Seeking clarification on SFD in AKM Brushless Servo Drives

[Ansari07]

Dear Community Members,

I am seeking clarification on the smart feedback device (SFD) installed in the AKM Brushless Servo series. There is limited information available about it in the specifications. It appeared that it provides a very high resolution, highly accurate rotor position information signals.

1. How exactly does the SFD provide the rotor position?

2. Is there a provision of getting low resolution Hall effect type feedback from SFD?

Thank you.

Ansari07

[Spicy_Mystery]

Hi Ansari07,

This is Kollmorgen-specific technology so I couldn't find too much info on them.  I think your best bet is to give Kollmorgen customer support a call and ask them directly.  Below is all the information I could find about them, but it doesn't quite seem to answer the question.  Sorry I couldn't be of more help.

http://sine.ni.com/ds/app/doc/p/id/ds-280/lang/en

http://www.ni.com/pdf/manuals/AKD_Install.pdf (p.102)

[Nate.H]

Hi Ansari07,

SFD is a resolver based feedback type. Resolver signals are converted to a serial protocol at the motor and then transmitted back to the drive. SFD also includes motor parameters in EEPROM memory to enable quicker/easier AKD drive setup with an AKM motor. Resolution is 2^24 count/rev. Details can be found here: http://www.wiki-kollmorgen.eu/wiki/tiki-index.php?page=SFD+Feedback&highlight=SFD

Let me know if you need further details. 

Best Regards,

Nate

[Ansari07]

Thank you Nate, Spicy_Mystery, for your valuable and helpful comments.

I have contacted KollMorgen to get specifics on the SFD Technology. Will share additional information that I receive from them. 

I found this document from NI website titled "Getting Started with the NI 9502 Brushless Servo Drive Module and NI AKM Motors". Going through the document (pp. 6-15), it seems that both the high resolution encoder and low resolution Hall effect options are available.

Regards,

Ansari07

[Nate.H]

Hi Ansari07,

So AKM motors can be configured with a variety of feedback types. The table below is from the Kollmorgen AKM Selection Guide. NI offers the complete Kollmorgen portfolio of drives and motors. The AKM motors offered with the NI 9502 are customized with a different winding configuration for the 24V supply voltage of the NI 9502 C series drive module. They are also offered with a quadrature encoder (Comcoder option 2-) instead of SFD feedback. As you read in the getting started manual, the NI 9411 is the preferred module to read this encoder feedback. Yes, the hall sensors are read back directly on the NI 9502 module, and the encoder is read back on the NI 9411. 

NI choose to implement hall sensors on the NI 9502 module and to implement feedback with the comcoder option and the NI 9411 as the recommended way to use the NI 9502. You can see examples showing Trapizoidal commutation and FOC commutation from the examples that install with SoftMotion. You will see that all the drive control code for the NI 9502 is implemented in the cRIO FPGA. Because of this, you could implement various different control schemes just using the current sense from the 9502, or combining with the halls to do a wake and shake algorithm, etc....

If you are using the NI 9502 module, you will not be able to take advantage of Kollmorgen's proprietary SFD feedback scheme. If you are using an AKD drive, I would highly recommend the SFD feedback option for the AKM motor you select. 

Best Regards,

Nate

[Ansari07]

Hi Nate,

Thank you for your valuable input.

My initial goal is to implement the trapezoidal commutation, utilizing low resolution Hall effect sensors. If I choose not to go with NI 9502 C series, and select an AKD drive instead, can I use a general digital input module to read in my Hall feedback and implement in my controller?

 Regards,

Ansari

P.S. I totally acknowledge your advice of using the SFD along with AKD drive, however my goal in this case is to establish a case for using the low resolution Hall sensors when high resolution encoder feedback is not available.

[Nate.H]

Hi Ansari07,

In that case, I would go with the NI 9502. The AKD will implement all the control loops in firmware on the drive (position, velocity, torque). When you buy the AKD, you are buying a ready-to-go-off-the-shelf drive. You can put the drive in torque mode and control with higher level loops by sending a +-10V analog command through a module such as the NI 9514 drive interface module. If you are looking to strip down the system components and do something simpler, you'll be better served with the NI 9502, where you can implement your own algorithms, as simple or complex as needed.  

Best Regards,

Nate

[Ansari07]

Hi Nate,

The limitation with choosing NI 9502 is that it only supports six AKM models that are all low power, low torque ratings. Whereas, I am looking to control a drive rated around 1kW. AKD is the only other alternative I could see from NI website. All I need is the ability to control the power electronic switches inside the drive according to a custom generated control signal. Please refer to the example below:

I implemented a simplest example of Hall feedback based control of a trapezoidal bldc motor in Simulink. Here, the controller senses the Hall feedback and generates six PWM signals based on the Hall feedback. Please refer to the figures below.

Figure 1 - Simplified Drive Schematic

Figure 2 - Rotor position, hall feedback signals (H1,H2,H3) and drive switch (Q1-Q6) states

So for example, if the sequence number is 0, the rotor is in position [ 0 - 60 deg ], the Halls will feedback [ 1 0 0 ] and PWM signal to be sent out should be [Hi , Lo , Lo , Hi , Lo , Lo] for the six switches [Q1 , Q2 , Q3 , Q4 , Q5 , Q6] (Hi being the 'switch on' position).

Can something like this be implemented on an AKD drive using NI 9514?

Best Regards,

Ansari

[Ansari07]

Sorry bad link. This is the link what I wanted to insert.NI 9502 C Series

[Nate.H · Accepted solution]

Hi Ansari07,

I don't believe this level of control is possible with the AKD. The drive is meant to close it's own current loop onboard to control commutation and phase outputs. The AKD offers torque, velocity, or position control modes, and you can talk to it over Ethernet to the service port, over a field bus such as EtherCAT or CANOpen, or with a +-10V analog signal, or with step/dir commands. 

I don't beleive you can bypass all of the internal firmware in the AKD and get direct control of how current is switched through the phases. 

Best Regards,

Nate