Tune & Calibrate Your Permanent Magnet Motor in a few Mouse-Clicks

The dream of any engineer designing a 3-phase inverter for an AC Brushless motor is to quickly get a running solution, tuned to his specific motor. An ideal design tool fulfilling this dream would enable automatic measurement of the motor parameters; ensure tuning of the system power stage; calibrate the coefficients of the Proportional/Integral current regulation block and guarantee a safe and reliable start-up process.

Renesas are now offering such a dream tool as an enhancement to their existing RX62T Motor Control Reference Kit (Partname: YRMCKITRX62T).

This Kit is based on the Renesas High Performance RX62T Microcontroller, which incorporates a 165 DMIPs 32-bit CPU with integrated Floating Point Unit ( FPU ). This enables the fast implementation of complex algorithms.

The RX62T also offers the ideal solution to control one or two invertors and can drive any 3-phase AC/DC Brushless motors (e.g. 3-phase Permanent Magnet Motors). By taking advantage of a sensorless field oriented control algorithm, the royalty-free Vector Control algorithm and VDE Certified MCU Self Test software use a small flash footprint to minimise CPU resources. The kit also comes with full Schematics, Gerber, BOM l, Datasheets and Users Manual.

In addition, interfacing to the hardware and developing with a motor is made easy using the highly featured PC Graphical User Interface (GUI).

In the enhanced kit the RX62T embedded control software and the PC User Interface have been updated with four new key benefits in mind:

1. Motor Parameter Auto-detection

By default, mathematical model parameters of the motor need to be defined before you can run the sinusoidal sensorless algorithm.

The most important parameters are: stator resistance; synchronous inductance (or the two axis inductances Ld and Lq in case of non-isotropic motors) and the permanent magnet flux linkages (e.g. the back Electromotive Force constant).

The measurement of these parameters is complex and usually requires a fully equipped laboratory with specific instrumentation.

Using the new Renesas motor design tool, the values of these parameters are automatically detected by the 3-phase inverter itself. In the case of non-isotropic motors, the medium value of the two axis inductance is provided. The PC User Interface displays the measured values in a few mouse clicks.

2. Inverter Non-linearity Auto-detection

In a 3-phase inverter, the modulation dead-times, the voltage drops on the transistors (e.g. switches) and the freewheeling diodes generate a non-linearity behavior. This will lead  to a difference between the voltage requested for the modulator and the effective output voltage of the inverter.

As a result, when the voltage reference is used for measurement (for example during flux estimation) there is an error. In many conditions this error can be very large compared to the real voltage level, affecting your estimation. The error depends on the output current, so it can be compensated for with a look-up table technique.

The main challenge is to define the values in the compensation table, as it requires a lot of measurements and mathematical calculations. The RX62T Kit is delivered with a standard power stage already incorporating the correct compensation table. However most designs use their own power stage hardware, meaning a new coefficients table needs to be derived.

Renesas’ motor design tool also allows the user to automatically get a custom compensation table filled with coefficients, which will enable the compensation of the inverter non-linearity.

3. Current Proportional-Integral (PI) Coefficients Tuning

Tuning the current PI controls normally requires the use of an oscilloscope, as well as the system performing a current step in order to analyse the step response. Such a feature is currently available on the RX62T Kit but it is a manual process.

Renesas’ motor design tool has two new features that allow PI controls to be tuned easily and quickly. This can be done without the requirement for any measurement system, using simply the board and connected motor together with the PC.

Firstly, automatic detection of the correct PI gain values is provided. This delivers the user working values for the proportional and the integral constants for the current. Secondly, the user can refine the tuning by imposing a desired current step and then analysing the step response directly in the User Interface without the need for an oscilloscope.

4. Improved Start-up

At stand still, the rotor position of the motor cannot be detected with observer or estimator algorithms. Position detection by means of signal injection can also be difficult in some cases and usually very noisy.

So normally a feed-forward start-up is required to reach a minimum speed. At  this point the observers or estimators can give the necessary results and the speed loop can be closed.

Usually during this phase the required current is equal to the maximum current, since the required torque is unknown. In many applications such behaviour is not acceptable. To address this problem Renesas’ has developed a complete new start-up procedure. This starts the motor with a very low current if no torque is required and automatically regulates the current according to the required torque.

The above enhancements to the RX62T Motor Control Kit enable design engineers to plug-in and tune their own motor within a few minutes, allowing speedy and efficient system optimisation.