DSPs, MCUs or Mixed Signal FPGAs in motor control? Making the choice. - Embedded.com

DSPs, MCUs or Mixed Signal FPGAs in motor control? Making the choice.

Electric motors convert electrical energy into mechanical motion. These motors are broadly categorized into two main categories: DC (Direct Current) and AC (Alternating Current). In turn, each of these categories comprises numerous sub-types, each offering unique capabilities and each targeted toward a specific range of applications.

Today, electric motors are ubiquitous, appearing in a seemingly-infinite variety of applications, including residential (refrigerators, fans, washers, pumps…), commercial (heating, cooling, ventilation…), and industrial (actuators, robotics systems…).

Consider the “How It's Made” program on the Science television channel, for example. A typical installment shows a product being processed by multiple actuators, manipulators, and robotic systems. As the product wends its way through the factory, it may be “touched” by hundreds of motors.

Few people, however, realize just how many motors there actually are and their impact on the environment. In fact, over 20 million motors are produced every day around the world, which equates to more than 7 billion new motors each year.

Furthermore, experts estimate that motors consume over 50% of the total energy production in the United States. In 2005, for example, the US consumed over 4,000 billion KWh (kilowatt hours) of electrical power, a staggering 2,000 billion KWh of which was consumed by electric motors.

The power output of electric motors is measured in horsepower (HP), where 1 HP equates to approximately 750 watts. Electric motors may be broadly categorized as small (less than HP), medium (1 to 99 HP), and large (100 HP and up). Some electric motors can be extremely large; NASA and Boeing use 60,000 HP electric motors in their wind tunnels, for example.

The larger motors tend to be the most efficient, because they are constructed from the ground up with efficiency in mind. The theoretical maximum efficiency for a motor is around 95%, and the larger motors typically achieve 93 to 94% efficiency.

Unfortunately, for every large electric motor there are tens of thousands of smaller ones, the vast majority of which are highly inefficient. The efficiency of small AC motors, for example, can be as low as 50%. What does this mean? Well, if a motor is only 50% efficient, then only half of the power it consumes is being converted into useful work; the other half is burned off as heat, which means each motor is actually acting like a small (or not-so-small) radiator.

This can add up to a huge amount of energy in an industrial setting like a factory, which actually receives a “double whammy.” This is because it is now necessary to provide cooling systems to remove the undesired heat, and these cooling systems use … you guessed it … yet more inefficient electric motors.

Users are becoming increasingly conscious of the rising cost of energy and the effects of technology on the environment. Also, there is ever-increasing pressure for greater efficiency from environmental regulators, and motor-driven products are more-and-more being required to meet the stringent environmental standards mandated by regulatory initiatives like Energy Star, the Kyoto Summit, and the U.S. Department of Energy Part 430.

One solution is to add intelligent load-matching and variable speed control, which can increase efficiency by anywhere from 14 to 30%. Implemented broadly, electronic motor control could result in savings of as much as 15% of the total electric power used in the US. This equates to an annual reduction in energy consumption of as much as 300 billion KWh, thereby saving $15 billion and reducing greenhouse gasses by more than 180 million metric tons a year. When extrapolated on a worldwide basis, the potential savings are staggering.

Who Is In Control?
For the purposes of this article, we are going to focus on AC electric motors, which account for 70% of the power consumed by industrial applications (they also account for 45% of the power in commercial applications and 42% in residential applications).

AC electric motors consist of a stator (stationary field) and a rotor (the rotating field or armature). Rotational speed and torque are produced by the interaction of magnetic flux and electric current. The key components for a system used to control an AC electric motor are sensors and an associated interface, a digital control system, and a power stage. For a large proportion of applications, it will also be necessary to provide some form of user interface (Figure 1 below ).

Figure 1. The main elements of a motor control system.

For closed-loop control systems, rotor position and/or tachometer inputs are needed. These may be provided by hall-effect sensors built into the motor or externally mounted optical position encoders, synchro-resolvers, or magnetic induction sensors. Each of these sensor types are available in various electrical topologies, and therefore may require unique analog or digital sensor interfaces to receive and format information in a manner that is appropriate for the digital control logic/system.

In the case of back-EMF detection, synchro-resolver positioning sensors, resistive position sensors, and so forth, an analog-to-digital converter (ADC) will be required. In some applications it may also be necessary to perform some form of signal conditioning (filtering, for example).

A motor's speed, torque, and direction are managed by electronically switching (modulating) voltages across the motor windings. In a Pulse Width Modulation (PWM) controlled system, the sequence in which voltages are applied to the windings determines the rotational direction and speed of the motor.

Based on the inductance of the winding, the frequency and duration (duty cycle) of each “pulse” the peak current and therefore the peak magnetic flux (torque) that is achieved in each winding is realized. The motor's winding inductance that is partially set by the number of turns used in the motor's windings, filters and smoothes out the digital PWM pulses transforming them into mechanical momentum.

By controlling the sequencing, frequency, and duty-cycle of the drive electronics, PWM systems can control direction, speed, and average torque of a motor. The PWM output from the digital control logic interfaces with the motor through the power stage. MOSFETs are the most commonly used power stage switching component for electronic motor control.

The user interface component in a motor control system allows users to issue commands for initializing, configuring, and controlling the control logic component. It can be as simple as a forward or reverse switch and a speed adjustment circuit, or as complicated as providing voltage, current, and temperature monitoring capabilities related to the control of the motor.

There are many different types of AC motors, each of which will require control techniques, sensors, power stage and algorithms that are tailored to the specific motor. Having a versatile, highly configurable, and highly integrated controller that can support a broad range of motor control techniques and motor types is desirable, and would improve cost, performance, and power efficiency.

Don't Slip!
The term “slip” is used for the difference in angular position between the location of the electric current in the rotor and the actual physical position of the rotor. In essence, slip equates to torque; the current has to lead the physical mass by some amount to cause the rotor to spin. Slip is not an unmixed blessing, because if the slip becomes too great power will be wasted. Fortunately, slip and wasted power can be minimized with electronic motor control while at the same time maximizing torque across a broad range of loading conditions.

Another big factor impacting AC motor efficiency is loading mismatch. An AC motor achieves its maximum efficiency when it is operating at or near its full rated load, but this is rarely the case. One consideration is that of “over engineering” in which the design engineer says something like: “I really only need a 1 HP motor, but I'll use a 2 HP unit just to be safe.” As a result, it is not unusual to find motors that are two to three times larger than they need to be for the load that they are driving, which is an expensive mode of operation.

And, even when the motor is correctly sized to meet the maximum load, in practice it will often be run at a lower, less efficient loading. In the case of an escalator whose motor is sized to carry some maximum number of people, for example, much of the time there will be relatively few people on the escalator causing it to run at a low level of efficiency, thereby wasting power.

By means of an electronic motor control system, the load can be intelligently and continuously sensed and exactly matched with the proper input power. Even small variations in the loading can be detected and power precisely applied to match it without affecting the speed of the motor. In effect, electronic control constantly sizes an AC induction motor to the job, so that it is always operating under ideal load conditions. This maximizes the efficiency of the motor over its full operating load range and minimizes its power consumption and operating costs.

Another consideration is that not all applications require the motor to be run at a constant speed. In some cases, it may be necessary to continuously vary the motor's speed to match it to current conditions. This is impossible to do with a standard AC motor, but an electronic control system may be used to continuously monitor the speed of the motor and adjust it to meet the requirements of the moment.

DSPs, MCUs, or FPGAs?
There are a number of potential solutions with regard to the digital control logic/system. One alternative is to use special-purpose DSPs, but these can be expensive and also typically require the addition of analog components, control elements and sub-systems.

Another common alternative is to use a microcontroller (MCU), many of which contain at least some of the required analog capabilities such as an analog-to-digital converter (ADC). Microcontrollers have the advantage of being relatively cheap (around $1 to $2 for an industrial motor controller application), but they are typically clocked in the range of 10 to 50 MHz, which limits the speed with which they can control the pulse width modulation.

Yet another consideration is that each microcontroller has sufficient analog capabilities and bandwidth to control only a single motor; there may not be sufficient bandwidth to support communications for diagnostic purposes, for example. Thus, if you have a robot application containing several motors, you will need several microcontrollers and perhaps another one or two to handle the user interface and communications adding more cost.

By comparison, a $5 mixed-signal FPGA (Figure 2 below ) contains sufficient analog and digital processing capabilities to control two motors simultaneously. In addition to the fact that such a device can be clocked at around 250 to 300 MHz, the digital portion of the FPGA fabric can be used to implement massively parallel processing of the motor control algorithms.

Furthermore, in the case of a mixed signal FPGA, a portion of the digital fabric can be used to implement a soft microprocessor core, such as the FPGA-optimized ARM Cortex-M1. In addition to handling the user interface and communications, this core can also be used to monitor and fine-tune the analog components “on-the-fly”.

Figure 2: A mixed-signal FPGA contains sufficient analog and digital processing capabilities to control 10 or more motors simultaneously.

The use of a mixed-signal FPGA with an integrated soft processor allows motors to be built with sensor-less sinusoidal current control, eliminating costly sensors and further reducing the price of the electronic controls.

In addition to monitoring the bus voltage, motor currents, and speed, the combination of a Cortex-M1 processor in a mixed-signal FPGA can also perform diagnostics and handle any user interface requirements. The ability to run diagnostics and respond intelligently to problems as they occur can significantly reduce damage and increase the life of the motor, further reducing the cost of ownership.

Last but not least, we should note that the conversion of AC motors to include electronic control systems doesn't necessarily require an expensive replacement of all of the motors that are currently in use. The Department of Energy estimates that the industrial sector alone uses 12.4 million motors larger than 1 HP. Motor replacement is an ongoing activity, with as many as 600,000 motor failures and replacements in the US each year.

This means that, over the next 20 years, most of the motors larger than 1 HP will need to be replaced. Replacing these units with highly-efficient, electronically-controlled motors will reduce ongoing industrial power requirements by as much as 18%, resulting in significant energy cost savings for the manufacturing sector and substantially decreasing the environmental impact associated with running these motors.

Mike Thompson is senior. Manager, IP & Applications Solution Marketing, Actel Corp. He holds a bachelor's of science degree in electrical engineering from Northern Illinois University, and a master's degree in business administration from Santa Clara University.

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