What is a Bearingless Encoder?

What is a Bearingless Encoder

An encoder is a device that converts an input count into a corresponding digital output. The input counts are not the physical distance but the number of movements. These counts are then converted to pulses that can be counted using a counter or scale.

A bearingless encoder is an encoder that does not use bearings as part of its design and feedback system. A bearingless encoder may have magnetic contact, mechanical contact, or optical feedback systems for determining position and speed of rotation. In a bearingless encoder, the output is the physical distance travelled. A standard encoder may be compared to a computer mouse, where the cursor movement only indicates the number of clicks made by an operator.

Incremental bearingless encoders measure and record changes in the position of a moving element.

Bearingless encoders are more compact and accurate than traditional bearing-supported encoders. The lack of bearings allows for smaller packaging, better heat dissipation, and lighter forces on output shafts.

In addition, these types of encoders have developed useful applications such as machine tool sensors (e.g., lathes), optical encoders, robotics, plotting devices for CNC machines, and other similar machines.

A change in position of the encoder’s measuring element generates an electrical pulse proportional to the change in status. The encoder’s output is usually a square wave, sinusoid (sine wave), or a step function resulting from a digital encoder with binary output.

There are two main types of bearingless encoders: rack and pinion or absolute (which detect rotation) and incremental (which measure position only).

A common type is the incremental magnetic encoder with an electronic sensor mounted on a rotating shaft, with magnets mounted inside the motor housing. As it spins, the motor moves past these magnets, which then magnetize metal portions within the motor housing.

The metal within the motor housing is part of a simple electromagnet, which is constructed with an additional winding that can be used to convert the magnetic pulse information into pulse-width modulated (PWM) output pulses with a specific period (a precise distance measured).

Using a magneto resistive sensor minimizes the system’s effects from external magnetic interference.

Another development towards high accuracy has been the addition of laser-diode sensing. This yields precision to one micrometer and, when reversed, engineered by industrial spies, have resulted in hard drives exceeding two terabytes (2 TB). The use of lasers means that these devices are much more immune to external magnetic interference than their predecessors.

Incremental optical encoders use a standard 2D linear sensor mounted to a shaft’s end. This shaft rotates on an axis perpendicular to the sensor’s, resulting in the sensor’s rotation relative to the encoder’s mounting point. A small group of plastic beam splitters mounted around the shaft cause multiple reflections between the two components as they rotate. When these reflections are modulated with an optical pulse, a high-speed camera (with a suitable image intensifier tube, but not necessarily) located adjacent to the rotating shaft detects this change.

The beam splitters can be manufactured using a micro-electromechanical system (MEMS) and a regular laser, though the former is less expensive than the latter. Components for this system can be manufactured using standard printed circuit board (PCB) techniques and assembled on the same equipment used to make the optical sensor.

Most incremental encoders use a photocell or an optical fiber that terminates in an LED. The fiber optic encoder is mainly used in robotics applications and allows pressure sensing, velocity sensing, and position detection at extremely low-price points.

The photocell is also commonly used as a subsystem in linear position sensor systems.

One area where incremental optical encoders have not been widely used involves the measurement of angular velocity (speed/direction of rotation). This can be accomplished using an optical rotary encoder. The optical rotary encoder uses a resonant ring-resonator to create a periodic signal that is then read by an opposing resonator. The standard rate for reading this signal is 200 Hz, but customized rates are possible to suit the application requirements.

What are the benefits of a Bearingless Encoder?

Bearingless encoders provide high accuracy, reliable operation, and long life, typically around 5 million counts per revolution. For example, a 300-rpm motor with a 24-bit resolution can move at 20 inches per second (10 cm/sec) and still achieve greater than 1,000 counts per revolution, meaning it’s possible to attain sub-angstrom accuracy.

Although these devices are fast—offerings up to 30,000 rpm translate into 30 million counts per revolution—they are also precise: the smallest increments of movement that an encoder can detect are determined by its resolution. For example, a 12-bit device will detect a distance of 0.0078-inch (0.2 mm) increments of space.

A bearingless encoder is an electromechanical device utilizing an optical, capacitive, or magnetic scheme for measuring the rotational position of a shaft or axle. When used with a motor or other rotating device, it provides feedback on the speed, work, and direction of rotation in the form of a digital signal that is proportional to the shaft’s orientation at any given time. An incremental encoder may be considered a type of transducer and an actuator since it produces both input and output.

Bearingless encoders can be either absolute or incremental. The two options are as follows:

The advantage of an absolute type is that it is accurate and very robust in operation. The disadvantage is that it requires more space on the mounting plate than the incremental type because the shaft has to be held still while the encoder counts the rotational distance. The equipment manufacturer must bear a high installation cost since it involves a special drive mechanism and some form of sensor alignment. The incremental type measures the shaft position in proportion to its rotation.

The disadvantages of an incremental type are that it is more expensive, consumes more power than an absolute type, and is less robust in operation. It can measure faster rotational speeds than an absolute encoder due to having smaller magnet sections. The advantage of this type of encoder is that a much smaller mounting plate space is needed since it does not require the shaft to be held still for measurement. This means that the equipment manufacturer can make an installation cost saving since only a basic drive mechanism is involved, and no form of sensor alignment or adjustment needs to take place.

When to use a Bearingless Encoder?

The bearingless encoder can be used to measure the shaft position, speed rotation, and direction. Many industrial machines are used in the production lines, such as food or textile processing equipment or manufacturing machinery. They are also used in scientific instruments where precision positioning is needed. Bearingless encoders can be applied to many applications, including machine tooling, robotics, and stamping lines.

A Transmission-type Encoder consists of a sensor that converts an angular position into electrical signals for transmission to a motor controller for conversion back into motion.

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