As automation is becoming miniaturized, we see a demand to produce high-performance encoders to fit on, or in, smaller and smaller motors. Today our smallest “standard” encoder uses a 17.5 mm OD target wheel and is typically mounted on 20 mm stepper or servo motors.

 
Small:  Timken M9 encoder used a 17.5 mm OD target wheel.  Example is a 1,000 CPR (4,000 edges per revolution) A, B, Z off-axis magnetic encoder.

The off-axis design of the Timken M9 encoder allows customers to mount the encoder on the front or back of the motor and to have the shaft pass through the encoder to connect to the end user’s application. The M9 platform offers up to 1,000 CPR (4,000 quadrature signals per revolution) and a once per revolution index pulse. The M9 is also available with a differential line driver output to operate in harsh industrial environments.  

 
Medium:  Ring kit designed for a wide range of shaft sizes.  Notice the large diameter of the target magnet.

When your motor shaft is larger or you need higher resolutions, choose a larger encoder.  When a magnetic encoder gets larger, the sensor circuit does not need to change.  At Timken we typically use MPS160 or MPS512 sensor chips and add optional line drivers, connectors and application specific circuitry. The diameter of the target wheel changes to allow for more magnetic pole pairs and to extend outside the motor shaft diameter. The larger the sensing diameter the more resolution is possible, as well as more accuracy.

 

Large:  The sensor ring is mounted in a drive motor on a mining truck. Notice the size of the 17.5 mm M9 target fits well within the mining truck target wheel. Both applications use the same MPS160 sensor chip with slightly different settings.
 
For more information on what you need to know about magnetic encoders for your large or small motor, reach me at 603.358.4760 or send me at email at [email protected]. Please join our LinkedIn Group to continue the discussion.

A. John Santos, Chief Engineer for Sensors

Let’s say your motor needs a rotary encoder to provide feedback to a controller. Which would you choose: a magnetic or an optical encoder?

In the past, an optical encoder might have been your only choice. Today, you have a second option. Magnetic encoders – once reserved for high-end process-industry applications – are now cost effective for virtually all encoder applications.

Magnetic encoders have benefited from overall advancements in integrated circuit technology. There is a wide range of Hall effect-based high-resolution encoder chips, modular encoders and kit encoders on the market.

Magnetic Encoder Advancements Outpace Optical Technology

The decision to use optical or magnetic technology for your next sensor will likely depend on the environmental and mechanical characteristics of your application. Magnetic encoder technology advancement has been outpacing optical technology for years. This has led to significant improvements in performance and pricing. While optics used to be the only choice for resolutions greater than 1,000 PPR, there are now 1.5 in. modular magnetic encoders and encoder kits available with up to 8,192 PPR.

Optical Encoders: How They Work

Traditional optical encoders have a light source, two or three light sensors, and a glass, metal or plastic code wheel placed between the light source and the sensors. These are the only functional components of an optical encoder. The other components you might find are used to keep contamination like dirt, dust and condensation out of the optical path or used to precisely position the optical disk in the working range between the light source and light sensors. A typical recommended tolerance range for an optical disk is +/- .003 in.

Traditional optical encoders have the optical components packaged in a sealed can with the code wheel precisely positioned using two ball bearings.  The packaged encoder is mounted using a shaft coupling or, in the case of a hollow shaft encoder, using a flexible tether. The housing, bearings and couplings are often the largest and most expensive parts of the assembly.

Magnetic Encoders: How They Work

Modular magnetic encoders can offer the same standard resolutions as optical encoders with greatly improved resistance to environmental conditions such as dirt, dust and humidity. Magnetic encoders consist of a system-on-a-chip encoder chip on a PCB and magnetic target wheel that is set to run with an air gap of between .020 in. and .060 in. above the PCB.

Magnetic encoders typically have much more lax assembly and shaft end play requirements. The ability of magnetic encoders to perform well over a wide range of assembly tolerances have eliminated the need for bearings in most magnetic encoders. This in turn has resulted in eliminating failure modes, lowering component cost and reduced overall axial length may applications.

On Axis
As the names suggest, the on axis design has the sensor located directly over the center line of the shaft.

On Axis encoders

• Sensor and magnet must be aligned with the center line of the rotating shaft.
• Button-shaped rare earth magnet mounted directly on the end the shaft.
• Can offer absolute position information and/or quadrature signals

Off Axis
The off axis design has the sensor sitting off to the side of the shaft with the magnetic target in the form of a ring.

Off Axis encoders   (Timken M15 shown)

• Do not block access to the end of the shaft.
• Shorter installed height; more tolerant of sensor and magnet position at installation.
• Higher accuracy and less latency than on axis designs.
• One chip works with a wide range of pole sizes and diameters.

For a more in-depth comparison see my paper on the same subject.

The charts below offer additional guidelines that may make your choice clearer.

If you still have questions and would like to discuss whether an off axis or an on axis encoder is the right choice for your application, call me at 603.358.4760 or send me at email at [email protected].

Rotary encoders provide position feedback in all sorts of systems. The demand for encoders continues to grow as customers demand higher efficiency and smarter motion control.

In the beginning
In the old days, encoders were baseball-sized metal cans containing a glass disk with opaque metal lines. Other parts included a set of bearings and a shaft used to mount the glass disk. One side of the disk had an incandescent or LED light source and lens assembly; the other reverse had a metal grating mounted in front of two or three light sensors.  More...