iC-MQF offers speedy sine/cosine evaluation
July 2, 2014. For speed control or positioning, drive controllers today demand high-resolution magnetic or optical position sensors, which require special integrated circuits for sensor signal conditioning and sine/cosine-to-digital conversion.
Employing an interpolation method for optical and magnetic length gauges featuring decimal gratings, iC-MQF offers a sine resolution of up to 4,000 edges. The circuit incorporates a signal-conditioning analog front end, a vector-tracking converter for real-time conversion without latency, and fault-tolerant RS-422 driver stages, which output the incremental signals. Reverse polarity protection is also embedded and covers all cable connections.
Differential or referenced input signals from a few millivolts upwards can be connected, such as those coming from MR bridge sensors or photo sensors, for which alternatively also a low impedance can be selected.
Interpolation accuracy is obtained from analog signal corrections: offsets, amplitude mismatches, and phase errors can be compensated. After initial adjustment, a controlled current-source output maintains the measurement accuracy during operation by tracking the sensor’s excitation. For example, the LED current of an optical encoder or the supply of a magnetic sensor bridge is regulated, so that temperature and aging effects are automatically compensated for.
All of the chip's major functions are monitored and can be configured for alarm indication. Typical sensor errors, such as loss of signal due to wire breakage, short circuits, dirt, or aging, are recognized. Further operational errors, such as exceeding the permissible operating temperature, exceeding the permissible input frequency through fast movements, or a line count error are detected.
The mode of error handling is configurable; an alarm can be displayed at the error output by an LED, can set the output drivers to tristate, or can be transmitted to the error memory for later diagnosis.
The encoder signals are supplemented by a generated index signal whose position, width, and logic can be set as required. At the same time a patented signal-conditioning unit provides glitch-free quadrature signals with hysteresis, which always maintain a preset minimum transition distance. This enables counting errors to be avoided and enhances the circuit’s EMI tolerance with sensor applications.
iC-MQF is pin-compatible to the iC-MSB/iC-MQ device family and is also available as a 20-pin TSSOP, taking board space of just 6.4 x 6.4 mm. It is configured using the integrated I2C multimaster interface, either by a microcontroller or a serial EEPROM.
With a single supply of +5 V the device functions within an operating temperature range of -40 to +100°C. Using an integrated protection switch, iC-MQF also protects the connected sensor circuit against a reversed supply voltage.
A ready-to-operate demo board with a GUI (PC software) is available for sampling; a device DLL is optional to support production tools. The device is available at $9.79 in 1,000 unit volumes.
A white paper on sine/cosine interpolation is available at www.ichaus.de/wp7_whitepaper_HighPrecision_SinCos_Interpolation_en.