SPINFamily Evaluation Software - BEMF Calculation

[Cooke.Ernest]

Posted on April 18, 2017 at 14:07

Hi,

When I am using the BEMF compensation configuration within the SPINFamily Evaluation Software, I input the Application Parameters and the Motor Parameters for my particular motor. 

When I press the �Evaluate� button at the bottom right of the menu, the following Device Configuration Values are calculated:

KVAL_HOLD

KVAL_ACC

KVAL_DEC

KVAL_RUN

ST_SLP

INT_SPEED

FN_SLP_ACC

FN_SLP_DEC

These calculated values can then be written to the device and can be seen in the Phase current control tab within the �Device Configuration� menu.

 

How are the Device Configuration Values calculated from the values that I have inputted?

Is there a particular calculation that must be used?

Kind Regards,

Ernest

#dspin #dspin-gui #powerstep01 #bemf #pcc009v2-eval6470h-spinfamily

[Enrico Poli]

Posted on April 19, 2017 at 15:09

Hi,

the calculations performed by the tool are the ones described in details into the

http://www.st.com/resource/en/application_note/dm00061093.pdf

.

[David Walker]

Posted on April 19, 2017 at 18:55

I came up with an alternative way to compute the motor parameters. It is here:

https://community.st.com/0D50X00009XkW2fSAF

[Cooke.Ernest]

Posted on April 20, 2017 at 14:39

Hi Enrico,

Thank you very much for the information you have sent me in relation to the calculations.  I found them to be helpful.

There are some further questions I would like to ask. 

From the values I have entered I was able to calculate the Hold duty-cycle, Intersect Speed & Starting Slope perfectly.

I got a value of 30% for the acceleration, deceleration and run duty cycles instead of 46.5% (See below).

However I was unable to calculate the Final Slope Values (Acceleration & Deceleration).  

Is it possible you could find the values which I am unable to calculate properly.  The parameter values to input are shown below.

Thanks,

Ernest

[Enrico Poli]

Posted on April 26, 2017 at 14:43

Did you considered the conversion between rms current and peak?

Can you write your calculation?

[Cooke.Ernest]

Posted on May 03, 2017 at 10:44

Hi Enrico,

Sorry for my slow reply.

So these are my input values for the calculations

Supply Voltage (Vs)

12

Holding Current

0.2

Running Target Current

1.2

Electric Constant (Ke)

0.055

Phase Inductance (mH)

2.8

Phase Resistance (Ω)

3.3

Steps/Revolution

200

Acceleration/Deceleration/Run Duty Cycle

3.3*(1.2/12)*100 = 33

When you mention rms current to peak, should I just simply multiply my value by 

√2, such as below:

(3.3*(1.2/12)*100)

*√2

= 46.67

[Enrico Poli]

Posted on May 03, 2017 at 13:54

Correct. The tool calculating the parameters look for the

rms current

, but the formula implies

peak current

value.  So the conversation factor sqrt(2) should be applied. For holding current, considering the current is constant and not sinusoidal, the rms and the peak are exactly the same.

[Cooke.Ernest]

Posted on May 08, 2017 at 12:33

I forgot to show you my calculations for the

 Final Slope Values (Acceleration & Deceleration)

These are my input values for the calculations

Supply Voltage (Vs)

12

Holding Current

0.4

Running Target Current

1.5

Electric Constant (Ke)

0.0262

Phase Inductance (mH)

2.6

Phase Resistance (?)

2.4

Steps/Revolution

200

Acceleration/Deceleration Final Slope Equation

(2*PI*2.6*1.5+(0.0262/(200/4)))/(12)=2.04%

As you can see from the GUI below, the value calculated is 0.127%

I am using the following equation from page 10 of AN4144

Could you please tell me what mistake I am making with this equation?

Kind Regards,

Ernest

[Enrico Poli]

Posted on May 12, 2017 at 10:05

The correct formula should be:

(2 * pi * Lm *

Ipeak

+ Ke) / 4 / VBUS = (2 * pi * 2.6e-3 *

1.5 * sqrt(2)

+ 0.0262) / 4 / 12

In your formula I see following discrepancies:

• The current is RMS instead of peak
• You are using a Lm of 2.6 H, not mH
• You are adding some extra conversion parameters in the Ke. It is right only if you measured the Ke as V/rps, but the tool expect the ke as V/Hz (where Hz is the electrical frequency of the motor, not mechanical)