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Optical encoder integration for brushless DC motor

Posted: 30 Dec 2013     Print Version  Bookmark and Share

Keywords:Brushless DC  BLDC  motors  motion control  Hall sensor 

With a sine/cosine to UVW interpolation unit the commutation signals can be generated for two, four or multiple pole BLDC motor types. In this case each commutation signal is shifted by 60? in phase. It can be used to control directly the BLDC-driver unit for block commutation. It can also be used by the motor controller to generate a sine wave commutation. An integrated single chip magnetic encoder has usually multiple output options to be used by the motor controller or a superior motion controller. But advances go far behind just the resolution.

Advances through single-chip encoder integration
The advances in single-chip encoder integration have taken them to a complete "system on-chip" with multiple output options for the BLDC motor. Figure 3 shows the BLDC motor feedback options for the iC-MH8 as one example. On top of the UVW signals other output options are provided, such as absolute position via the SSI/BiSS interface, ABZ incremental and analogue sine/cosine signals.

Figure 3: Motor control with absolute magnetic encoder with outputs options.

The chip includes a Hall array, analogue signal conditioning, digital sine/cosine interpolation, error monitoring, automatic gain controls, multiple encoder output formats, UVW motor commutation outputs, digital configuration, line driver capability, and in-system programmability.

The signals from the Hall bridge are conditioned and amplified by a PGA with auto gain control to compensate for different operation condition, like temperature, supply voltage or magnetic field changes due to temperature or ageing. The on-chip sine/cosine signals are amplified to 1 Vpp and provided through a differential analogue output driver for external monitoring or independent interpolation. They also drive the 12-bit real-time Sine-to-Digital converter/interpolator with a very low latency time of less than 1µs.

12-bit provides a resolution of better the 0.1?. An absolute position can be readout through the serial SSI (Synchronous Serial Interface) or BiSS-Interface (Bi-directional Synchronous Serial Interface) by the motion controller. The open standard SSI/BiSS provides a high speed serial interface also for configuration at the production line. If needed, integrated RS422 line driver support also longer cable length to the motor or motion controller. The ABZ-signals are updated at a 2MHz frequency and have a latency time of less the 1µs. The zero position can be programmed in 256 steps (1.4?) for the incremental and 192 steps (1.8°) for the UVW interface.

Important is also the ability to setup and adjust the analogue signal conditioning. This allows for a higher quality encoder output signal. Selecting the BLDC motor commutation pole setting enables the device to be used with various BLDC motor types. The adjustable settings reside in the onboard RAM of the encoder chip and can be programmed into the onboard nonvolatile PROM read-on at power-up.

Figure 4: Optical single-chip motor encoder ICs with UVW commutation.

Optical integration also possible
Magnetic encoder ICs can be better for very harsh, dusty and rigorous environments. However optical single-chip encoder ICs with commutation outputs have become available through optical system integration as well. The performance can be higher, but comparisons indicate more and more a head on head race between the two technologies. Figure 4 shows two single chip optical encoders with incremental and UVW outputs. Here the resolution is defined by the code disc and uses three optical sensors for the UVW generation. The number of pole pairs of the motor is defined the code wheel design. An array of four photodiodes can provide up to 20,000 counts per revolution at a code disc diameter of 33.2 mm, for instance. Special packaging such as optoQFN is required for this optical solution.

Today's mixed signal integration capabilities can provide single-chip encoder ICs offering reliable, highly flexible and configurable magnetic encoder feedback options with 12 bit resolution. This can compete with traditional Hall sensors/ switches at the system level with higher performance integrated into the motor housing. On optical encoder ICs with integrated UVW-output options follow also the trend toward a single chip solution. These trends support the increasing performance required to improve energy efficiency in electronically commutating motors through best in class motor feedback solutions.

About the author
David Lin is responsible for magnetic sensor products and applications at iC-Haus.

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