility of USB. For instance, the QSB quadrature to USB adapter from U.S. Digital is a low-cost USB data acquisition device that can count quadrature and index signals from an incremental encoder, provide digital I/O, perform A/D conversion or act as a stepper/motor controller. Its USB-bus powered and comes in a slim, compact package that’s easy to install and wire up. The QSB appears as a COM serial port to the PC, so any application that can read/write the COM port can be used to control the QSB as well.
Four variations allow for a range of applications. Models are available as single-ended or differential quadrature interfaces with varying numbers of 4-bit or 2-bit digital I/O and also can include step/direction stepper motor control.
On the linear side of things, encoders continue to push the barriers of position measurement. For instance, the latest Mercury II encoders from MicroE Systems boast resolutions of up to 1.2 nm and are ideal for X-Y microscope stages, nanometer and sub-micron positioning systems, fine control of optics, and piezo-motor stages.
The small sensor, measuring 8.7 mm and tape scale 6 mm wide, allows for compact system designs. The models also include programmable interpolation of up to 16,384 and there is the option of either analog or digital output from the sensor as well as a variety of connector options such as customer-specific connectors.
Ultra-high speed applications are supported by faster A-quad-B output, as well as serial word versions of up to 10 m/s with 1.2 nm resolution. The encoder’s specifications include vacuum models up to 10-8 torr, long-range accuracy up to ±1 μm, short-range accuracy up to ±20 nm, rotary resolutions up to 268M CPR, and linear resolutions from 5 µm to 1.2 nm.
The AMT203 series represents the ‘next generation’ of absolute encoders claims CUI. “With our patented technology, we offer a solution that is more reliable than an optical encoder and less susceptible to magnetic interference compared to a magnetic encoder,” said James Seiler, CUI Encoder Product Manager.
The encoder generates absolute position information using the company’s patented, capacitive code generation system coupled with a proprietary ASIC. Using TCL code, the on-board PIC 16F690 MCU operates at up to 2 MHz. Zero position may be set by SPI command or ground trigger, removing the need for mechanical alignment in the mounting process. Additionally, the encoder provides a non-magnetic index pulse and may be configured to output incremental position data after zero position is established to approach throughput of up to 10 MHz.
Typical Causes of Failure of Rotary Encoders
Here are some of the most common causes of rotary encoder break down:
Worn-out ball bearings due to poor installation:
Rotary encoders can be connected to the motor shaft by a coupling (typical for shaft encoders) or by plugging onto the shaft (typical for hollow shaft encoders), whereby the rotary encoder is prevented from rotating by means of a torque support. If the specified minimum tolerances are not met, imbalance results, which causes premature wear to the ball bearings. The result: a wobbling of the increment-disc. Individual areas lose contrast, which in turn causes the loss of several pulses. Specifically, this means that the encoder still functions, but the entire drive unit becomes irregular as the frequency inverter attempts to compensate for these fluctuations.
The loose contact:
Imbalance of the drive unit may put excessive strain on soldered joints and terminal contacts, so that bad contacting causes sporadic faults.
Soiling in the rotary encoder causes dust particles to be deposited on the increment disc:
When dust particles are present on the incremental disc, the rotary encoder detects two increment lines as one, producing one pulse too few. This can happen, for example, with assembly under difficult conditions with high levels of dust if the connection cover is opened. (In such cases, it’s advisable to use versions with external plug connections.)
Moisture in the connector or in the housing:
If, for example, cables are too thin, moisture can penetrate into the rotary encoder through the cable gland and cause sporadic malfunctions.
As the rotary encoders are often installed behind a fan, the exhaust air from the motor passes over the encoder. If a motor bearing is faulty, 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 of the rotary encoder is only implemented in the frequency inverter. However, between the inverter and the rotary encoder there are enough cables and terminals to cause problems. For example, a crushed cable can lead to an interruption of the signal, which is mistakenly interpreted as an encoder fault by the inverter and results in the user exchanging a component, without the actual fault being remedied. (courtesy of Leine & Linde.)
Leine & Linde
BEI Industrial Encoders
Filed Under: Factory automation, Encoders • linear, Encoders • optical, Encoders (rotary) + resolvers, Encoders • absolute, Motion control • motor controls