March 6, 2017 08:49
Can you spin up your AC brushless motor in 45 seconds?
Ever wanted to spin up a 3-phase permanent magnet synchronous motor (brushless motor), but just haven’t got round to it thinking it would be a lengthy project to undertake? Well, when we got our hands on the Renesas YROTATE-IT-RX23T kit, it looked too good not to. In this article, we take you through the process providing written details, with some images and screen shots to help along the way. The field oriented control algorithms used will work with external sensors or sensor-less using the on-board current shunts. A motor is supplied with the kit and connects directly to the board with just 3 phase wires, simplifying the motor wiring with no need for external sensors. A complete 3-phase inverter is on-board using a 3-phase bridge which incorporates current reading shunts allowing the powerful sensor-less algorithm operation on the RX23T 32-bit microcontroller.
These motor types are becoming very popular as they offer greater efficiency and reduced maintenance with no brushes to wear out. They operate at synchronous speed as the permanent magnets stay aligned to the rotating field. The rotor position needs to be known in real operating time. This can be achieved by using external hall sensors, however, this kit uses the integral current shunts (which can operate in single or 3 phase configuration).
Inverter board overview
The RX23T microcontroller is a 32-bit device with a built-in floating point processing unit allowing complex inverter control algorithms to be easily run. These process the current pulse information in order to work out the required rotor speed position and the drive requirements needed in real time.
Low voltage power stage
The power stage uses a 3-phase bridge with 6 high current MOSFETs which can be seen in the image below. The 3 on-board current shunt resistors are in the lower arms and can be seen below the MOSFETs. They measure the motor current and allow the motor timing information to be derived without the need for external sensors or wiring.
Communication is via USB to your PC which runs the control GUI.
Is via USB for set up and communications. It’s an external 24V power supply for improved motor driving, covering the full operating range.
This 3-phase brushless AC motor platform uses field-orientated control algorithms, offering sensor-less operation where just the 3 phase supply connections to the motor are needed, or can be configured to use external sensors.
- A graphical user interface (GUI) running on your PC to provide the control window
- Royalty-free embedded source code
- Offers auto tuning, calibration and motor parameter identification
- Supplied complete with a 3 phase brushless motor allowing you to quickly get started.
The kit can be used with motors up to 48VDC at 5A peak current when using an external power supply. The default settings are for using the 3 on-board current detection shunts which simplifies operation with the supplied motor.
Using the GUI makes it easy to get the supplied motor set up and running with this board.
The YROTATE-IT-RX23T kit (125-3763) contains the following items:
- RX23T Inverter board
- AC brushless motor Nanotec DB42S03
- Mini USB cable
- Quick start guide
The first step was to connect the motor into the socket J7 on the inverter board. The motor connector is a 3 pole cable-mounted socket which is polarised. It is worth checking the motor leads are connected with the red to U, yellow to V and black to W connections.
The RS (189-6026) Phoenix Contact cable-mounted connector is ideal when you want to attach your own additional motors.
For initial set up, the RX-23T board must be USB powered, so it is important to check that the 2 red jumpers JP1 and JP2 are both linking pins 4 to 6.
The latest version of the PC GUI software should now be downloaded from the Renesas website:
ZIP PC Control GUI
After registration on the Renesas site, the YROTATE-IT_PC_GUI_XXXXX.zip should be downloaded.
Click to install the motor control demo on the installer opening screen and follow the instructions to finish. At this stage, do not click ‘Start Demo’.
Now, the USB cable is connected to the host PC first, then the RX-23T board.
The board uses an FTDI FT232RL (040-6580) to communicate with the PC. If the board isn’t picked up by Windows as a COMs port, first ensure you download and install the latest drivers from FTDI (http://www.ftdichip.com/Drivers/VCP.htm). In our case we found that running the board in Windows compatibility mode fixed the COMs port issue, likely due to how our machines have been set up by our IT department.
Once we ran the GUI through WCM we didn’t face that issue again until we turned off the power, then we had to run through the trouble shooter again to get the COMs port to connect.
Following driver installation, we closed the GUI and switched off the PC.
We now switched to using an external 24V 2A power supply, which allows the motor to run over a wider operating range and allows a better resolution for the auto tuning and extraction of the motor parameters.
Firstly we swapped the 2 red jumpers JP1 and JP2 so that they both linking pins 1 to 3.
This removes the USB power supply and allows a separate 24V supply to be connected to the J4 connector. We used an RS (189-6010) connector and cable to bench power supply observing the correct polarity as marked on the RX-23T board. The supply had been set to 24V and switched off before the connections were made.
Next we connected the USB cable to the PC and then the RX-23T board, and switched on the external 24V supply.
The PC GUI is launched and communication started. This can be verified by the board LED DL1 rapidly flashing.
Click the ‘Set Up’ button, select RX-23 kit and click ‘Auto detect’. This connects the PC to the RX23-T.
The PC GUI display has various windows and a range of set up buttons down the left hand side. These allow the motor parameters to be set, tuned and monitored.
For AC brushless motors driven in sensor-less mode, the important parameters which need to be tuned are:
Current PI parameters, Proportional Kp and Integral Ki
Motor Parameters of Stator resistance, Synchronous Inductance and Permanent magnet flux.
We now go through the following process which will ensure that the driver is tuned to drive the motor correctly.
We will use the following
‘Cur PI tuning’
‘Cur PI tuning (Auto)’
1) Before starting we needed to ensure that the RX23-T board had the default parameters set. This was done by clicking on ‘Parameters Setting’ and entering 33 into the ‘00 Operation Select’ status. Then clicking ‘Write’, followed by pressing the yellow reset button P1 on the RX23T board.
2) On the ‘Parameters Setting’ screen, the motor maximum current in mA was set for the supplied motor (3500) which is entered into ‘07 Maximum Current’, followed by clicking ‘Write’ to save into the EEPROM, then the ‘Parameters Setting’ screen is closed. This step is vital for auto calibration and ensures highest resolution.
3) Automatic PI tuning was performed by clicking the ‘Cu.PI tuning (AUTO)’ button. The software generated a step voltage and measuring the motor response generated the 2 PI Current coefficients. These were displayed as below and clicking ‘Yes’ saved these into the EEPROM.
4) Manual current tuning was then performed by clicking on the ‘CurPI tuning’ button. The current tuning window appeared. When clicking on the ‘Apply Current Step’ button the screen shows the step response and it is possible to zoom in to see the transient details.
Adjustment of the values is now possible in order to obtain the cleanest step response. The step current can be increased from the default value of 50% to 90%. When the cleanest step is achieved, the proportional and integral PI current coefficients are tuned and the window can be closed.
5) Auto identification of the motor parameters was performed by clicking on the ‘Motor Identification’ button and then ‘Start’. The motor started rotating and was left load free for this operation. The results were accepted and stored by clicking yes.
This test measured the motors stator resistance, inductance and permanent magnet flux and tuned for this.
We now went into the ‘Parameters Setting’ screen and entered the number of pole pairs (which is 4 for the supplied motor) into the ‘05 Polar Couples’ and a minimum speed of 1000 rpm. The start-up current is 25% maximum current (25% of 3.5A is 875mA), so 875 was entered into ‘06 Start Current’. This was then written into the EEPROM by clicking on ‘Write’ and the window was closed.
6) We then started to run the motor using the RPM control and testing the motor to 1500 rpm, 1.5 times minimum speed.
Using the ‘Oscilloscope’ function we could display the motor current against time and we also checked the phase using the phase selector. We explored changing the values of the 4 parameters below, noting the effects.
The 2 speed parameters 13 Speed loop Kp and 14 Speed loop Ki
The proportional integral terms 11 Current loop Kp and 12 Current loop Ki
The effect at different speeds was also tested and the optimum values were identified.
It was possible to observe extremes with the motor showing instability and vibrations as can be seen in the video below.
Very stable operation was achieved when using values close to the original tuned values and this can be viewed in this video.
Going through this process provided a detailed overview of control techniques and the parameters that need to be optimised in order to get the best performance from this type of motor. With the higher levels of efficiency they offer, application areas are rapidly expanding. Using this kit makes it easy to drive custom motors with the wide range of output power and options available.
RS offers a wide range of brushless DC motors, such as Faulhaber (873-4811) which could be used with this kit.
So the answer to the original question is yes! With the board set up you can perform the auto tune functions very quickly and achieve a good stable running motor.
When will you give it a spin?
Nanotec DB42S03 motor specifications
3-phase AC brushless
Max current 5.4A
Rated speed 4000 rpm
No Pole pairs 4
Board LEDs status indication
DL1 USB communication status, flashes when data is exchanged between the PC and board.
DL2 RX23T microcontroller status slow blink for control running normally.
DL3 Free for use, is on when main control interrupt is active.
DL4 Indicates presence of the 15V switch drive supply from step-down DC-DC converter.
DL5 Indicates the logic power supply is OK.
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March 16, 2017 09:59
To expand on this :- the whole motor drive system is operating in a continuous closed loop mode with a default sampling frequency of 8KHz. It needs to do this in order for the field oriented control algorithm to monitor the 3 phase currents and calculate the motor pole positions to provide the correct drive signals. It will also take positional information from motor sensors if preferred which are hard wired to the board. Hope this helps.
The PC GUI is the HMI to the board and allows parameters to be monitored and changed. It also allows speed to be set this will be controlled within the control algorithm in order to keep constant within the normal operating range for the motor. In an end product / application as this whole drive is implemented using the Synergy RX23T platform microcontrollers then other remote sensors could be implemented into the overall control loop required by the end application.
March 8, 2017 09:52
Nice article, very clear informative.
Will this drive handle closed loop systems?