A Hollow Shaft Encoder—Easy Way to Accurate Motion Control
An encoder is a hollow-shaft or shafted measuring instrument capable of providing miscellaneous response by measuring parameter values and converting them into electrical impulses. Encoded parameters are diverse—velocity, motion orientation, travel, revolution count, angular position, depending on the specifics of an application system.
When integrated into a bottle-filling line, the devices enable the machines to always know the exact location of a bottle. In elevators, the sensors signal the control equipment when a car ascends or descends to the desired floor and its doors are aligned with the flooring. When linked to an actuator, a hollow shaft encoder enables controlling how the actuator shifts from one point to another, varying revolution rates.
Shafted vs. hollow shaft encoder
The type of the device you use—shafted or a with a hollow shaft—dictates how you should proceed with its integration into your application. Inside a shaft encoder, there is a rod that enables attaching it to an engine. Matching the rod width and length to motor dimensions is of primary importance in these installations. Otherwise, misalignment can lead to performance deterioration.
Another issue is accommodating additional fittings, such as a flex couple and an external mounting flange. Due to the extra fasteners, shaft encoders are typically larger in size than configurations with a hollow shaft.
A hollow shaft encoder is linked to a drive shaft with an elastic tether. The mounting arrangement allows for flexibility in choosing mounting arrangements, while doing away with the need to size the feedback hardware to actuator dimensions.
Such installations are easier to implement, while also being less sensitive to shock loads and vibrations causing misalignment issues. As no external fittings are required, the overall dimensions of hollow-shaft assemblies are smaller.
Relative vs. absolute
Both shafted and hollow shaft encoders output either incremental or absolute values. An incremental mechanism is the preferred choice for induction engines with alternating current supply. Absolute sensing is mostly applied in combination with brushless direct-current actuators.
Incremental response is a series of pulses generated in one or more streams. The number of pulses is equal to that of encoder steps per turn referenced to a starting point. Based on the frequency of pulses, it is also possible to determine the rotational speed.
Whereas a single stream of pulses enables monitoring speed and relative position, it gives you no information about motion direction. To solve the problem, manufacturers introduce a second stream with a 90-degree offset relative to the first one.
To understand the direction of rotation, you have to look at which of the pulse stream takes the lead. As illustrated below, if channel A is ahead, an engine moves is in the clockwise direction, whereas channel B leading indicates an actuator rotating anticlockwise. Sometimes, an incremental feedback mechanism has a third track producing so-called index or zero impulses. The third track allows for setting a precise reference to count pulses.
Though featuring a simple design and low price, incremental encoders have their own drawbacks. With this type, you will only know the extent to which an engine has moved relative to a previous point, but not its actual angular location at a given moment. The detectors are sensitive to vibration. Because of frequency limits, their resolution is likely to deteriorate as a drive approaches the velocity maximum. After a power failure, you need to restore referencing against the same starting point, e.g., with a reference drive.
Absolute output gives the user a chance to obtain true angular position data in real time. To get an actual location of a motor, the user needs not to know all the previous pulses, which reduces overall latency.
A mechanism of the type comprises two marked disks—stationary and rotating. As the movable disk rotates, its markers form various configurations with the marking on the fixed disk. Each configuration corresponds to a unique bit code standing for a specific angular position of the attached engine.
Unlike incremental encoders, absolute devices are non-volatile: no risk of losing position and no referencing required after a power outage. You will get desired return right at startup, including shaft rotations during supply interruptions. Their response is compatible with a range of communication protocols, such as BiSS, SSI (grey, extended, etc.), as well as DeviceNet, Profibus, Interbus, and CAN.
Optical and magnetic sensing
Optics-based and magnetic sensing are the most popular technologies. An optical encoder relies on light to gather positioning data. It incorporates the following elements:
- A wheel fitted onto a spinning shaft with alternating opaque and transparent tracks, arranged in a concentric pattern, like grooves in a long-playing record. The more concentric tracks a disk has, the better accuracy the optical feedback device provides. The wheels are made from Mylar, etched metal, emulsion on tempered glass, as well as a chrome-glass combination.
- A diode (LEDs) emitting light rays projected to the grooves of the moving wheel. The wheel disk either blocks the light with non-transparent segments or passes it through.
- A light detector (e.g., a silicon photodiode, a photovoltaic cell) that receive a sequence of light signals. The detector functions similar to a solar cell: the more light they receive, the more intensive electron flow they produce. The output is a sine or cosine waveform.
- A signal conditioner converting the light detector output into a square wave form.
An optics-based mechanism goes out of order quickly when exposed to liquids or particulates, which is why they require extensive sealing. They are also no good for application where environment temperature varies greatly causing humidity accumulation inside its housing.
As a means to enhance precision and resolution, the encoder leverages phased-array sensing—averaging a number of output impulses to acquire a single measured value to adjust engine operation. Phased-array devices boast a greater response stability, which enables their utilization in heavy-duty environments.
A magnetic encoder relies on the same operating concept as the optical mechanism, except that it uses magnetism instead of optics. Its components are as follows:
A rotating wheel disk with a sequence of poles on its outer edge
A sensor (e.g., Hall-effect or magnetoresistive) monitoring the varying strength of the magnetic field as the wheel spins and transforming the measured values into a sine waveform.
A conditioning circuit converting the sensor response into the required form by applying interpolation and other mathematical conversion methods.
The mechanisms are considered to be more robust than the optical sensors, showing reliable operation even at high temperatures, excessive vibration, and shock. The more sensors and poles such a feedback mechanism has, the greater its resolution is.
Apart from the above technologies, it is also possible to mention laser, mechanical, as well as capacitive sensing. In mechanical feedback devices, a set of sliding contacts brushes against a metal disk with a set of concentric openings, producing current signals measured by sensors attached to each contact spot. The capacitive technology relies on capacitance measurements between two electrodes, whereas the laser technology—on laser emission.
Encoder in RDrive servos
Rozum Robotics has enhanced its RDrive servos with two rotary encoders—linked to a hollow-shaft rotor to the output shaft of the frameless drive. They are already fitted into servo housing for your benefit. The mechanisms leverage the magnetic design, which gives them sufficient robustness to survive the harsh loads in industrial environments. Relying on the well-established technology, the built-in hardware provide absolute positioning data with the resolution of 19 bit.
The feedback devices have compact dimensions and are inexpensive, which makes it possible to fit them into servomechanism without tangible impact on their dimensions or consumer price. They communicate with an integrated RDrive controller via a bidirectional synchronous serial (BiSS) interface, which enables quick recovery after supply interruptions.
Enhance your machine with accurate feedback from encoder-equipped RDrive servos!