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Background:
I have a Lorenz-actuator (coil+magnet) that I need to control precisely using a computer. The computer signal is output from a dSpace microLab box as an analog ±10V signal. This signal needs to be scaled to a current of ±5A at roughly 24V. Given the phase shifting properties of coils as well as the underlying physics of Lorenz-actuators, the output current needs to follow the input signal. The system being controlled is highly dynamic and therefore requires a very accurate step response from the current controller. (Perfect tracking up to 200Hz) Four of these actuators need to be controlled so price and size are a factor to consider.

Attempted solutions:
Using a shaker current amplifier: Shakers cant handle any DC, therefore the amplifier has irremovable high-pass filters. My application requires some DC-component in the signal. The ones available to me are also very large and expensive.

Using a voltage amplifier and hoping for the best: Not getting the desired force trajectory from the actuator because current is not being controlled directly. Also can't tune the PID parameters of the control loop.

Using a current controller: I haven't been able to find any online that can drive a current of 5A dynamically. A constant current source is not useful for this application.

Using elmo servo drivers: The output current follows the input signal perfectly, however the drives are completely overkill. They are meant to take a digital signal and control the velocity, position and current of an actuator. Using them as an analog current amplifier is a waste of money.

Question:
Does anyone know of an off-the-shelf solution to control the current to the actuator? Or possibly a paper describing how to build a circuit that could amplify an analog current signal?

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  • \$\begingroup\$ I think it will he hard to find an off-the-shelf solution. Perhaps a solution using two voltage controlled current source (see daycounter.com/Circuits/Current-Servo/Current-Servo.phtml for example), one for positive current and the other for negative current could be good for your application. \$\endgroup\$ – Wheatley Oct 10 at 10:48
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    \$\begingroup\$ PA amplifiers in the kW range would output that current easily, maybe with enough voltage to track to 200Hz, and are relatively cheap. You may find one with a circuit diagram that allows you to remove the highpass coupling and get it to work to DC, then wrap a current sense loop round it. \$\endgroup\$ – Neil_UK Oct 10 at 11:26
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    \$\begingroup\$ A SubWoofer Amp ought to have the power and 500+ Damping factor needed. What is the DC coil Resistance? 2 OHms? Even an old Crown DC300 should work (audio PWr Amp) \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 10 at 12:25
  • \$\begingroup\$ To get Current to follow Voltage with <1% error, you sense voltage drop with 50mV Current shunt R =0.05V/5A=10mOHm then amplify by 20 to get 1A per volt input with a 2kHz BW for low phase error at 200 Hz. Heat loss for Linear drive needs 50W of cooling with CPU cooler. Use +/15V supply or +/14.2 battery power and Audio DC coupled power Amp. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 10 at 12:45
  • \$\begingroup\$ You might also consider something like this. \$\endgroup\$ – jonk Oct 10 at 14:50
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schematic

simulate this circuit – Schematic created using CircuitLab

Class D audio amplifier would be a good choice. The one that you can eliminate the blocking capacitor at the input. Your current requirement seems to be a close match for what an D class amp is made for.

But, the audio amp would amplify the voltage in certain relation. It compares input voltage and feedback voltage at amps output. Not a voltage to current converter.

You should put a shunt resistor in series to a load, then measure this current and close the loop with a P controller - supposing that you have successfully eliminated the DC blocking capacitor at the input. You would need an opamp, a small dual power supply and couple of resistors.

EDIT:

The current is measured with Rsh. A voltage drop is created on this resistor Vsh=I*Rsh. This voltage is then filtered with a low pass filter 1kohm + 100nF. Without R6 mounted, the loop gain is very high and could make the whole thing unstable, you should adjust the gain. You could use also one another opamp to separate the LP filter and gain setting.

IRAUDAMP9 no blocking capacitors at input signal

IRAUD

This is a schematics of IRAUDAMP with IRS2092, you will find many cheap boards with this setup, most of them require strictly the voltage power supply as declared. Instead of the speaker, you should attach your load. The ground has to be the same for IRAUDAMP and for the P controller.

schematic

simulate this circuit

Fixed LP filter and separate gain setting:

schematic

simulate this circuit

EDIT2 :

Supposing the input and output voltages are out of phase by 180 degrees. You can see that both are summed by inverting input of opamp in IRS2092, so the conclusion is that they are shifted, and so the current is also shifted. The P controller has to get new shape:

schematic

simulate this circuit

You might need to attenuate the +/-10V setpoint value to a lower value also to fit to the amplifier input voltage:

schematic

simulate this circuit

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  • \$\begingroup\$ but on your schematic, because of the filter, the shunt resistor is not in series with the load ? Can you explain the circuit pls \$\endgroup\$ – Wheatley Oct 10 at 14:02
  • \$\begingroup\$ @Wheatley OK, I shall draw a new schematics. \$\endgroup\$ – Marko Buršič Oct 10 at 14:10
  • \$\begingroup\$ thank you very much \$\endgroup\$ – Wheatley Oct 10 at 15:16

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