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EETE NOV 2013

OPTOELECTRONICS angular position within a 360˚ turn is continuously available. Figure 1 shows this Hall arrangement on the right side. A small permanent magnet 4-6mm in diameter is attached to the rotor axis and generates a rotating field which is picked-up by the integrated Hall bridge. The sensor arrangement allows the generation of a differential sine/cosine signal which is insensitive to common mode magnetic fields. The sine/cosine signals can then be converted by a sine-to- digital converter to an absolute position. This interpolation is done by calculating the arctangent of the sine value divided by the cosine value. It delivers an absolute position of the rotor with a configurable resolution of 6-12 bit. Today’s advances in mixed-signal integration allow the Hall array plus all sine/cosine signal conditioning and interpolation for the absolute position to be on one encoder IC. Instead of the three discrete Hall sensor/switches, a single 5x5mm package can be assembled on the same PCB (see figure 1). From the absolute position also incremental ABZ signals can be generated to monitor fast position changes with a very low latency. Figure 2 shows the up/down coded AB-signals for incremental operation. When the direction of the motor is reversing the ABsignals shift its phase. The Z-signal marks the zero position of the rotor and allows in a simple manner to count from the ABZsignals the absolute position in the motor control or motion control system. 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. Fig. 2: Generating UVW and ABZ from Sine/Cosine. Fig. 3: Motor control with absolute magnetic encoder with outputs options. 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. 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. 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. Fig. 4: Optical single-chip motor encoder ICs with UVW commutation. 32 Electronic Engineering Times Europe November 2013 www.electronics-eetimes.com


EETE NOV 2013
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