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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 

Oversized motors and fixed speed drive systems generate a huge bulk of the industry's wasted electrical energy. Thus energy efficient motion control system should adapt in the future to the actual load demand from the application. Brushless DC (BLDC) motors meet this requirement through electronic commutation and variable speed control. Commutating the motor pole winding at the optimal rotor position is essential for reducing electrical losses when managing variable speed and load situations. The following article discusses different Hall sensor arrangement and technology trends in integration.

Reliable feedback of the rotor position is important for the performance of the total motion control system. It allows a precise commutation of the stator windings and minimizes electrical losses in the motor. Typically the 120? in phase shifted UVW signals are used to activate commutation in the BLDC motor driver. Different options are available today to generate the UVW signals.

This could be using Hall sensors or switches which are built into the windings or mounted on a small PCB; calculation by software with data based on the back-EMF from the stator winding; attaching an optical or magnetic encoder to the motor axis; or the integration of advanced single-chip optical or magnetic encoder ICs into the motor housing.

Hall sensors or switches are widely used in BLDC motors due to their low component cost. The sensorless approach requires effective algorithm to calculate UVW from the measured back-EMF. Also a fast microprocessor or DSP is needed to reduce execution time and minimize the additional latency time introduced. The limitation with sensorless UVW generation can be seen on fast load changes, at low speed and out of sync operation. Sensing the absolute rotor position in hardware is regarded as the most reliable option. Attaching an optical or magnetic encoder unit to the BLDC motor is advantageous when very high precision dynamic positioning is required and if the application is not cost sensitive.

Figure 1: Options of BLDC motor position sensing for commutation.

Hall sensor for commutation
Using three discrete Hall sensors/switches in a BLDC motor generates UVW signals based on the sensor mounting position, either in the stator windings, or assembled on a small PCB at 0?, 120? and 240?locations opposite the rotors permanent magnets. In some cases a magnetic pole ring attached to the axis can be used. Figure 1 shows on the left side the mechanical position of the three Hall sensors/switches and the resulting UVW signals generated. The position accuracy of the UVW signals in relation to the actual rotor position depends on the mounting tolerances and matching of the Hall sensors/switches sensitivity and stability. Since the magnetic field will vary quite a lot over temperature, rotor speed and operating life time (permanent magnet ageing) a position error can add-up easily to ±3? or more.

Another approach uses four integrated Hall sensors and signal conditioning to generate a sine/cosine signal, where the 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.

Figure 2: Generating UVW and ABZ from Sine/Cosine.

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 (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.

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