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Pre-commutation technique in new rotary position sensor ensures accurate measurement at high rotation speeds

June 06, 2012 | Paul Buckley | 222902292
Pre-commutation technique in new rotary position sensor ensures accurate measurement at high rotation speeds ams has launched a magnetic rotary position sensor IC offering accurate angular measurement even at high rotation speeds.
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The AS5132 is a system-on-chip, combining integrated Hall elements, an analog front end and digital signal processing in a single device. The device is well suited to industrial and automotive brushless DC (BLDC) motors, where it offers designers a small, robust and easily assembled position sensing solution. The device is highly resistant to interference from stray magnetic fields.

The new device improves on previous generations of magnetic encoder by dynamically compensating for angle errors attributable to propagation delay, which are most noticeable at high speeds. Error compensation is achieved through integrated pre-commutation functions. These are easy to configure in the AS5132, and do not require external software routines to be executed, therefore unburdening host microcontrollers or electronic control units (ECUs). The configuration can be changed while the device is operating, in response to changes in operating conditions such as variations in rotation speed.

In addition, the advanced signal-processing circuitry in the AS5132 claims to give it an excellent propagation delay figure of <22 s, which means that even before pre-commutation is implemented the angle error is small.

In combination, the small propagation delay and advanced pre-commutation deliver extremely accurate angle measurement even in motors rotating at speeds up to 80,000rpm: angle measurements are accurate to within 3 (maximum). This enables BLDC motor manufacturers to achieve high and constant levels of torque even in high-speed applications over the whole dynamic range.

The AS5132 offers increased sensitivity to allow operation in a magnetic field input as weak as 20 mT. This enables the sensor to be paired with a cheap ferrite magnet.
The device simplifies the interface for easier integration and includes an external clock mode, synchronizing sensor and controller systems. The device has implemented a 3 V interface mode (in addition to a 5 V mode) eliminating the need for a level shifter.

When paired with a simple two-pole magnet, the AS5132 provides absolute angle measurement at a resolution of 8.5 bits, or 360 positions per revolution. This measurement is available as a serial output and as a pulse width modulated (PWM) signal.

An additional U,V,W output can be used for block commutation in a BLDC motor; alternatively an incremental signal (ABI) is available. In addition to the angle information, the strength of the magnetic field is represented as a 5-bit value.

The zero position can be set by software on the production line. This simplifies assembly, as the magnet does not need to be precisely aligned with a fixed starting position.

The AS5132 is suitable for contactless rotary position sensing, rotary switches (human machine interface), AC/DC motor position control and BLDC motor position control in automotive applications such as pump BLDC motors (stop-start system) and steering column BLDC motors. The AS5132 is also particularly well suited to position sensing in double-clutch BLDC motors.

Availability and Pricing

The AS5132 rotary position sensor is in volume production now. The price for the device is $3.27 in 1000-piece quantities.

More information about the AS5132 rotary position sensor at
www.ams.com/Rotary-Encoders/AS5132

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