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When subject to rough automotive operating conditions, digital motor control (DMC) with position sensing and speed acquisition calls for inexpensive and highly integrated solutions. In order to meet the specific requirements of active motor feedback, iC-Haus has developed a system-on-chip (SoC) with a resolution of 12 bits.
The objective of the iC-MH design was to integrate a complete incremental and absolute encoder, an interface for direct commutation signals (for the three motor phases—U,V, and W), and RS422 line drivers onto a single silicon chip. This architecture would ensure that an entire motor feedback unit could be built into small motors using just one device measuring 5x5 mm and a diametrically magnetized permanent magnet with a diameter of 3-6 mm.
Requirements for active motor feedback
The energy-saving digital control of an electric motor requires a precise logging of rotor position to calculate the angle, speed, and acceleration. This can be determined either directly—without a sensor—by measuring the current and voltage, or by using a motor feedback device with an incremental encoder, resolver, or Hall sensors in the motor. Often, however, the cost of active motor feedback is higher than that of the motor itself—especially with small motors. Thus, for many high-volume applications active feedback is not feasible as low costs are a priority.
The chief technical demands made of a sensor in a feedback loop are accuracy and a certain robustness against negative "industrial" effects, such as dust, dirt, or "disturbance" fields, and high temperatures. In critical applications error protection is also a decisive factor. The systems engineer is thus challenged when it comes to effecting an optimum balance between the cost of the feedback loop and a necessarily high degree of precision.
Hall sensors have long been in successful use in automotive applications. For a while now integrated Hall encoders have proved their worth in position sensing applications where cost is an issue. By effecting a complete CMOS system integration (SoC) of the motor feedback loop these basic demands have been met in iC-MH, with a resolution of 12 bits.
Single SoC functionality
In addition to the Hall sensor array and a programmable preamplifier with amplifier power control (APC) and automatic offset compensation, five other functions have been integrated on the iC-MH, resulting in a one-chip encoder. These functions are:
Absolute encoder with an interpolator
Commutation signal generator
Serial interface
Programming for calibration
Error monitor
RS422 output drivers
The figure below shows these functions as a block diagram with the interfaces which are accessible externally. The incremental encoder interface (upper right) consists of A and B quadrature signals and a programmable index pulse (Z output). The 8 MHz step rate permits motor revolutions of 120,000/min at the highest resolution.
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The integrated interpolator is freely adjustable and can output up to 4,096 angle steps per revolution. Even at the highest number of revolutions the absolute value of the angle position can be read out with a resolution of 12 bits. An adjustable angle hysteresis prevents loss of pulses when the direction of rotation is reversed. The position of the index pulse can be set in 256 steps (1.4° per step) using an 8-bit register via the serial interface (lower left).
Another important functional block is the commutation control unit (at right). This section can be used in parallel to the incremental interface for the commutation of two- and four-pole motors and generates the three phase signals U, V, and W. The rising edge of the U signal marks the zero position and can also be programmed in 192 steps with a step angle of 1.9°.
To effect fast communication with the DMC controller an SSI (synchronous serial interface) commonly used with encoders has been integrated onto the device as one of its basic functions. As the SSI is only unidirectional and intended for simple applications, an advanced serial interface has also been included, which is suitable for systems with data rates up to 10 MHz.
This advanced interface serves for the readout of the absolute angle position and for the programming of the encoder system. The programming of the 16-byte one-time programmable ROM (ZROM) is intended to correct mounting tolerances and is used to calibrate the device (e.g. the offset) and control the parameters and mode of operation. Programming takes place after the device has been mounted in order to achieve maximum accuracy.
To keep system costs and space to a minimum the RS422 line drivers are on chip. This is the case with the UVW and ABZ signals. If the incremental interface is not required, the UVW commutation signals can also be transmitted differentially to the power electronics. The type (push-pull or tristate) and driver current of the RS422 driver outputs can be configured so that they can be adjusted to suit the line impedance.
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