I am working on a project related to online auto tuning of a BLDC motor (speed control). The rating of the motor is 24V, 100W, 1500rpm. For this,I need to formulate a control loop to limit the current to safe values. The speed loop comes outside the current loop.

To measure the current in the three phases, I have put a 0.1 ohm 10W resistor in series with the phases. The voltage drop across the resistors is measured with an AD-620 instrumentation amplifier from Analog Devices. The problem is that the output voltage is showing me only positive values, even though current flows in either direction through a phase. The Ad620 IC has been given a supply voltage of +-10V. If I reverse the input, then it is giving me only negative values. This same problem is there in the other three phases as well.

I would be grateful if anyone comes forward with a solution.

Thanks and regards,
Control_expert

 

It's hard to say without seeing your circuit, but it sounds a lot like you are taking one or both of the inputs outside the common mode voltage range.

If the driver and the amps share a common ground, you might put your sensing resistors in the low side. The idea is to keep the amp inputs inside it's power supply rails or common mode voltage range if that is lower. They really should share a common ground or else you have no reference for the CMV.

Regards
cww an itinerant automation guy.

 

Thank you for your prompt reply....I was out of station for a couple of days..hence could see it only today....

I am sorry that I could not understand your statements. As far as the circuit is concerned, the instrumentation amp is a 8-pin amp. Two of the pins accept the V+ and V- differential inputs. Two pins accept the +- supply voltages. There is a resistor across two of the other pins for gain adjustment. And finally, two pins provide the output with respect to a reference pin. What I have done is that I have tied this reference pin to the analog ground. Thus, my amp, the power supplies and the motor controller share the same ground.

The max ratings of the instrumentation amp AD620 are as follows:
Supply voltage :+-18V
Input Voltage (Common mode): +-Supply voltage
Differential input voltage: 25V

The maximum current that will be allowed by the controller is 6A. Right now, the motor is put on no load and thus it is drawing a current of only 0.5 A. The gain of the amp is set to 23.45. This means that 0.5A flowing through a 0.1 ohm resistor produces a voltage drop of 0.05V, which is amplified by 23.45, giving an output voltage of around 1.7V. I am getting this 1.7V in the positive direction, but 0 V when the current is supposed to flow the other way.

Control_expert

 

Another difference amplifier from Analog - AD629 - maybe strongly recommended instead AD620 used by you. This IC can overcome key problem you have - low common mode rejection ratio even for your application with 24VDC supply.

 

Thanx....

 

Hi

What I am saying is that if you measure the voltage to ground from either end of your current sense resistors, if it is more than the amp's power supply voltage, you have exceeded the common mode voltage limit and the amp will at best, do funny things. This is unless you have voltage dividers that lower the voltage at the actual inputs. I'm sorry I missed this, but I'm trying to get employed and must have skimmed past it. I also use the email version of the list which doesn't thread things conveniently.

Regards,
cww

 

Isn't so that an AD 620 can be powered by both plus and minus? Then it should give negative values also!