Maintain constant current

[ethen99]

HI I would like to use NI Daq to produce current output that corresponds to desire magnetic field strength, then I would like to maintain the desired magnetic field strength constant in either magnitude, phase or frequency in a closed loop system.

Could you please let me know if I need to  implement a PID control algorithm or any specific control design toolkit in NI LabVIEW for this project?

Thanks

[johnsold]

The first half of your first sentence sounds like you want to use LV and DAQ AO to generate a setpoint. The second half sounds like a description of a power source which can generate the necessary current for a magnet to maintain the field at the desired setpoint.  It also appears that you wish to be able to select one parameter (magnitude, phase, or frequency) to regulate. Is this an accurate assessment of your statement?

1. It should be relatively easy to generate a signal to use as a setpoint.

2. Unless your magnet is very small, the magnet power source may need to be very specialized.  It is unlikely that standard DAQ devices will be adequate.

3. Until you specify the range of the field parameters, the electrical chracteristics of the magnet, and the equipment you will be using to measure the magnetic field strength, no one can give you meaningful advice about the control or feedback system.

Lynn

[ethen99]

yes . I would like to generate a current setpoint using DAQ and Power supply. As you mentioned this is the necessary current for a magnet to maintain the field at the desired setpoint. ( magnitude, phase or frequency) so I will use Multimeter to measure current and Gaussmeter to measure the strength and direction of the magnetic field for the closed loop system to make sure I am following the current and field strength set points

the equipments I am going to use are:

Power Supply
HP6652A
20V, 25A

---

Multimeter
HP3478A

---

Gaussmeter
Hirst 908

and NI DAQ

I know the relationship between the magnetic flux density and current.

I have the LV driver of all of them

Could you please advice me about the control or feedback system

[johnsold]

This looks like a DC system, so I do not understand your references to phase and frequency in your first post.

Do you know the inductance and resistance of your magnet? How fast do you want to change the field strength? What are the principal causes of drift for your fields?

All of those can affect the choice of a control system.

The power supply appears to be controlled by GPIB.  What does the DAQ have to do with the setpoint? -- OK.  I found a reference in the specs for "modulation" which allows analog programming.

Lynn

[ethen99]

To be honest I don't have the specs and requirment list but I think the program should be able to produce current output(s) that corresponds to a desired AC/DC
magnetic field strength (magnitude, phase, & frequency)

I don't know the  the inductance and resistance of the magnet but I can provide this information later.

and you are right the powersupply will be controlled by GPIB but I am not sure about the DMM and Gaussmeter

Could you please let me know what would be your advice for the control system based on the provided information?

As I mentioned the design should be

1- Able to produce current output(s) that corresponds to a desired AC/DC
magnetic field strength (magnitude, phase, & frequency)
2-  Able to maintain the desired magnetic field strength constant in either
magnitude, phase or frequency ( closed loop feedback)

[johnsold]

I only looked at the power supply specifications briefly but it appears that the GPIB control will only be able to set DC or very slowly changing values. The analog input was described as a "modulating" input but did not indicate bandwidth.

I would start with a very simple system. Set current to some value. Read Gaussmeter. If field strength is not what you want increase or decreas current accordingly. Let it run for a time long enough to see whether it is stable or has drift problems. This will be a simple proportional control. Then think about adding modulation to get the AC behavior you want.

My suspicion is that you will need to run some tests to characterize the frequency response of the equipment before you can design a control system.

Lynn