Using Field-Oriented Control for Electric Drives in Vehicles
Texas Instruments’ InstaSPIN-FOC solution, combined with the company’s FAST software sensor algorithm, is an ideal solution for this dilemma. Enabling a straightforward entry into the world of sensor less field-oriented control, it can be used for robust, highly reliable and energy-efficient motor designs in the e-mobility segment.
Electric motors are indispensable parts of today’s vehicles. A medium-class car currently has an average of ten pumps and compressors and around 40 electric motors in total. Depending on equipment specifications, this number can rise to more than 100 in premium-class cars. Apart from water and fuel pumps, electric drives are used for adaptive suspension controls, blowers, compressors, power steering, windshield wipers and window lifts.
Approximately 40 electric motors can be found in medium-class cars, including drives for pumps, compressors and power steering. Premium-class cars frequently include more than 100.
The e-mobility sector is another promising application for electric drives. This field includes hybrid vehicles powered by a conventional combustion engine in addition to the electric drive as well as purely electric vehicles. Current examples of electric passenger cars include the BMW i3, the VW E-up and the Opel Ampera. In the commercial segment, electric motors can be used for electric busses operated by local transport authorities.
Requirements for Electric Motors Used in Vehicles
Electric motors used in vehicles must provide the following features:
- High efficiency
- Small size and low weight
- High robustness
- Reliable operation and low maintenance requirements
- Minimum noise
- Low manufacturing costs
These requirements apply to traction motors in vehicles (e. g. wheel hub motors) as well as motors driving pumps, ventilators, window lifts or power steering.
FOC Increases the Efficiency of Electric Motors
FOC is an established method to increase the efficiency of any electric motor, including traction motors, as well as the auxiliary drives required in an electric car. The FOC approach offers several advantages:
- Motors can be operated at their optimum torque and speed at any time
- FOC offers precise and fast speed regulation characteristics, which are important for applications frequently subjected to dynamic load changes (e. g. pumps or ventilators)
- Reduced torque fluctuations lead to a smoother motor rotation. This, in turn, results in less noise emission, which is particularly significant for electric vehicles which do not have a loud combustion engine. Furthermore, smoother motor operation also reduces bearing wear, resulting in improved durability and reliability
On the other hand, FOC has several disadvantages, the most significant being the necessity to use a rotor sensor. This can be implemented mechanically or as a complex software sensor (observer). The sensor is needed because FOC must determine the exact rotor position in order to generate an appropriate magnetic field in the stator for maximum torque.
The use of sensors and observers, however, leads to increasing costs and a higher error rate especially in vehicle applications. According to e-Traction, a Dutch manufacturer of motors for electric busses, FOC sensors turned out to be the most unreliable parts of the entire system setup. The company reports that most motor failures could be attributed to difficulties with these sensors.
On the other hand, conventional sensorless implementations – including the Sliding Mode Observer (SMO) – require extensive expertise on the part of the designer. Furthermore, the associated controls feature insufficient stability at low speeds, which is particularly troublesome for low-speed motors or during the acceleration phase.
The Solution: InstaSPIN-FOC
InstaSPIN-FOC was developed by Texas Instruments as a field-oriented control approach that minimises the drawbacks of sensorless controls. InstaSPIN-FOC was combined with two technologies, including the FAST software sensor algorithm (Flux – Angle – Speed – Torque) and the C2000 Piccolo microcontroller (MCU).
C2000 Piccolo includes the entire sensorless FOC solution in its ROM. For better resolution click here.
Using InstaSPIN-FOC, FAST and the MCU, designers can tune and control any type of three-phase synchronous or asynchronous motor across variable speed and load levels within minutes. The associated software has been integrated into the MCU’s ROM by TI, and the cost of the software is included in the price of the MCU. For development and evaluation purposes, TI provides motor-control libraries (modules, drivers, reference systems and documentation) within MotorWare based on the latest trends in object-oriented C programming and API-based coding. Experts can use the free GUI Composer Tool included by TI for the straightforward development of a graphical test environment, which can be used for lab testing.
This is complemented by the free InstaSPIN Simulation Tool (https://www.ti.com/tool/instaspinsim#2), which can be used for interactive on-line simulations of InstaSPIN-FOC. Using the parameters of various motors and different load scenarios, designers can thus get insight into the functionality of InstaSPIN-FOC and the FAST algorithm under real-world conditions.
Optimise Any Kind of Electric Motors
InstaSPIN-FOC offers many advantages as opposed to traditional techniques. These include the Luenberger observer or the Sliding Mode Observer (SMO). For instance, it enables the design of robust, low-speed/high-torque motors for compact, low-noise direct drives suitable for e-bikes, golf carts and electric scooters. Additional applications include electric or hybrid passenger cars and commercial vehicles. Because the FAST algorithm can make precise estimations of the magnetic flux, rotor angle, speed and torque at any time, rotary encoders can be eliminated in many cases.
InstaSPIN-FOC is currently available for the 90MHz/32-bit floating-point Piccolo F2806xF MCUs and the ultra-low-cost Piccolo F2802xF range.
InstaSPIN FOC by Texas Instruments represents the next evolutionary step in the field of sensorless motor control.
Motors supported by InstaSPIN-FOC include:
- Brushless DC motors (BLDC)
- Permanent-magnet synchronous motors (PMSM)
- Brushless interior permanent magnet motors (IPM)
- AC induction motors (ACIM)
Steppers will follow soon.
Increased Energy Efficiency and Reduced Component Count
The energy consumption of Texas Instruments‘ solution is positively affected by the fact that the angle information is preserved even when rotor speeds are significantly below 1Hz (typ.) at full torque, when the direction of rotation is reversed or when the motor is stalled (with straightforward removal of the stall condition). The built-in start-up modes provided by InstaSPIN-FOC ensure that the rotor angle is determined within less than one electrical cycle. This is important for vehicles featuring a start/stop function or purely electric vehicles because start-up problems can be resolved efficiently.
As an additional advantage, InstaSPIN-FOC requires no complex wiring if used with a software sensor like FAST. In control solutions using mechanical sensors, the longer distances between the motor and the associated control sometimes require complex shielded wiring in order to eliminate interferences (noise etc.).
Electric Vehicles and E-Bikes as Reference Applications
InstaSPIN-FOC used with the FAST algorithm and PowerWarp particularly lends itself to applications requiring high torque at low motor speeds, including electric vehicles and e-bikes. For instance, Royal Dutch Gazelle uses InstaSPIN-FOC for the motors driving their electric bicycles. While conventional sensorless controls cannot provide the desired torque at speeds below 3 – 4km/h, InstaSPIN-FOC is operational immediately after start-up. According to Gazelle, significantly reduced design times are another advantage of TI’s solution, enabling the bike manufacturer‘s designers to focus on other tasks, including optimised battery management.
Another Dutch company, e-Traction, is manufacturing drives, electric motors and additional components for electric vehicles (including city busses). The direct drives of the company’s TheWheel series provide a torque between 800 and 10,000Nm. Furthermore, e-Traction is using InstaSPIN-FOC for pumps, compressors and starter-generators based on asynchronous motors as well as synchronous wheel hub motors.
The reason e-Traction selected InstaSPIN-FOC is because it enables the motors to provide the desired torque immediately after start-up. As an additional advantage, practical experience of the manufacturer has proven that the sensorless operation positively affects the motors‘ safety and reliability. Furthermore, no complex calibration is required anymore.
Electric drives based on sensorless field-oriented control can also be used for a wide variety of additional applications. These include walking and climbing aids for elderly people or persons with walking impediments. Additional applications include electric scooters, electric wheelchairs, golf carts and electric motorbikes and scooters. Electric drives for these small-scale vehicles represent a market that is almost as promising as electric drives for passenger cars, busses and trucks.
Speed Control with InstaSPIN-FOC Motion
In addition to InstaSPIN-FOC, electric vehicle drives require robust and highly efficient speed control in order to damp the steep accelerations and accompanying jerky movements of electric vehicles. TI’s InstaSPIN-Motion is a sensorless solution for applications requiring precise speed control and reliably smooth operation. Additional uses include applications featuring multiple state transitions or dynamic changes.
Using InstaSPIN-Motion, the stiffness of the drive system can be tuned across the full operating range using the bandwidth as the only variable.
As an additional component of InstaSPIN-Motion, SpinTAC – developed by LineStream Technologies, Inc. – enables robust control functionality across the dynamic speed and load ranges of an electric drive system. SpinTAC achieves maximum control precision using the moment of inertia. The inertia calculator automatically determines the system’s inertia by driving the motor and measuring the feedback.
By using a single tuning parameter, InstaSPIN-Motion enables the speed control to be tested and tuned for soft to stiff response. This single controller gain (bandwidth) works across the full speed and load range of an application, resulting in less tuning effort as opposed to PID-based systems using multiple variables. These systems frequently require a dozen or more speed and load-tuned coefficient sets in order to cover the full range of conceivable dynamic conditions.
Execution of Motion Profiles
SpinTAC also simplifies transitions between different speed steps. Contrary to pre-defined look-up tables, SpinTAC is executed by the processor in order to achieve a smooth, configurable transition between two steps.
SpinTAC automatically generates the optimum curve satisfying the customer-specific jerk and acceleration constraints that are applied to the curve type selected by the designer:
- Standard trapezoid curve (constant acceleration, no jerk limit)
- S curve (smooth, limited jerk)
- Proprietary ST curves developed by LineStream (very smooth, permanent jerk)
Based on the data or curves, resp., designers can individually tune the transition between two speed steps. For instance, a softer transition can be implemented for a scooter designed for elderly people, while a more aggressive setting can be chosen for more sportive vehicles including e-scooters or electric motorbikes.
Using InstaSPIN-Motion combined with SpinTAC, it is thus possible to mimic the behaviour of a combustion engine (especially the soft torque increase) in an electric vehicle. This, in turn, can increase the acceptance of electric vehicles by drivers of traditional cars.
Using PowerWarp to Increase the Efficiency of AC Induction Motors
The PowerWarp software algorithm included in the InstaSPIN-FOC solution can be used to further increase the efficiency of AC induction motors especially at low loads. PowerWarp allows balancing the dynamic torque and speed profile in order to keep the control system stable. This is based on reduced copper losses in the motor’s stator and rotor. According to real-world tests conducted by Texas Instruments, energy consumption can be reduced by 28 per cent using PowerWarp for asynchronous AC drives. This is important for electric vehicles including the Tesla. This roadster, manufactured by Tesla Motors, is powered by a three-phase AC induction motor.
Much of the savings can be achieved in the partial-load range, while savings are lower at full load. As the majority of the motors rarely operate under full-load conditions, overall substantial savings can be made using PowerWarp.
Therefore, InstaSPIN-FOC is a viable alternative to conventional technologies especially for designers of control solutions for AC induction motors. This type of motor provides the features required by electric vehicle drives, including robustness and long operating life, complemented by low cost and independence of rare earth metals.
Conclusion
Field-oriented, sensorless control approaches, including InstaSPIN-FOC, offer significant advantages compared to sensor-based solutions. This particularly applies to applications in the e-mobility segment. Robustness and the instant availability of the full torque are significant factors for electric vehicles and motors used in compressors, pumps and air conditioning systems. Thanks to InstaSPIN-FOC, sensorless field-oriented control can be used in these applications as well.
About the author:
Michael Seidl is a Business Development Manager at Texas Instruments. After studying electrical engineering with a focus on communications at the Munich University of Applied Science, he spent 18 years in the semiconductor industry, working in various positions in the DSP software development, application, product marketing and business development sectors.
All images courtesy TI
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