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Design News, March 2013

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Automation & Control Simplify Your Motor Drive Solution Integrating motors into your design needn't cause sleepless nights. Source: Texas Instruments D By ryan Kehr, Texas Instruments riving medium-voltage motors, especially a look at the interface (Figure 1). stepper and brushless dc (BLDC) motors, The Step and Direction pins can be connected to stancan be a control engineering nightmare. As dard general-purpose input/output (GPIO) pins on your these motors become more popular in everyprocessor. A rising edge on the Step input indexes one thing from radio-controlled cars, electric razors, and car position into the logic table and increases (or decreases) the water pumps, it's a challenge that must be met with minicurrent in each phase, as needed to commutate with the mal cost, board area, and complexity. Stepper motors also motor. An example of this lookup table is shown in Table 1. can be found in everyday products like video surveillance The resulting current in the winding for a given step input products, micro-printers, and cash machines. In addidepends on the microstepping mode you chose for the tion, stepper motors are a hobbyist's dream for pursuing a device. Speed is determined by how fast you issue the step variety of home projects like computer numerical control pulses. The Direction pin is simply set high, or low, depend(CNC) machines or 3D printers. Today's integrated circuits ing on which way you move. have accepted the challenge Finally,Vref is a voltage that with highly integrated control can be derived from a simple engines that reduce developvoltage divider to set the ment time, cost, and board area. maximum, or full-scale, curAt the heart of a stepper rent in each phase. This is the motor is an H-bridge for directreference for the internal DAC ing bi-directional current across that dynamically changes the the motor windings. For stepper Figure 1: Example of a stepper motor driver system block voltage on the internal comdiagram. motor applications requiring parator. By comparing this some degree of microstepping, it's necessary to implement voltage with the voltage across the external sense resistor, some sort of current control to accurately and smoothly current is regulated at each step per Table 1. This technique step the motor from one position to the next. In a disis commonly referred to as current chopping. Choose your crete solution, you implement this with a gate driver, some microstepping mode with a set of high-low digital input MOSFETs in the bridge, current sense resistors in series pins and you are spinning in record time. It really is that with the low-side MOSFETs across the winding, an opera- easy, and evaluation boards with a graphical user interface tional amplifier (op amp) to measure the voltage across the (GUI) makes it that much easier. sense resistor, and finally a microcontroller (MCU) capable of measuring this voltage (integrated analog-to-digital con- BLDC Motor Drive verter, or ADC) and acting upon it. Close the loop with a Moving to a more complicated beast, the BLDC motor proportional-integral controller in a microprocessor with requires all of the previously mentioned discrete compohigh-resolution pulse-width modulation (PWM) outputs, nents necessary for a stepper motor, plus a way to detect and you're done. This sounds expensive, area-consuming, back electromotive force (BEMF). This is commonly done and most certainly time-consuming — unless you stayed with zero-crossing comparators to detect when the trapawake during all those control-theory classes. And we ezoidal voltage waveforms of each phase cross through haven't even begun to include protection on the MOSFETs a zero voltage point. After all the discrete blocks are in for shoot through, thermal run-away, and over-current. place, it's time to delve into the dark world of firmware Integrated solutions are available that can simplify all of development in your MCU. Here's where you generate the above to a simple pulse input from a microprocessor 120-degree phase-shifted PWM bursts to move the electric when you want your stepper motor to take a step. Let's take field precisely around the stator of your motor to lead the Design News | march 2013 | www.d esign n ews.com –53–

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