“I was working on a new running control algorithm and noticed that my startup routine (forced ramp-up to speed) was being tripped up by something,” says Ward Brown, staff applications engineer with the Security, Microcontroller and Technology Division at Microchip Technology. “On closer inspection, I noticed that my running algorithm was trying to take control before my startup routine was completed. On a hunch, I took out the startup routine altogether, and voila! Unexpected success. I discovered that my new running control algorithm was self-starting. Not only that, it started much better than my forced startup ever did.”
CHIPS TARGET MOTORS
These days, designers usually choose from microcontrollers designed specifically for motor-control applications. Some have PWM timers with a granularity in the picosecond range. Dead-time control, another feature that’s common for multiphase PWM support, separates the start and top times of timers (Fig. 4). This requirement is common in an H-bridge application, where the pair of PWM signals controls complementary transistors. Turn both transistors on at the same time even for a fraction of a second, and the short circuit can fry the electronics.
Microcontrollers that use feedback mechanisms as sensorless BLDC motor-controller designs often synchronize peripherals. Though this can be done in software, going the hardware route will reduce processor overhead and can lower performance requirements, allowing for a less expensive core. Dedicated hardware configurations abound, but programmer-configurable systems are becoming more common.
Companies such as Atmel, Energy Micro, Microchip, and Silicon Labs have peripherals with programmable trigger interconnects. This enables scenarios with an analog comparator input that triggers a timer that, in turn, triggers an ADC to capture an analog signal and convert it to a digital value. The processor doesn’t get an interrupt until the value is ready. In a software-only implementation, the processor would be interrupted three times and have to manually start each peripheral in response to the interrupt.
Cypress Semiconductor’s PSoC 3 and 5 lines include programmable analog and digital blocks in addition to an 8051 or Cortex-M3 processors (see “Field-Programmable I/O Augments 8-Bit Microcontroller”). These blocks can be linked together, including basic state machine support for standalone operation (Fig. 5). Cypress doesn’t have a separate product line for motor control since every chip is customizable.
Don’t overlook speciality controllers. For example, the Renesas 100-MHz, 200 MIPS, 32-bit SH7286 is designed to control a pair of motors (see “Mechatronics Means Motors”). It includes hardware to make the job easier.
The bottom line when choosing a micro is to check out all of the options. There are many motor-control applications that even the sophisticated programmable system-on- a-chip (PSoC) block configuration can’t handle, such as where floating-point precision is required. In this case, the high-end DSPs are often the best solution for multiphase motor-control applications.
COORDINATED NETWORKED MOTORS
It’s easy to overlook other aspects of a system when concentrating on motor-control designs. But these days, communications play a big part. Tim Resker, product marketing manager for Blackfin in industrial instrumentation, notes that Analog Devices is “seeing a trend toward integration of real-time Ethernet controllers with motor control.” Developers are becoming more aware of timing-related standards such as IEEE 1588 on Ethernet.
Resker also sees more intelligence per node and sometimes multiple motors being controlled by one chip. Adding communication to the mix adds overhead. But with augmented hardware, the processors often have plenty of headroom to handle even a Web server. Coordinated communication between motor-control nodes for emergency shutdown and with a central controller tend to be standard fare.
“Software engineers who focus strictly on writing motor-control code will be out of work within a decade,” says Dave Wilson, motion products specialist for Freescale. “As ‘model-to-code’ tools become more sophisticated and reliable, more engineers are turning to the CAD tools to actually generate the code. In contrast, motor-control system engineers will be in high demand, as there will be a need for individuals who understand the design problem from a system level and to input the problem to the computer.”