As part of a project, I am using a h bridge motor driver ( http://www.mouser.com/ds/2/427/sip2100-519775.pdf ) to drive my magnetorquer, which is essentially a solenoid within a metal coil( The magnetorquer has a resistance of 30Ohms and an inductance of 0.15H). The attached images show my connection.
I am powering the chip with a 5V DC supply ( represented by the batteries in the image ). I also have a digital multimeter(DMM) to measure the current through the magnetorquer, the positive lead of my DMM is connected to the Pin OutA of the motor driver, and the negative lead is connected to the magnetorquer, thus making this a series connection to measure current. I will be controlling the input pins (InA and InB) using PWM signals from my arduino. As a test, I am just using a simple arduino program that varies the duty cycle of the pwm output in Pin 5, and sets Pin 6 to digital low. The bottom left pin of the motor driver is pin 1.
So here's the issue:
The current passing through my magnetorquer is significantly lower than what is expected. Eg, when I set my duty cycle to 50% , the average voltage is 2.5V , so theoretically 2.5/30 = 83mA of current should be going through my magnetorquer, however, I am observing that only 53mA of current goes through the magnetorquer, as measured by my DMM. Also, once the duty cycle is increased past this point, the current seems to have saturated and does not change much, and at certain duty cycles past 50% even goes lower, to about 40+mA.
I tried connected my magnetorquer coil directly to my DC Supply ( Set to 2.5V DC ), and it really does draw around 80mA of current. So I am quite sure that this current drop is due to my motor driver.
What I suspect:
My arduino uno has a pwm frequency of around 800Hz. I'm guessing that this drop is current is probably due to the switching losses by the mosfets in the motor driver. But at 800Hz, I doubt it should be this significant?
Or are there other sources of power losses that I am missing out? And if so is there anyway to reduce these losses to increase the current through my magnetorquer?
Thanks!
I am powering the chip with a 5V DC supply ( represented by the batteries in the image ). I also have a digital multimeter(DMM) to measure the current through the magnetorquer, the positive lead of my DMM is connected to the Pin OutA of the motor driver, and the negative lead is connected to the magnetorquer, thus making this a series connection to measure current. I will be controlling the input pins (InA and InB) using PWM signals from my arduino. As a test, I am just using a simple arduino program that varies the duty cycle of the pwm output in Pin 5, and sets Pin 6 to digital low. The bottom left pin of the motor driver is pin 1.
So here's the issue:
The current passing through my magnetorquer is significantly lower than what is expected. Eg, when I set my duty cycle to 50% , the average voltage is 2.5V , so theoretically 2.5/30 = 83mA of current should be going through my magnetorquer, however, I am observing that only 53mA of current goes through the magnetorquer, as measured by my DMM. Also, once the duty cycle is increased past this point, the current seems to have saturated and does not change much, and at certain duty cycles past 50% even goes lower, to about 40+mA.
I tried connected my magnetorquer coil directly to my DC Supply ( Set to 2.5V DC ), and it really does draw around 80mA of current. So I am quite sure that this current drop is due to my motor driver.
What I suspect:
My arduino uno has a pwm frequency of around 800Hz. I'm guessing that this drop is current is probably due to the switching losses by the mosfets in the motor driver. But at 800Hz, I doubt it should be this significant?
Or are there other sources of power losses that I am missing out? And if so is there anyway to reduce these losses to increase the current through my magnetorquer?
Thanks!
