DC Brushed Motor Current Sensing (and Limiting)

A

Andrew

Guest
Hi,

I have a quick question regarding current sensing in DC motors. I know
this topic is beaten to death so I will make this quick.

I need to get the current reading into my microprocessor A/D (0-5V). I
want to sense up to 15A. I have heard of many ways, but one seems
easiest to me, the MAX4372H current amplifier. It seems that with a
properly sized sense resistor, I would be able to sense the current in
the fashion that I would like.

I have one question though. Since the motor I am working with reverses
direction, will this cause a problem with the current sensing using
this chip? I want to make sure that having a negative voltage across
the RS+ to RS- will perform the same as a positive one from RS+ to RS-
as shown in the typical application here
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2104/ln/en. The spec
sheet is found here
http://pdfserv.maxim-ic.com/en/ds/MAX4372F-MAX4372T.pdf, and describes
the internal working of the chip. It seems that it would function
properly, but I want to make sure.

One more thing, just to clarify, if I am using this chip with my motor
driver, how would it be connected? When looking at the typical
application, I am guessing that the 24V battery (it is a 24V motor)
will be connected to Vcc, and the "Vin" will be the output of the motor
driver (M+), and the "load" will be the motor, and on the other side of
the load, where in the typical application it is ground, there will be
the M- fromt he motor driver?

As I mentioned, the voltage developed across the RS+ and RS- (sense
resistor) will be both positive and negative depending on motor
direction, and this is my main concern for this setup, to ensure that
this is possible. If not, are there any similar circuits that I could
use?

Thanks for any help!
 
Andrew wrote:
Hi,

I have a quick question regarding current sensing in DC motors. I know
this topic is beaten to death so I will make this quick.

I need to get the current reading into my microprocessor A/D (0-5V). I
want to sense up to 15A. I have heard of many ways, but one seems
easiest to me, the MAX4372H current amplifier. It seems that with a
properly sized sense resistor, I would be able to sense the current in
the fashion that I would like.

I have one question though. Since the motor I am working with reverses
direction, will this cause a problem with the current sensing using
this chip? I want to make sure that having a negative voltage across
the RS+ to RS- will perform the same as a positive one from RS+ to RS-
as shown in the typical application here
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2104/ln/en. The spec
sheet is found here
http://pdfserv.maxim-ic.com/en/ds/MAX4372F-MAX4372T.pdf, and describes
the internal working of the chip. It seems that it would function
properly, but I want to make sure.

One more thing, just to clarify, if I am using this chip with my motor
driver, how would it be connected? When looking at the typical
application, I am guessing that the 24V battery (it is a 24V motor)
will be connected to Vcc, and the "Vin" will be the output of the motor
driver (M+), and the "load" will be the motor, and on the other side of
the load, where in the typical application it is ground, there will be
the M- fromt he motor driver?

As I mentioned, the voltage developed across the RS+ and RS- (sense
resistor) will be both positive and negative depending on motor
direction, and this is my main concern for this setup, to ensure that
this is possible. If not, are there any similar circuits that I could
use?

Thanks for any help!
Click to expand...
Put the sense resitor in series with the power feed instead of the motor
itself. This will give the same polarity regardless of which way the
motor is running.

This circuit measures only a threshhold, but an op amp connected to the
resistor would give a measurable voltage to use to determine current
thru the motor.

http://members.cox.net/berniekm/toycar1.gif

--
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm
 
The only problem with doing this through the power source is that the
motor driver is sourcing two motors (in parallel), meaning that I would
get the current draw for the total of the two. I really need it to be
for each motor. What would happen with the MAX 4372H chip if I put it
in series with the motor? When the voltage at the sense resistor
swings negative, will this cause a problem?
 
Andrew wrote:

The only problem with doing this through the power source is that the
motor driver is sourcing two motors (in parallel), meaning that I would
get the current draw for the total of the two. I really need it to be
for each motor. What would happen with the MAX 4372H chip if I put it
in series with the motor? When the voltage at the sense resistor
swings negative, will this cause a problem?
Click to expand...
Sorry, I'm not familiar with that chip.

--
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm
 
In message <1110464251.164443.89790@z14g2000cwz.googlegroups.com>,
Andrew <xxragexx@gmail.com> writes

(snip)
One more thing, just to clarify, if I am using this chip with my motor
driver, how would it be connected? When looking at the typical
application, I am guessing that the 24V battery (it is a 24V motor)
will be connected to Vcc, and the "Vin" will be the output of the motor
driver (M+), and the "load" will be the motor, and on the other side of
the load, where in the typical application it is ground, there will be
the M- fromt he motor driver?
Click to expand...
I take it these are lead-acid batteries you are using. If so, you might
find the IC dies from over-voltage. The maximum rated voltage is 28
Volts and a 24 Volt battery can exceed that at times.

My approach would be to amplify the volt drop across a section of the
supply cable in a similar way to Luhan Monat's circuit. He is running at
much lower power so he has used a 1 Ohm resistor. At 15 Amps a short
length of cable should drop a few milliVolts which ought to be usable.
--
Graham
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top