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signals such as those shown in figure 3. In the following examples, two alternative scenarios are presented, and the performance of the IC detection method is demonstrated: Hard (locked) stall scenario: A spinning rotor was stopped approximately 2ms before the stall detect signal indicated a fault by going low. Note how the phase current maintains its shape even though the number of PWM cycles has increased – see figure 2. Soft (partial) stall scenario: Fig. 4: Typical soft stall behaviour. Fig. 5: Typical application circuits. In many cases the rotor stall is not locked, and the rotor vibrates as the drive currents are applied. In these cases it can be difficult to detect a stall as it appears the motor is still moving. The Allegro stepper motor drivers, however, achieve stall detection by using a differential technique. When a partial stall is applied, the fault output continuously changes state, indicating that the rotor is going in and out of locked condition – see figure 4. Magnetic sensor measures down to 0.1% of Earth’s field PNI Sensor has introduced a geomagnetic sensor that can measure fields at 0.1 percent the intensity of the Earth’s magnetic field and claimed to be 20 times more sensitive than some competitive technologies including Hall Effect sensors. The RM3100 measures signals that less than 10nT and is suitable for applications that require absolute heading or orientation data, including augmented reality and location tracking. The component includes three magneto-inductive sensor coils together with an ASIC for drive and to interpret the data. The RM3100 delivers improvements in gain, resolution and power consumption over previous PNI products with the added flexibility of both SPI and I2C digital interfaces. PNI’s proprietary technology provides more than 23 times better resolution and 27 times less noise than commonly used Hall Effect magnetic sensors. PNI Sensor Corp. www.pnicorp.com EMF generated by the fields of the magnetic poles passing over the phase windings acts against the supply voltage and reduces the rise time of the phase current, causing the PWM current control to take longer to activate. Assuming a constant step rate, this results in fewer PWM cycles for each step of the motor. This effect can be seen in figure 1. Two phases of the winding current are shown, and are offset so each step is shown overlaid. Phase B is delayed by 90 electrical degrees. This allows direct comparison of the winding current. When phase B current is rising, the motor is still running normally and back EMF acts to limit the current rise time. The stall is applied at time t = –4 ms. A visual comparison shows that phase A current rises slightly faster, causing the control IC to apply more PWM cycles to control the current. These additional cycles provide the count difference necessary to detect a stall condition. The stall is detected at time t = 0. Method for determining a stall Each motor winding phase has a PWM counter that accumulates the number of current limit events at each full step, from zero to full current. The allowable difference in counts is programmed into the IC’s onboard diagnostic register. A stall is detected when the count falls below the programmed value. There are a few conditions required for electronic stall detection to work properly. Before the stall, the motor must have been stepping fast enough for the back EMF to reduce the phase current slew rate. In addition, the motor cannot be in full-step mode; the phase current scheme must conform to 0% and 100% currents at steps 0, 16, 32 and 48; and both phases must have the same profile. Stall detection scenarios There are many factors which can contribute to a stall, so it is important to use an advanced IC that properly evaluates stall Moortec improves on-chip thermal sensors Moortec Semiconductor has announced improvements to its range of on-chip temperature sensors for advanced CMOS technologies. With an uncalibrated accuracy of +/-3C and a calibrated accuracy of +/-1C, across the temperature range of -40C to +125C, the range also offers a resolution of 0.06C. The enhanced accuracy temperature sensor range is available on TSMC28LP and TSMC28HPM technology nodes. By licensing and deploying this sensor IP IC designers will be able to monitor temperature on different regions of processors and SoC chips to higher accuracy. This is now a fundamental method by which the use of IP blocks within complex chip designs at the leading edge is now controlled. Better management of IC heating issues on advanced-node ICs allows for higher gate and power densities than would otherwise be possible. The sensor IP can be used within performance optimization schemes such as Dynamic Voltage and Frequency Scaling (DVFS), thermal monitoring to create alarm conditions and also device security against hacking. Moortec Semiconductor www.moortec.com www.electronics-eetimes.com Electronic Engineering Times Europe January 2014 27


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