Fundamentally, all electric motors convert electrical energy into rotational kinetic energy by exploiting the laws of electromagnetism. But these physical rules have given rise to a variety of motor architectures, which offer very different performance characteristics. In this article, we take a look at the two most common motor designs: brushed and brushless motors.

Brushed Motors

The relatively simple brushed motor was the first type of electric motor to attain widespread use.

A brushed motor essentially consists of two parts: a stator and a rotor. The stator is made up of a stationary ring of permanent magnets. The rotor, which sits inside of the stator, consists of a ring of electromagnetic windings whose ends are connected to a commutator. The commutator is in constant contact with brushes that are fixed on opposing sides. Supplying direct current to the electromagnetic windings in the rotor induces a magnetic field, and it will naturally rotate until it aligns with the magnetic field of the stator.

To enable constant rotation of the rotor, the polarity of the electromagnetic windings must be alternated by cycling current through the different phases of the windings. This process is known as commutation. In brushed motors, DC current is supplied from the fixed brushes to the commutator. By turning on and off the current in a particular sequence, you can induce constantly rotating magnetic fields, which cause the rotor to spin.

Brushless Motors

Brushless motors do away with brushes; instead using electronics to commutate the motor. In brushless motors, an electronic circuit (example: Optical encoder or Hall-Effect sensors) senses the position of the rotor relative to the stator and supplies current through the three phase pairs of the stator windings, maintaining a 120° phase offset between each, to ensure smooth rotation and low torque ripple. Brushless motors are a comparatively recent motor design, made possible by the development of solid-state electronics in the 1960s.

Though the electronics involved in brushless motors are simple by todays’ standards, they represent a radical departure from the mechanical commutation systems found in brushed motors. This design change gives brushless motors a surprising number of advantages.

1. Quieter Motor Operation

Friction and electrical arcing between brushes and commutator plates in brushed motors produce substantial motor noise. In brushless motors, the job of commutation is carried out by an electronic circuit, resulting in much quieter operation.

2. Less Heat Production of the Motor

As well as producing sound, friction between the brushes and commutator plates in a brushed motor produces a significant amount of heat. This can be a serious problem in many applications. In brushless motors, the only friction that occurs is in the rotor bearings. This means heat production is much less of an issue in brushless motors.

3. Higher Motor Efficiency

This is a particularly important advantage of brushless motors. The sound and heat produced by a brushed motor essentially represent power losses from the device, taking energy away from the rotor itself – which would be used to drive the load. In brushless motors, the amounts of sound and heat produced are greatly reduced, resulting in significantly higher efficiency.

4. Longer Motor Life

The brushes in brushed motors are gradually worn away with use, since they are in constant contact with the commutator – it is only a matter of time until the brushes need to be replaced. Brushless motors do not face this problem, which drastically reduces maintenance requirements, and enables a range of applications where brush replacement would be impractical, such as in outer space satcom equipment.

5. Better Power-to-Weight Ratio of a Motor

Fewer mechanical components means brushless motors have lower mass than brushed motors. The result: brushless motors offer a better power-to-weight and torque-to-weight ratio than brushed motors.

Direct Drive Frameless Brushless Motors

All of these advantages mean that, aside from a few legacy uses, brushless motors reign supreme for present-day applications. Contact a member of the Celera Motion team to learn more about our Applimotion range of direct drive brushless motors.

Industrial metrology is a central component of high-reliability quality assurance and control (QA/QC). It plays an end-to-end role in manufacturing, where metrological instruments are used prior to part production for machine calibration and in post-production for verification. However, modern production methods and techniques increasingly push the frontier in terms of new manufacturing capabilities, which puts a strain on standard metrological systems like coordinate measuring machines (CMMs).

3D metrology is widely envisaged as a solution to these limitations, while presenting new opportunities for metrological accuracy in industrial QA/QC.

Part of the attraction to 3D metrology is based on the extremely smooth motion control used. This article will explore the basics of 3D metrology and the benefits of frameless, slotless motors for extremely smooth and accurate motion capabilities.

What is 3D Metrology?

3D metrology is scanning technologies that exceed the performance characteristics of standard CMMs. Traditional metrological instruments use contacting probes to acquire dimensional information from test parts along X-Y-Z axes, yielding a series of coordinates that are compared to established data. Contact-based 3D measurement solutions are still used, but non-contacting 3D metrology solutions based on optical technologies (lasers, structured lights, photogrammetry, etc.) are preferred for their speed, resolution, and accuracy.

Challenges in 3D Metrology

Optical 3D metrological surface inspection enables users to create a fully three-dimensional representation of the test part which is subsequently rendered as a 3D color map with clear visual indications as to whether components are out of specification. Individual problem areas that fail to meet tolerances can be easily identified, streamlining defect inspection and general QC applications. The challenge therein is ensuring that machine capabilities align with those of the optics.

Without precise motion control, it is impossible to guarantee accurate readings which may lead to increased defect rates, higher volumes of rejected materials, component failure, and product recalls.

Frameless Motors for Metrology

Frameless motors that are slotless are an ideal solution for the challenges posed by stringent positioning requirements in 3D metrology. Unlike slotted motors, slotless motors have stators whose copper coils are wound and potted in epoxy rather than being wound around iron teeth. By removing the ferrous material in the stator, cogging torque is eliminated. Cogging torque is torque ripple that is caused by the slots in a slotted motor. As the rotor spins, the inherent attraction between the iron teeth is reduced at every slot, causing inconsistent torque. This effect is most noticeable at low speeds in slotted motors. The very smooth motion and direct-drive principles offered by frameless slotless motors make them ideal candidates for metrology applications. High quality imaging is enabled and scanning data is able to be collected from the accurate, smooth motion delivered.

Interested in slotless motors? Learn more about the Agility™ Series direct drive frameless slotless motors.

Motors for 3D metrology

3D Metrology Component Solutions from Celera Motion

At Celera Motion, we have developed a series of zero-cogging direct-drive motors that are ideal for 3D metrology applications. Our AgilityTM series of zero-cogging direct drive motors is based on the slotless motor technology described above, and offers exceedingly smooth velocity control and low vibration.

Are you looking for frameless motors for 3D metrology applications? Contact a member of the Celera Motion team today for full specifications on our AgilityTM series motors.

Position sensors are very diverse, but in the world of high performance automation, the encoder is a critical element. Position sensor encoders provide feedback for both rotary and linear servo motor control, as well as position information for metrology applications.

Rotary Position Sensors

Rotary position sensors monitor movement along an axis and generate an output signal that varies depending on underlying technology and principal of operation. It is based on the same basic principle as a linear position sensor, except the device translates angular positions into useful output signals. Rotary encoders may be absolute or incremental. Read the article linked below to learn more about the working principles of rotary position sensors:

How Rotary Encoders Work

Position Sensor Technologies

Rotary position sensors can be further defined based on the underlying technologies. Several sensor types can be used for both absolute and incremental position monitoring. The following is a brief selection of some of the primary types of sensor technologies:

  • Capacitive position sensors
  • Inductive sensors
  • Magnetic position sensors
  • Optical sensors
  • Potentiometric position sensors
  • Ultrasonic sensors

Although different sensing paradigms are preferred for different applications, optical position sensors typically offer the greatest accuracy and the highest resolution.

Optical Rotary Encoders

Optical encoders use photosensitive detectors and optical gratings to precisely indicate absolute or incremental position for rotary sensors. Interferential systems eclipse both transmissive and reflective encoders in terms of accuracy, compactness, repeatability, and resolution. They generate precise position data by impinging a laser beam on a diffraction grating printed on the rotary scale. This diffracts the incoming beam, generating a high contrast interference pattern on the detection array. Optical encoders require a readhead, which is used regardless of the scale being used.

Mercury I Interferential Optical Rotary Encoders

At Celera Motion, we offer compact interferential optical encoders with the MercuryTM Series encoders. The Mercury™ Series optical encoders are position sensors that can be used for rotary position sensing. The miniature rotary sensor fits into tight spaces and has wide alignment tolerances for easy installation. Mercury Series M1000 has analog outputs that can be interpolated by the customer’s electronics for resolution up to 5 nm.

Mercury™ Series

Mercury™ M1000 Rotary Grating Specifications

Highest Accuracy Class+/- 2 arc-seconds
Diametersup to 108mm
IndexYes
MaterialSoda lime glass
Coefficient of Thermal Expansion8ppm/°C
MountingHub

Mercury™ Series M1000 Rotary Scale Specifications

 Mercury II Optical Rotary Encoders

The Mercury II™ Series can also be used for rotary applications.  This technology achieves the highest resolution in its class, up to 1.2 nm. Low cyclical error, surpassing other 20μm encoders, enables smooth velocity control, high precision, and fast positioning. Low jitter and low power consumption are important specifications when choosing a position sensor for your application, and the Mercury II delivers this with optimal performance. Broad alignment tolerances, a full +/-2 degrees, makes Mercury II the easiest and the fastest rotary position sensor to install and align – in under 30 seconds!

Mercury II™ Series

Mercury II™ 5800 Rotary Grating Specifications

Highest Accuracy Class+/- 2 arc-seconds
Diametersup to 108mm
IndexYes
MaterialSoda lime glass
Coefficient of Thermal Expansion8ppm/°C
MountingHub

Mercury II™ 5800 Rotary Scale Specifications

Mercury II™ 6000 Rotary Grating Specifications

Highest Accuracy Class+/- 2 arc-seconds
Diametersup to 120mm
IndexYes
MaterialSoda lime glass
Coefficient of Thermal Expansion8ppm/°C
MountingHub

Mercury II™ 6000 Rotary Scale Specifications

Rotary Inductive Encoders

Inductive position sensors work on the basic principles of resolvers using laminar windings to generate inductive fields. Rather than using adjacent conductors (coil windings), induction sensors use PCB traces which are excited by frequencies in the nominal range 1-10MHz. There are enormous benefits to using printed traces rather than adjacent windings, including lower cost, smaller device formats, and greater device integration into multi-layer PCBs.

At Celera Motion, we supply inductive angle encoders with the Ultra IncOderTM Series. This pre-calibrated rotary position sensor is a non-contact system trusted by aerospace, industrial, and medical device manufacturers around the world.

 

Mini Ultra IncOder

Mini Ultra IncOder™ – 58 mm

For more information on which rotary position sensor would best suit your application needs, please contact a member of the Celera Motion team today.

Encoders provide precise position feedback for both rotary and linear servo motor control, as well as accurate position information for applications. Rotary encoders are truly at the heart of high-performance automation. This article offers a detailed insight into the working principles of rotary encoders. 

Types of Rotary Encoders

Rotary encoder technologies include optical, magnetic, capacitive, and inductive. The optical encoder is preferred for applications requiring the highest resolution and accuracy. Magnetic and inductive encoders excel in harsher environments. The primary focus in this article is the rotary optical encoder.

Celera Motion has developed a suite of precision rotary encoders for cutting-edge applications. Miniature components are employed to meet the needs of the smallest electromechanical systems. The latest generation is the Aura™ Series.

How rotary encoders work

Selecting a Rotary Encoder

As mentioned, there are different types of rotary encoder technologies. The optical rotary encoder is preferred for applications requiring the highest resolution, accuracy, and repeatability. Magnetic and inductive rotary encoders excel in harsher environments, but magnetic encoders are sensitive to external magnetic fields and can suffer from accuracy drift over the specified temperature range. The remainder of this article narrows the focus to the optical rotary encoders.

Optical Rotary Encoder Theory of Operation

Rotary encoders can be incremental or absolute devices. Incremental encoders generate position change information only. An additional index/marker signal defines zero position, which is detected during a homing routine. Absolute devices provide actual physical position –  eliminating time-consuming homing, which requires movement to locate the index.

Incremental Rotary Encoders

Incremental rotary encoders employ optical scanning of a rotary scale. The scale is made of reflective and non-reflective lines of precisely equal width. Incident light from an LED creates a projection of the scale (light/dark lines) which can be detected by a photoelectric sensor. As the scale moves relative to the sensor, the lines can be counted to provide incremental position information.

Incremental rotary encoders

Incremental Rotary Scale

The number of reflective lines on the scale corresponds to the resolution of the rotary encoder. With careful design of sensor geometry, however, the variation in light intensity from line to line appears sinusoidal. This enables the generation of sinusoidal signals which can be interpolated to much higher resolution.

Left to right: A rotary scale and a linear scale

Absolute Rotary Encoders

Absolute rotary encoders also typically incorporate an incremental scale. To determine absolute position, an additional pseudo-random pattern of reflective lines is illuminated and projected to a second sensor. This essentially creates a barcode which, at startup, is used to identify a specific line on the incremental track i.e. the rotary encoder is now reading line 128 which is at exactly 43.5°.

The interface to an incremental rotary encoder is known as ABZ. A and B are square waves, phase shifted by 90°. Whether A leads B or B leads A indicates the direction of motion. The controller counts transitions on the AB signals. The Z signal is the index signal or zero reference. Absolute rotary encoders typically have a high speed synchronous serial interface such as the open standard BiSS-C.

 Installing a Modular Rotary Encoder

A modular rotary encoder solution requires that the scale disc be mounted to a hub. Ensuring the scale centre is concentric with the axis of rotation is critical to minimize angular error. Eccentricity (difference in centres of rotation) can have a significant impact on angular error. As can be seen from the following formula, the error is magnified for smaller scale discs.

Angular error = arctan (eccentricity-error/radius) degrees

For high performance applications, an eccentricity of less than 25 microns is preferred. This can be challenging and two readheads are sometimes employed to average out the error.

Eccentricity compensation using two redheads

 The sensor in the readhead must also be aligned correctly relative to the scale. Wide alignment tolerances can significantly reduce installation time, reducing production cost.

Rotary scales mounted to hubs

Aura™ Series Rotary Encoders

Celera Motion Aura™ ­­Series rotary encoders provide 18 to 22 bit resolution, corresponding to as many as 4,194,304 discrete positions per rotation. Accuracy is ± 0.01 degree. The small format encoder is also easy to install with wide alignment tolerances. This Celera Motion rotary encoder delivers advanced features including scale eccentricity compensation, eliminating the potential need for two averaging readheads.

Looking for rotary encoders?

Aura™ Series rotary encoders consume minimal power. Comprehensive connectivity includes low-latency BiSS-C as well as SSI and SPI. An incremental ABZ output with configurable resolution adds additional interfacing flexibility.

Aura™ Series Encoders

Aura™ Absolute Optical Rotary Encoder

Want to learn more? Contact a member of the Celera Motion team today to learn about our new flagship absolute rotary encoder.