Motion Controls Move Vehicles In A New Direction

July 26, 2011
The first generation of EVS and HEVs are on the road or about to be introduced. Now carmakers have changed their focus from get to market to get more efficiency. The latest simulation tools can evaluate improved motor controls and power electronics.

Fig 1. Custom micro-inverter modules from TT electronics will provide the integrated power electronics in Protean’s in-wheel electric drive system. (courtesy of TT Electronics and Protean Electric)

Fig 2. Ansys Simplorer simulations in the motor control system include physics-based models for solving thermal, magnetic, mechanical, electromagnetic compatibility (EMC) and electromagnetic interference (EMI) problems, and more. (courtesy of Ansys)

Fig 3. A SimPowerSystem simulation model shows the data flow in a motor and generator in an EV motor control. (courtesy of the MathWorks)

Fig 4. Infineon’s motor control block diagram identifies the relationship between key high-level components in an EV or HEV. (courtesy of Infineon Technologies)

Fig 5. With improved packaging and silicon technology, Infineon expects to achieve higher output and still provide a 30-year lifetime for its power modules. (courtesy of Infineon Technologies)

The rapidly growing electric and hybrid segment of the automotive industry is driving considerable changes. While these vehicles still lag far behind the sales of their fossil fuel counterparts, carmakers have learned quite a bit from their efforts. As a result, automobile designers no longer can settle for motion control components developed for industrial applications.

Carmakers are exercising their combined purchasing power to get new products more in line with their needs. Concurrently, changes for electric propulsion are occurring in power electronics, microcontrollers, system design and development tools, and more.

Motoring Along

Electric motors provide propulsion, and those with dual functions (motor-generators) recover energy from a variety of new vehicle classifications. In addition to the well-known Prius hybrid pioneered by Toyota, several carmakers are now developing or offering plug-in hybrid, mild hybrid, and even micro hybrid (no propulsion) as well as electric vehicles (see the table). While the terminology each manufacturer uses may vary slightly, the permanent magnet synchronous motor (PMSM) is the machine of choice for most carmakers.

The motor or motors are the central item of a motion control system in vehicle propulsion, so carmakers and suppliers are continuously improving existing designs and exploring new alternatives. There are at least two departures from current vehicle trends. One involves a new motor design, and the other uses more familiar approaches.

On the familiar side, the rising cost of neodymium magnets, the key enabler of the high performance in permanent magnet motors, is instigating carmakers and their Tier 1 suppliers to reevaluate alternate motor designs, specifically induction motors.

At the 2011 SAE Congress in Detroit, Jon Lutz, vice president of engineering at UQM Technologies, developer and manufacturer of high-efficiency electric motors, generators, and power electronic controllers, discussed alternatives to permanent magnet motors.

Based on the cost almost quadrupling and availability concerns for the neodymium magnets used in permanent magnet motors whose main supply comes from China, induction and wound field designs that had previously been viewed as less desirable are getting more attention, Lutz said.

Lutz also identified ongoing material research that could minimize the amount of neodymium required for a particular motor rating and alternate magnet materials. Improvements from power semiconductor technologies including silicon carbide (SiC) and gallium nitride (GaN) were among his reasons for expecting improved performance from the alternate motor technologies.

On the more radical side, in-wheel motors (IWMs) will provide a significant improvement to electric and hybrid vehicles according to Andy Watts, chief technology officer for Protean Electric. The company’s Protean Drive is a fully integrated, three-phase, permanent magnet direct-drive solution for vehicle propulsion. Each motor has a built-in inverter, control electronics, and software (Fig. 1).

The company has installed four IWMs in a Ford F-150 battery electric vehicle (BEV). Operating at 400 V dc, a Protean Drive PD18 with integrated inverter technology, an 18-in. IWH, achieves more than 300 Nm of torque at 1300 rpm with a peak power of 84 kW and 54-kW continuous power.

Simulating Motor Control

Simulation tools are an integral part of every carmaker’s and many suppliers’ tool boxes to get new vehicles to market sooner and avoid costly development mistakes that could spill over into production, causing quality and reliability problems. Because of the increased complexity of EVs and HEVs, these tools are required whether the next-generation design involves a new motor, new components, or system-level improvements.

Ansys Inc. can provide new simulation approaches thanks to its acquisition of Ansoft in 2008. At this point, permanent magnet motor evaluations are still increasing in interest, according to Scott Stanton, technical director for advanced technology initiatives at Ansys.

“We have customers looking into ways of modeling the electric machine in more detail and providing the system engineers with that detailed model,” Stanton said.

Prior modeling techniques that relied on providing the motor’s parameters, its inductance and resistance, offer too rough of a cut for today’s simulations. Now, Ansys engineers work with customers to get a map of the performance of the machine using its field solvers.

“They would analyze the problem using our electromagnetic field solvers, characterize the behavior of the machine, and then create a model from that,” said Stanton. Using the new model accounts for much more of the motor’s characteristics (Fig. 2).

Improved system analysis capabilities are an integral part of the reevaluation of alternative motor technologies. Stanton said he has a lot of customers investigating the replacement of permanent magnet motors.

“We have customers who have been using permanent magnet designs since we first started talking to them six, seven years ago,” he says. “Now all of a sudden they are coming to us and they are saying, ‘We are now solving induction motor problems and we are solving synchronous reluctance problems.’”

In addition to comparing the performance of the machines, the customers also use the simulation tools to establish a new control strategy for the induction machine, which is much more difficult than controlling a permanent magnet machine.

From what Stanton has observed in working with Ansys clients, the power semiconductors will continue along familiar paths. “IGBTs are going to be the semiconductor of choice for a long time,” he said. However, with the Ansys approach, SiC and GaN could certainly be evaluated in conjunction with a system supplier to determine the advantages and issues that these new technologies could provide for hybrids and EVs.

Synopsys Saber provides a suite of physical modeling and simulation tools that automakers and Tier 1 suppliers use to design, verify, and optimize power electronics for traction motor control. “The Saber tools give our customers the ability to analyze and optimize individual subsystems standalone or integrated into a complete system,” said Lee Johnson, senior manager of business development for the Synopsis Saber product line.

Synopsis engineers anticipate the importance of new device materials such as SiC and GaN to the EV and HEV designs. “We’ve released our first SiC device models to allow customers to start understanding the impact of these new technologies on their motor control designs,” said Johnson.

While the cost of rare earth magnets has instigated investigation into other motor technologies, Johnson insisted that several other costs, including the complexity of control electronics, thermal and electromagnetic compatibility (EMC) behavior, and machine reliability must be taken into account. “The Saber simulation tools provide a platform for our customers to evaluate new motor types and control strategies without the expense of hardware prototypes and physical testing facilities,” he said.

Model-based design (MBD) and MathWorks tools have been extensively used for traction motor control in EVs and HEVs, for production programs as well as rapid prototyping. The increased complexity of these systems is a compelling reason to use MBD.

“We see widespread use of executable models to capture designs, use of simulation for design tradeoff, and use of automatic code generation for implementation,” said Wensi Jin, automotive industry manager at the MathWorks.

Chris Fillyaw, application engineering manager at MathWorks, credited the ongoing development of SimPowerSystem with its library of electric drive and power electronics component models for simplifying EV and HEV system development. “The ability to quickly build a system level simulation of a motor drive using library blocks for both design tradeoff and controller verification has proven to be very valuable to motor controller developers,” said Fillyaw (Fig. 3).

The increased system complexity produces a higher demand on processing power for both simulation and execution of the embedded controller. As a result, Fillyaw is seeing increased interest from motor control developers for ASICs and FPGAs to provide the processing bandwidth required to execute electric machine models for real-time testing of their controller in a hardware in-the-loop (HIL) environment. 

“Simulink HDL Coder can be used to generate HDL (hardware description language) code from models that can be simulated and synthesized using industry-standard tools and then implemented on FPGAs and ASICs,” Fillyaw said.

Computing Power For EV Motors

Control of the motors in the initial EVs and HEVs relied on expertise and, in many cases, available products from traditional automotive MCU suppliers. For example, as the leading automotive MCU supplier, Renesas has been involved in EVs and HEVs for many years and is currently working on design-ins for next-generation systems.

Amrit Vivekanand, director of business development for Renesas, has seen a rather dramatic shift from first- to second-generation designs.

“In the first generation, cost was not so much a concern from the carmakers,” Vivekanand said.

While cost is always an issue, the volumes were expected to be small so time-to-market and creating a green image was more important. These systems were “not optimized for motor control or even HEV” applications, according to Vivekanand.

For the generation in the model year 2016-2018 timeframe, the focus is really shifting toward more of a mass market product. This has the carmakers asking several system-related questions. “How do we get costs down? How do we increase efficiencies? How do we reduce battery sizes?” asked Vivekanand. “How can we convert energy as efficiently as possible?”

Vivekanand noted a definite shift in strategy between first- and second-generation design criteria. “The first generation was all about controlling the motor safely and that was their primary concern, but now it has become an efficiency issue,” he said. Now, the motor control has to be extremely sophisticated to manage the energy conversion efficiency and deal with the system tradeoffs.

Freescale Semiconductor provides microcontroller technology to Tier 1 and OEM designs for the motor control electronics. “Thus far, Freescale, and most of the market, has been reusing microcontrollers designed for other applications such as chassis control and powertrain,” said Steven Rober, segment operations manager for microcontrollers at Freescale Semiconductor. In the future, Freescale plans to incorporate a more full-system approach.

The company’s next-generation 55-nm microcontrollers are expected to include the custom motor control peripherals from its chassis control devices that are capable of driving three-phase motors with current measurement and control and combine these capabilities with the high-throughput micros and high memory content of powertrain. Rober expects these changes to improve the controllability of the system by improving the control signals, allow higher-complexity motor control algorithms, and reduce cost.

Rober anticipates many of the low-end to mid-range vehicles to be designed with electronic control systems to control both the IC engine and traction motor. “Our new 55-nm microcontrollers are being designed with the memory, throughput, and control peripherals to enable these designs,” he said. 

Texas Instruments sees a role for its recently announced dual-subsystem C2000 Concerto microcontrollers in hybrid vehicles as well as other non-automotive applications. The series combines a C28x core with an ARM Cortex-M3 core for real-time control and connectivity.

“The C2000 has always had a strong play in automotive control and digital motor control,” said Michael Wei, C2000 MCU marketing manager at TI. “Our value really comes out in ac induction because of the capabilities that we have.”

One of the changes that Texas Instruments sees in future EV and HEV motor controls is more comprehensive safety requirements due to ISO 26262, the functional safety standard being developed for automotive systems.

“The TMS 470M fits in really well in conjunction with a C2000 that is doing the traction motor control driving,” said Anthony Vaughan, marketing manager for the safety MCU group at TI.

In this architecture, the TMS 470M provides the safety functionality. At one customer, the previous-generation system uses a C2000 for electric motor control. For the next-generation system, the customer will add the TMS470M. “For example, all the flash memory has ECC (error correction code) protection on it so it is capable of detecting single bit errors and correcting them on the fly,” said Vaughan.

Companies with products that have been used in powertrain applications could be expanding into motor controls for hybrids and EVs. For example, Minal Sawent, a product marketing manager for Microsemi, said that Microsemi has had AEC Q100 qualified products from its recent Actel acquisition designed into powertrain applications.

The newest SmartFusion products include an FPGA with a hard ARM Cortex-M3 and programmable analog technology. A motor control development kit helps users demonstrate the product’s capabilities for motor control applications. While these products are not currently automotive qualified, Microsemi could pursue this qualification in the future.

The Real Power

Power semiconductor devices in the inverter directly interface to the permanent magnet, inductive, or other motors in an EV or HEV. The inverter takes the dc energy in the battery and converts it to the ac power required to run the motors.

“Getting the inverter to be more efficient has a direct impact on the battery size,” said Carl Bonfiglio, segment marketing manager for hybrid and electric vehicle technologies at Infineon Technologies, North America.

Bonfiglio has observed that carmakers’ requests for higher power levels have reached a plateau. “We don’t see that they are going to be increasing the amount of power they want out of an inverter, so the push is on to shrink the inverter,” he said.

In his presentation at the SAE 2011 Hybrid Vehicle Symposium & EV Symposium in Anaheim, California, Jochen Hanebeck, president of Infineon‘s Automotive Division, discussed many changes that can be expected in power semiconductor technologies for future EVs and hybrids.

Compared to today’s standard technology, with 200°C junction temperature operation, Hanebeck said it’s possible to achieve more than 60% higher output power per unit silicon area at the same lifetime and more than 500% larger lifetime per unit silicon area at the same output power. Another alternative is more than a 40% reduction in silicon area at the same lifetime and output power with higher temperature coolant capability to eliminate the low temperature radiator (Fig. 5).

Gate driver IC improvements can have a huge benefit on the system. Being able to quickly diagnose different fault conditions and then take action and communicate with the main microcontroller so it can respond appropriately is occurring now. “The industry as a whole used gate drivers that were developed for industrial motor control where diagnostic requirements aren’t so high,” said Bonfiglio.

On-board diagnostics (OBD) requirements alone are sufficient to drive these improvements. With the revisions, the current approach can be optimized for lower cost and reduced system complexity.

TT electronics developed the custom micro-inverter/modules for Protean Electric’s IWM. Steve Jones, director of Global Technology at TT electronics, sees significant advantages for integrated in-wheel electronic drive systems that include providing sufficient power and torque for larger SUVs and saving packaging space in the vehicle. He has also been involved with alternate motor and semiconductor technologies.

“We’ve seen some interest in inductive control with hybrids, where the car is driven electrically within a city and with an internal combustion engine elsewhere,” said Jones. “This is a modification that can be done quickly with a short time-to-market.”

TT electronics has implemented several technologies in its development of new products for the EV/HEV market. “The use of SiC devices and compatible packaging provide application advantages including greater power density, lower switching losses, higher operating frequencies, and higher operating temperature,” said Jones.

Alternate motor technologies and advanced semiconductor devices certainly aren’t the only potential changes for powering future EVs and HEVs. According to Dave Torrey, vice president of engineering at Ioxus Inc., motor drives are one of the vehicle systems that can benefit from ultracapacitors, which can supplement the battery voltage under acceleration conditions.

“If you had an ultracapacitor, you really have an energy system that is hybridized between batteries and ultracapacitors,” said Torrey. “And the ultracapacitors could support that power.”

The Right Direction

Carmakers are moving toward lower fuel consumption, greater electric driving range, and longer time between charges. To head in that direction, they need help from a range of suppliers with enabling technologies. Any trip begins with the first step.

“I still see the whole hybrid market really in the infancy stage,” said Ansys’ Stanton. “To me it seems like the wild west of the automotive industry 100 years ago. You have all these small suppliers and OEMs, guys in garages coming up with motors and designs that they are trying to sell to the automotive industry.”

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