Toshiba says tools are key for motor design
Field-oriented control has become the preferred option for many of the motor drives used in industrial applications. However, despite an abundance of suitable IP being available to developers, they need intuitive tools to help visualise the motor’s behaviour, writes Roland Gehrmann.
Appliance designers are increasingly implementing variable-speed motor drives to achieve superior energy ratings as product marking schemes for refrigerators, washing machines and other appliances focus buyers’ attention on energy consumption.
However, the more complex nature of these drives requires more design effort and an increased focus on software – which means that solutions which minimise the development and programming overhead are particularly attractive to engineers. And the most effective development environments increasingly bring together flexible prototype hardware, the means to easily control and visualise motor behaviour, and tools that simplify testing and fine tuning.
Field-orientated control (FOC), also known as vector control, has become the preferred strategy in many appliance designs, enabling vendors to meet demands for efficiency, quiet operation and long-term reliability at a competitive selling price.
By manipulating motor currents and voltages with reference to the rotor axes, FOC provides smooth and responsive control without the high demand on compute power imposed by traditional sinusoidal control techniques at high rotor speeds.
IP for implementing FOC has developed quickly in recent years; designers can choose to implement the algorithms in software, in register-configurable hardware, or using a combination of proprietary algorithms and IP supplied by a processor or power-semiconductor vendor.
Fully software-based FOC can be implemented in a DSP or a high-performance microcontroller such as devices built around the ARM Cortex-M3 processor core.
Some microcontrollers, such as the Toshiba TMPM37x family, implement important FOC algorithms in hardware, which allows the CPU to be used for other application-level tasks.
The TMPM37x provides separate programmable motor drive (PMD) and vector engine (VE) IP blocks for controlling a 3-phase brushless dc (BLDC) motor.
A firmware-based approach gives flexibility for designers to combine their own algorithms with vendor IP to achieve an optimal blend of cost, performance and differentiating features. It also allows developers to use CPU sleep modes most effectively to minimise energy consumed by the control circuitry.
The TMPM37x PMD block implements a 3-phase PWM generator, dead-time controller, protection circuit and ADC timing network.
Developers are free to combine functions from the PMD block with proprietary motor-control IP if required.
Alternatively, the hardware vector engine is used in combination with the embedded PMD, acting as a co-processor to offload the main CPU.
Within the VE, there is a scheduler for event and priority control, a calculation core and decoder, an operation unit, a multiply-accumulate unit, and FOC modules to perform the algorithm with 3-phase current data from the in-built ADC.
When the PMD and VE are used together, only few register settings are required to manage motor-control functions including three-phase PWM waveform generation (at 16-bits resolution), as well as speed control and position estimation.
Compared to typical software-based FOC, which requires the CPU to generate data on each PWM period, this approach reduces the processor loading to one computation at every rotor-position update: that is at 60° intervals only.
The microcontroller architecture also provides a 12bit ADC for high-speed PWM-synchronised measurement, as well as an on-board comparator to detect emergency-stop conditions. Programmable amplifiers are also integrated to set gain for phase current measurement.
To build a motor drive using a TMPM37x, the designer must develop the application, configure the PMD and VE blocks and integrate any proprietary IP, and subsequently test the application and verify satisfactory motor performance using prototype hardware.
Application development is aided for microcontrollers using the ARM Cortex-M3 processor architecture by a number of established development environments, including tools such as Atollic TrueStudio , IAR KickStart or Embedded Workbench, and Keil MDK.
A motor-control starter kit containing will provide prototype hardware on which developers can begin building their applications.
The CPU board typically connects to a host PC via a USB link, allowing the user to download applications compiled using the chosen development environment.
For ease of probing, PWM output signals and other control signals may be bought out to a connector.
Toshiba’s prototype hardware has separate processor board and driver board, the latter containing the power electronics including a mosfet bridge for driving the motor.
Separating the boards allows developers to use their own power stage if required, which may be preferred if a proven power design is already available, and to connect different power boards to target a variety of motors having different current and voltage ratings.
Although electrical isolation may be provided between processor and driver by optocouplers, separate boards assists in confining PC tools and logic development within a safe voltage area.
Engineers ready to begin development using motor-control microcontroller application development tools and starter kit hardware, also need a convenient means of setting up motor parameters, controlling motor parameters, and of visualising behaviour to fine-tune performance.
To satisfy this requirement, Toshiba has developed a PC-based graphical tool called MotorMind.
Using this, engineers can transfer parameters to the evaluation board and immediately run the motor without having to recompile the firmware if there are new settings. In most cases, customised set-up of the motor can be achieved in under an hour, and values can be changed on the fly if necessary.
The tool provides a graphical user interface (GUI) for configuring motor parameters, adjusting speed settings, and monitoring status in real time via a graphical display showing target speed, current speed and torque. It also incorporates a type storage oscilloscope function with suitable trigger settings.
As illustrated in 030ct12Toshiba1, motor parameters, such as pole pairs, current ratings, rotational directions, acceleration and encoder details can be entered and transferred to the CPU board using click-to-upload controls. Proportional-integral (PI) control settings and system parameters such as dead-time, PWM frequency, and shutdown/restart behaviour can also be set via the GUI. Settings can also be captured from the CPU board, either by clicking or by automatic download when the GUI is started.
The GUI presents several windows, including a system-load indicator that shows CPU usage, as well as a motor configuration window that provides a start/stop button and a slider control for setting rotation speed. In addition, two windows provide a statistics view showing motor speed, torque and current against target speed, and the oscilloscope display
The oscilloscope presents captured signals with resolution of up to 50µs, at a PWM frequency of 20kHz.
Up to eight VE and firmware parameters can be displayed at any one time, which the user can choose from a list of 32 possible signals. These include phase voltages, phase currents, d- and q-axis currents, reference currents, proportional coefficients, and other internal signals from the microcontroller’s integrated vector engine.
The user can also select trigger sources, and set trigger conditions such as rising or falling edge, centre or left trigger, as well as the threshold value for the trigger, as with a conventional storage oscilloscope.
In addition, the function can be programmed to log every nth event for longer recordings, up to n = 256. Tick duration and total recording time are displayed at the bottom of the window.
When viewed as a whole, the MotorMind GUI allows the designer to observe in real time how modifying various parameters affects motor behaviour and MCU performance during acceleration, operation at target speed, and deceleration.
This allows designers to identify and resolve bugs and problems during prototyping and testing. Parameters can be exported to a header file for subsequent compiling into firmware. A storage and loading function allows the software to be used with different motor applications.
The motor-control evaluation kit and MotorMind configuration/analysis tool are used in conjunction with on-line resources such as a descriptive start-up guide detailing software and hardware setup, sample applications and code, and application briefs covering subjects such as connecting Hall sensors.
Schematics for assemblies such as the CPU board and power module are also available.
Combining a high-performance microcontroller with field-oriented control firmware provide a flexible platform for designers to build three-phase brushless dc motor drives.
Using an industry-standard microcontroller development environment, and with the support of dedicated graphical configuration and analysis tools, it is possible to build, test and fine-tune motor drives for consumer or industrial use targeting a variety of applications and power ratings.
Roland Gehrmann is consumer and industrial IC marketing manager at Toshiba Electronics Europe.