Closed Loop Current Sensor: Magnetics Question

Thread Starter

llamadave

Joined Oct 17, 2016
2
Hi There,


I have been building a closed loop AC/DC current sensor, with the end goal to detect 500 mA of current at the full scale output. I have successfully designed circuits from 25A to 400A with acceptable error. The holy grail, though, is low-level current measurement, with a high degree of accuracy. It appears I have figured out much of the electronics, so this is more about the physics of the magnetic circuit, for which I am undoubtedly less skilled.

For a depiction of the high-level circuit I am talking about, see here: https://upload.wikimedia.org/wikipedia/commons/thumb/c/c4/Closed_loop_hall_effect_current_sensor.svg/431px-Closed_loop_hall_effect_current_sensor.svg.png.

At a sensor sensitivity of 5A, a problem emerges that I am hoping someone could help me with. The present configuration of the sensor is so sensitive that the measurement is notably disturbed by:

1) Bringing steel/ferrous objects near the sensor. I found that waving a pair of pliers in front of the core could cause the circuit to completely saturate (Vout >= Vmax)

2) Merely rotating the sensor 90 degrees (presumably into/out of the Earth's magnetic field) causes as much as 500mV of error in the quiescent offset voltage (Voq = Vout @ (Ip = 0A))!

Does anyone have any guidance on things to try?

1) The present magnetic core is made out of a ferrous (iron-based) material called "kool mu." That is: 85% Iron, 9% Silicon, and 6% Aluminum alloy. Would using a ferrite (NiZn or MnZn) reduce or eliminate the problem? Any other material suggestions? I'm not sure that this would, as any magnetic material would presumably respond undesirably, too, but I figured I'd ask.

2) Reducing the diameter of my magnetic core. Right now the inner diameter is 1.2". Should I expect lesser coupling to the external environment using a diameter of say, 0.8", or 0.5"?

3) reducing the ratio of primary to secondary turns. Right now I am at n = 1000. Presumably, reducing to n = 100 would reduce external, coupled magnetic fields by a factor of 10?

4) ???


Thanks in advance for any suggestions you may have!


Dave
 

ebeowulf17

Joined Aug 12, 2014
3,274
I'm not entirely clear on what you've got going so far. That image looks to me more or less like the usual current transformer setup (except for the amp symbol?)

If it's a current transformer, then I don't think you can do DC measurements with it, but AC from 50A down to tens of mA with one transformer is pretty straightforward. I played with them for the first time pretty recently and was really impressed. I've got one each of the following two transformers:

http://www.crmagnetics.com/high-frequency-current-transformers/wire-lead/voltage-output/solid-core/cr8448

http://www.crmagnetics.com/ansi-metering-class-current-transformers/wire-lead/voltage-output/solid-core/cr8449

http://www.crmagnetics.com/Assets/ProductPDFs/CR8400 Series.pdf

Just add a burden resistor and connect it to meter, oscilloscope, microcontroller, etc. Surprisingly easy, and seems to be remarkably accurate over a wide range.

For non contact DC current sensing, I'm at a loss. I know it's done, but I don't understand it yet. If that's what you're up to, best of luck, and maybe I can learn something from you!
 

Thread Starter

llamadave

Joined Oct 17, 2016
2
ebeowulf17, yes, thank you! I suppose I should have been more clear for folks not familiar with the rather niche application. So here's a more detailed explanation of what's going on.

So those CR magnetics CTs are nice, very accurate and are capable of sensing very low level currents, due to the high primary to secondary turns ratio. But they only measure AC. As an alternative, the closed-loop current sensor actually uses the hall effect to sense a magnetic field displacement in the core (both AC and DC). Further, the "closed-loop" feature actually refers to a feedback circuit that is used to reduce the nonlinear effects of temperature, DC bias, and various other imperfections. The primary components of a closed-loop current sensor are: 1_ a magnetic concentrator (such as a gapped, toroidal core), 2) a magnetic sensing element (such as hall effect sensor, or "field probe"), 3) a gain stage, and 4) a secondary compensating or sensing coil (with driver circuit) that provides feedback current to cancel the magnetic field in the magnetic core, caused by the primary current. The goal of the feedback is to completely cancel the field in the core, thereby preventing large hysteresis errors, and enabling a high degree of accuracy without allowing the core to saturate (even with very high primary currents). The secondary coil current is passed through a precision shunt resistor to convert the current to a voltage signal as the output. The general relationship of the closed-loop sensor, given an amplifier with sufficiently high open-loop gain is used, is: Vout / Ip = Rs / N, where:

Ip = input current (A)
Rs = shunt resistance (ohm)
N = primary to secondary turns ratio
Vout = output voltage (V)

One final thing I might add, is that the closed-loop sensor also has a bandwidth from DC up to many 10's of kHz, or possibly more, due to a phenomenon called the "CT" effect. What happens is that at higher frequencies the secondary current is actually directly generated from the magnetic coupling to the primary circuit, rather than the current being generated by the hall-effect sensor and feedback loop. In all, the closed-loop hall effect sensor is extremely accurate, and versatile, but certainly more complicated to design than a CT or open-loop hall effect sensor.

My questions on how to eliminate the external magnetic interference still apply. Today I will try changing the number of CT turns, as well as the coil material (I have NiZn core laying around), and we'll see what happens. Let me know if you think of anything else!

Regards,
Dave
 

ebeowulf17

Joined Aug 12, 2014
3,274
ebeowulf17, yes, thank you! I suppose I should have been more clear for folks not familiar with the rather niche application. So here's a more detailed explanation of what's going on.

So those CR magnetics CTs are nice, very accurate and are capable of sensing very low level currents, due to the high primary to secondary turns ratio. But they only measure AC. As an alternative, the closed-loop current sensor actually uses the hall effect to sense a magnetic field displacement in the core (both AC and DC). Further, the "closed-loop" feature actually refers to a feedback circuit that is used to reduce the nonlinear effects of temperature, DC bias, and various other imperfections. The primary components of a closed-loop current sensor are: 1_ a magnetic concentrator (such as a gapped, toroidal core), 2) a magnetic sensing element (such as hall effect sensor, or "field probe"), 3) a gain stage, and 4) a secondary compensating or sensing coil (with driver circuit) that provides feedback current to cancel the magnetic field in the magnetic core, caused by the primary current. The goal of the feedback is to completely cancel the field in the core, thereby preventing large hysteresis errors, and enabling a high degree of accuracy without allowing the core to saturate (even with very high primary currents). The secondary coil current is passed through a precision shunt resistor to convert the current to a voltage signal as the output. The general relationship of the closed-loop sensor, given an amplifier with sufficiently high open-loop gain is used, is: Vout / Ip = Rs / N, where:

Ip = input current (A)
Rs = shunt resistance (ohm)
N = primary to secondary turns ratio
Vout = output voltage (V)

One final thing I might add, is that the closed-loop sensor also has a bandwidth from DC up to many 10's of kHz, or possibly more, due to a phenomenon called the "CT" effect. What happens is that at higher frequencies the secondary current is actually directly generated from the magnetic coupling to the primary circuit, rather than the current being generated by the hall-effect sensor and feedback loop. In all, the closed-loop hall effect sensor is extremely accurate, and versatile, but certainly more complicated to design than a CT or open-loop hall effect sensor.

My questions on how to eliminate the external magnetic interference still apply. Today I will try changing the number of CT turns, as well as the coil material (I have NiZn core laying around), and we'll see what happens. Let me know if you think of anything else!

Regards,
Dave
Thanks so much for the background info.

Although I really love magnetism, and Hall Effect sensors in particular, I'm way out of my depth here.

I've gotten help on several previous magnetism-related projects here from a number of people, including @bertus , @MaxHeadRoom , @MrAl , @OBW0549, and many others. Hopefully someone who's better at this stuff will have some insights. I'll be watching this thread to see what I can learn!
 

MrAl

Joined Jun 17, 2014
7,762
ebeowulf17, yes, thank you! I suppose I should have been more clear for folks not familiar with the rather niche application. So here's a more detailed explanation of what's going on.

So those CR magnetics CTs are nice, very accurate and are capable of sensing very low level currents, due to the high primary to secondary turns ratio. But they only measure AC. As an alternative, the closed-loop current sensor actually uses the hall effect to sense a magnetic field displacement in the core (both AC and DC). Further, the "closed-loop" feature actually refers to a feedback circuit that is used to reduce the nonlinear effects of temperature, DC bias, and various other imperfections. The primary components of a closed-loop current sensor are: 1_ a magnetic concentrator (such as a gapped, toroidal core), 2) a magnetic sensing element (such as hall effect sensor, or "field probe"), 3) a gain stage, and 4) a secondary compensating or sensing coil (with driver circuit) that provides feedback current to cancel the magnetic field in the magnetic core, caused by the primary current. The goal of the feedback is to completely cancel the field in the core, thereby preventing large hysteresis errors, and enabling a high degree of accuracy without allowing the core to saturate (even with very high primary currents). The secondary coil current is passed through a precision shunt resistor to convert the current to a voltage signal as the output. The general relationship of the closed-loop sensor, given an amplifier with sufficiently high open-loop gain is used, is: Vout / Ip = Rs / N, where:

Ip = input current (A)
Rs = shunt resistance (ohm)
N = primary to secondary turns ratio
Vout = output voltage (V)

One final thing I might add, is that the closed-loop sensor also has a bandwidth from DC up to many 10's of kHz, or possibly more, due to a phenomenon called the "CT" effect. What happens is that at higher frequencies the secondary current is actually directly generated from the magnetic coupling to the primary circuit, rather than the current being generated by the hall-effect sensor and feedback loop. In all, the closed-loop hall effect sensor is extremely accurate, and versatile, but certainly more complicated to design than a CT or open-loop hall effect sensor.

My questions on how to eliminate the external magnetic interference still apply. Today I will try changing the number of CT turns, as well as the coil material (I have NiZn core laying around), and we'll see what happens. Let me know if you think of anything else!

Regards,
Dave
Hello,

The primary operating principle of the hall effect current sensor is flux balance, as i think you said but i'll point out here.

The primary winding has the current to be measured and is usually just one or two turns, and the current could be several amps. The secondary winding has many turns and the current direction is opposite in polarity to the primary current, and can be much lower because of the large number of turns. The secondary current 'Is' balances the flux in the core through N*Is while the primary has current 'Ip', and the flux is balanced when N*is=Ip assuming one turn on the primary. The HED senses the flux and provides feedback and the amplifier drives the secondary winding current. When the flux gets close to zero, the secondary current no longer increases as then N*Is=Ip. The current measured is then equal to N*Is. It's a negative feedback system.

If an external magnetic field interferes with this process, then the device either has to be shielded or electrically balanced again. Shielding could be hard to do but the main idea is to provide a low magnetic impedance path around the object. To electrically balance, a second HED device is used to sense the interfering field and provide a signal that can be subtracted from the original. That would require a second HED device oriented the right way and a adder/subtractor op amp circuit, or it may be able ot be incorporated right into the HED error amplifier. The advantage of the second HED device is it also helps to cancel out effects of temperature change on the original HED device.

So there are a couple ideas you might find useful.
 
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