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The driving forces in industrial electronics

The Carbon Trust has estimated that 40% of global electricity consumption is down to electric motors. Within the UK's industrial sector, motors consume around two thirds of electricity generated.

As this electricity consumption was calculated to be the equivalent of 8.411million tonnes of oil (DECC's 2012 report), or about $6billion – and a figure that is rising – there is clearly potential for huge savings, whether the main consideration is financial or environmental, by running motors efficiently.

David Somo, ON Semiconductor's vice president for corporate strategy and marketing, pointed to the broader issues for electronics designers in the industrial sector. "The driving forces in industrial electronics are consistent with the trends emerging across the entire electronics industry; electronics devices and systems are increasingly becoming intelligent, connected and energy efficient. Examples range from smart lighting systems to entire smart buildings to new energy efficient variable speed motors and high efficiency power supplies."

There are some things that factory managers can do at relatively low cost – fitting a variable speed drive to a motor can have a payback period of less than two years, just in terms of energy savings – but replacing or upgrading industrial equipment is capital intensive. Dr Stephan Kubisch, head of R&D at TRINAMIC Motor Control, commented: "As long as the mechanics and motorised parts are working, it will usually not be changed. This makes it a little bit conservative, since there is risk in every change, but nevertheless there is evolution. Companies like TRINAMIC need to provide game changing and mind blowing features and solutions to the market to really justify a change."

It is also the case that shifts in global manufacturing are no longer all towards the East. Increased shipping costs and wages in China means that production costs are not as polarised as they once were and highly automated UK factories can be globally competitive. This drive towards productivity through automation is also causing factory production managers to constantly review their processes.

So what should they be looking for from the electronics at their disposal? First of all, asserted Dr Kubisch: "Electronic design should neither become more complex nor cost more." Also power efficiency represents a strong and growing market in its own right, and Somo commented: "Motor drives are evolving from fixed speed to variable speed to reduce energy consumption as efficiency becomes increasingly important in response to government regulations and operating cost considerations.

"New intelligent motor drive techniques, such as the use of inverters and sensorless control, are being adopted to bring improved efficiency and cost. For example, the use of inverters for motor control can reduce power consumption by 30% or more compared to damper based control. ON Semi's inverter Intelligent Power Modules provide this superior efficiency and are used in a variety of motor control applications."

Meanwhile, Trinamic addresses power efficiency with 'self sensing' features embedded in its range of pre-drivers, of which the TMC2660 is one of the most recent. This device integrates both a pre-driver for real time calculation of motor coil current values, and power mosfets for amplification of coil outputs to directly drive an external motor.

With an Rds(on) of 65mO, the TMC2660 dissipates 2.8W at 4A, claimed by Trinamic as the lowest power dissipation currently available for a 4A stepper motor driver. This eliminates the need for a heat sink, enabling highly dense board designs and reduced component count and cost.

Dr Kubisch explained the reason for developing such technology. "Sensor technology brings great advantages to efficient and reliable motor control, but adds cost and space, both of which are not acceptable in many applications. Unfortunately, most applications do not allow for expensive/big sensors to be integrated, and it cannot be good that a sensor is more expensive than the motor and electronics. In general, smaller and cheaper sensors would be nice. This is why TRINAMIC provides a lot of integrated 'self sensing' features."

Torque ripple
A further consideration for controlling a motor is torque ripple – essentially a vibrating feedback which is unwelcome in many industrial processes. If the application has to be optimised for high efficiency and low torque ripple while supporting variable speed control over a wide range of loads, then vector control of a permanent magnet synchronous motor (PMSM) is a good solution, according to Taylan Mucuk, field applications engineer for distributor Future Electronics.

"Vector control was invented decades ago, but has only become popular recently," he claimed. "This is because the control algorithm requires a huge amount of mathematical processing and the execution of these control loops at very high speed. In the 1980s, a dsp capable of supporting such functions would have cost around $600. Now, devices providing technologies far superior to those of the first dsps are available for as little as $3. Evolving technology has thus enabled vector control to enter the mainstream."

The purpose of vector control is to adjust the current through the stator's windings in real time – or as close to it as possible. In the development of a motor system, then, it is essential to be able to extract real time data from the prototype board. Mucuk said: "In the past, this would have required a costly real time emulator. Now, the same capability can be provided by a pc with a USB port and a standard programmer. Freescale's new tool, FreeMASTER, enables users to debug a motor control application without stopping the controller. It is also easy to link a FreeMASTER project to a CodeWarrior project (CodeWarrior being the integrated development environment Freescale supplies to support its microcontrollers), and to plot all the variables in order to evaluate the performance of the motor and the firmware controllers."

Upgrade path
The fundamental requirements for motion control – like ramp profile generation, motor control and motor current regulation – will not change and, according to Dr Kubisch, should be set in hardware. However, when considering the upgrade path, he asks: "What changes in electronics most often? Microcontrollers, for example, and the high level interfacing. Thus software, since it is flexible, should cover this." Somo, concurs, although with a degree of caution: "It's important to consider future upgradability via software or other means as most industrial systems have a life span of several years or more, but designers still need to be cost conscious in making design trade offs."

Mucuk claimed that it has fallen to hardware suppliers to create a full set of design tools, including this flexible software, for the motion control environment. "Sensorless control of PMSMs would not be economically viable without the existence of powerful, cheap microcontrollers," he concluded. "Freescale is helping mainstream designers to take advantage of the technology by providing libraries, firmware and hardware peripherals that ease the implementation of a motor control system. While designers' evaluation of controller hardware will rightly attach a high priority to performance and hardware features, software and tools available from the controller manufacturer are now an important additional factor for the design engineer to take into account."

Author
Tim Fryer

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