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RESOLVED - Calculating Current Loop Gains Manually
#1
Hi,
I wanted to calculate the current loop gains for my stepper empirically according to the instructions given in the Power PMAC User's Manual from page 270 onwards.
Therefore I need I_sat. And to get I_sat I need to know K_c, the current sensor gain.

So my question is: - How do I get K_c or how can I derive it from measurements,

best regards,
hannsx
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#2
This is a specification of the current sensor used as the feedback element in the Current Loops, typically two of three phases on the amplifier output.
For instance, an ODT GPH-052 amplifier shows 16.26 amps (dc) full scale w/ 12bits resolution.
This value is obtained from the amplifier documentation.
It is implied by your question that you are closing the current loops in PPMAC - using an ODT amplifier - GPH/L, or Brick, or Copley (?) with Direct PWM interface.
If you are using a 'current mode' amplifier (eCat, analog +-10v, Yaskawa, etc) then the current command comes from PPMAC but the loop is closed in the amplifier.
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#3
I work with a PowerBrickLV with a maximum ADC current of 33.85A and yes, I close the current loop in the PowerBrickLV. The Maximum Voltage is 80V - Is that equal to the Maximum the ADC can read? If I got it right the Resolution of the ADC is 16 bits.
Quote from Manual:
"I_sat Maximum (saturated) current reading from phase-current A/D converter (Amps).
This is a DC value, not an RMS AC value.
This value can be derived from the current-sensor gain K_c (volts/amp) and the maximum
voltage in volts that the A/D-converter can read V_cmax: I_sat= V_cmax/K_c"
Is it as simple as this:
K_c = 80V / 33.85A ?
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#4
Not exactly
The 80v you refer to is the DC Bus voltage for your motors, correct?
The current sensors read phase current and output a DC voltage signal proportional to the current. They are not concerned with motor voltage at all.
I looked briefly and didn't find what I was looking for, but lets just say the output of the current sensor is +-10v (I may be wrong, perhaps it is +-12 or 15 or ???). The gain of the sensor would then be 10v/33.85A.
The voltage output of the sensor is then digitized and sent as current feedback to the control loops.
Note that what this means is that the control loops don't care what the actual current is; they only care where it is in the range of -max|0|max+.
Certainly the output transistors and heat sink care about actual amperage though :-)

But I have to ask why you wish to manually make this calculation? It is generally considered optimum to use the tuning tools and tune this by hand. The manual calc will give a theoretical result that could be considered a starting point.
It can be argued that this is one area where ODT allows the user to exceed other systems. Most suppliers use a set of 'safe' current loop parameters in a 'one size fits all' approach. ODT provides the means to truly optimize the torque response of the motor-amplifier-cable-etc combination.
That said, I find the ODT technical manuals are excellent in providing the theory necessary to better understand their products. So good luck with your project, you're using the best controls available.
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#5
Ok - I see. So it is only the current sensor.
I took the 80V from the maximum allowable Bus Supply Voltage.
I wish to fully understand what I am working with to be able to fit all the pieces together from the technical manuals etc.
I want to understand this in particual because I sometimes obtain current loop gains that don't work. In cases like that it would be helpful to have a manual estimate as to which values are sensible.

Is ODT short for Omron Delta Tau?
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#6
yes on the ODT
It may take a while but once you get a good understanding of the subsystems, the bigger picture will come together like an avalanche.
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