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Developing Electric Actuators with Model-Based Design

Author: Mahendra Muli, Director of Marketing & New Business Development, dSPACE Inc.

July 1, 2014: The recent innovation focus in the Aerospace industry on more electric aircraft is increasingly shifting mechanical, hydraulics-based actuation systems to electric. These actuators are being applied across the spectrum, from propulsion to control surfaces and from landing gear to space launch vehicles. This ubiquitous choice towards electrical actuators is due to its ability to perform highly dynamic, fast and precise control of systems.

It is often noted, though, that this move towards electric actuation increases complexity due to the integration of electrical hardware, software and mechanical systems. This complexity, added with fast response time of the actuation, leads to complexity in the development of controls technology − not only at the actuator level, but also at the system level.
Model-based design (MBD) has proven to be the technology to manage complexity in system design. The advantages of using MBD to increase efficiency in development and reduce costs are quite well known. Combining the advantages of MBD together with advances in powerful semiconductor technology with flexible, fast-computing Field Programmable Gate Arrays (FPGA) open up a new field of possibilities, with very fast execution speeds leading to precision controls, for engineers to explore. This paper explores tools for developing embedded software, specifically for electric motor drive control units using the MBD process. 
 

Electric Actuators
The two main types of electric actuators are linear and rotary actuators. The linear actuators are further classified as either direct drive or geared drive, depending on the coupling with the end effector. The types of sensors used in the system, particularly for motion sensing, will vary from linear variable differential transformer (LVDT) to Resolver depending on the type of actuator. The other components of the actuator – motor and power electronics – will remain the same across the actuator type, but will of course be appropriate for the power level of the system.

Various types of motors (e.g. Induction, DC, Brushless DC, Permanent Magnet Synchronous) are used in these actuator applications. High-power applications requiring higher torque and RPM commonly use AC Induction Motor, and higher precision, lower-power applications often use brushless DC motors. The power inverter-based controls can be applied in both cases, even though the actual techniques, such as only speed control vs. speed, power and position control, may vary depending on the motor and application.
 

Use Cases for Embedded System Tool-Chain in Actuator Development
Rapid Controls Prototyping (RCP) and Hardware-in-the-Loop (HIL) technologies for simulation of controller hardware and software, as well as simulation of actuators, including motor and overall system, are commonly used in the development of actuation technology. These technologies can be applied not only for software development and testing, but also for running rigorous testing of hardware components.

 
Use CaseType of Development System
Actuator Acceptance TestRCP
Controller and Actuator PrototypingRCP
Actuator SimulationHIL
Actuator Simulation + System SimulationHIL
System Integration TestingHIL
 

 

 Graphic 1: RCP and HIL Simulation for Motorized Actuator System

Use of a Rapid Controls Prototyping platform in development for controller simulation could use one or more components of either processor or FPGA (i.e. computation platform, and power electronics). On the other hand, with HIL applications, the development platform could include simulation of an entire system or subsystem, all the way up to the power electronics of a controller unit (see graphic 1).

Graphic 2

Motor Control Techniques
Depending on the motor, the control systems could use speed and position control for Brushless DC (BLDC) motors or field-oriented controls for Permanent Magnet Synchronous (PSM) motors. The sensor interfaces and measurements required for developing controls vary and have to be taken into account in development of control systems.

Graphic 3

BLDC motor applications typically require measurements of rotational speed with resolvers, additional position sensors depending on application and typically current sensors from power electronics for power-control loops. In the case of PSM motors, additional sensors for current and voltage measurement are required (see graphic 3). 

Graphic 4: dSPACE Processor and FPGA based platform MicroAutoBox-II for Motor Controller Development

Rapid Controls Prototyping (RCP) Development Platform
For precise control of highly-dynamic systems, the control loops are desired to run at very high execution speeds of over 100 kHz. Advanced RCP systems, such as MicroAutoBox II from dSPACE, augment the processor-based computation with special FPGA-based computation and sensor interface capabilities to meet these requirements. 
By combining the processor and FPGA platforms together in a single unit, dSPACE RCP system allows engineers to develop slow and fast parts of the control systems separately, yet simultaneously. The FPGA layer of the system provides all necessary interfaces for sensors and to connect this system to drive power electronics stage.

By combining the processor and FPGA platforms together in a single unit, dSPACE RCP system allows engineers to develop slow and fast parts of the control systems separately, yet simultaneously. The FPGA layer of the system provides all necessary interfaces for sensors and to connect this system to drive power electronics stage.

 

HIL System for Motor and System Simulation
In order to validate the controller algorithm in the MBD paradigm, it is necessary to have the motor model that can be computed at a rate higher than the controller model computation to provide meaningful inputs to the controller model. Various approaches for motor modeling like the one shown below for PMSM, can be used in the HIL scenario.

Graphic 5: Model of Permanent Magnet Synchronous Motor

Motor Model
The PMSM model is based on a stator and magnetic field with a configurable number of magnetic poles. The generated back EMF in the stator windings is sinusoidal, so the machine is modeled in d-q coordinates (also known as the rotor reference frame). The real-time capability, with respect to calculation time and I/O, is considered during the model development. The non-linearity of the PMSM motor, with respect to inductances and magnetic flux, are modeled using look-up tables. The rotor does not have any electrical connection, and therefore, it is sufficient to model only the stator windings. It is important to realize the computation differences using different solvers and to choose appropriate solver techniques. An appropriate combination of Mixed-Backward Euler and Tustin approaches are used in this simulation model.

Graphic 6

Typical motor models are computationally intensive. Again, similar to the approach with control model simulation, the fast and slow parts of the motor model (e.g. motor temperature model computation) can be at a much slower rate compared to back EMF calculations, and can be split between computation on the FPGA and the processor. dSPACE’s FPGA solution (XSG Blockset) allows easy targeting of motor model components to FPGA and residual portions (i.e. slow dynamics part of the simulation) onto the processor board (see graphic 6).

HIL Test System for BLDC Motor with Electronic Loads for a Hydraulic Pump System

dSPACE HIL systems allow the actual power electronics of the controller unit to be included in the overall test system, while simulating the rest of the system. Such power-level testing becomes necessary when there is no access available to separate the controller and power electronics portion. In such cases, drawing current through the power electronics through the use of high-speed motor simulation, combined with highly-dynamic power loads, is the only way to test. dSPACE solutions, ranging from smaller 60V systems to high power systems, have been deployed for such applications.
Advanced high-fidelity simulation techniques, such as finite element simulation, are available to simulate non-linear characteristics, based on position or current. dSPACE tools enable the use of such technologies to bring high-quality simulation to your actuator development and testing programs.

 

Summary
Developing modern, electrified actuation systems requires sophisticated simulation capabilities in the tool chain to effectively develop and validate control strategies. Advanced high-speed computation of control algorithms and plant models on FPGA-based platforms are true enablers for advancing the actuator technology. dSPACE hardware platforms and software enable easy programming of these models through model-based design and can expedite your controls development project, while saving time and costs.

Further Information