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Motor Load detection

Gr3mlin

Jul 29, 2011
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Hi there, I have been asked to make up a very simple, yet efffective load detection circuit for a motor, 12/24v DC, the motor extends and is timed to how long it is running for, but the resistance against this motor, sometimes alters the distance it extends.
So i though, the motor has to come to rest and its home and fully extended stops.
If the motor is prevented from moving, it will then increase the current requirement.
if this could then be detected, the timing feature can be bypassed.

Any ideas on how this could be achieved? i had the though of a low ohm resistor inbetween the supply and motor, and messuring the difference on either side. but sorta got stumped at how to use this.

Thanks.
Chris.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Yes measuring the current is probably a good way to go.

You may be able to use an analog hall effect sensor which will give you the advantage of a completely isolated signal. This is handy where the motor runs off a high voltage or a very noisy source.
 

poor mystic

Apr 8, 2011
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Hi
The way you'd use your resistor is by measuring the voltage that gets developed across it when current flows through it.
You'd connect that voltage, and a stable voltage reference on a potentiometer, to a comparator like an LM311. When current got too high, the output of the LM311 would switch, and this could be used to switch the circuit off.
I really like Steve's idea though. It's usually a good thing in this sort of work to do the same thing twice over in different ways, and his alternative method of checking to see that the motor is moving is very good, if not even better than the resistor.
 

duke37

Jan 9, 2011
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A resistor and comparator can detect when the motor is stalled but the current will be turned back on unless a latch of some sort is employed. The latch would need to be reset when the power is removed.
If the motor is reversed by reversing the polarity of the supply, then this will also need to be taken into account.
A bit more information on the details would be a help.
 

daddles

Jun 10, 2011
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You're gonna find that resistor has a lot of noise across it, so you're going to probably want to put some kind of low pass filter on it. Personally, I'd first check to see if the motor driving the thing has any kind of tachometer output from it. If not, I like the idea of a Hall effect sensor detecting the rotation.

Another thought is to put limit switches in to detect the end of movement.
 

Gr3mlin

Jul 29, 2011
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Thanks all for the advice, i had a play, and the LM393N does a very good job at detecting the V loss across the resistor, with little noise..

"If the motor is reversed by reversing the polarity of the supply, then this will also need to be taken into account." very good point, i worked around this by going straight for the supply before the switch in polarity.

Now, my little exp. involved a quick setup on a breadboard and test.
i went for the h-bride out of transistors, instead of the double pole relay swapping the polarity. but. well, im either tripping or my transistors are mis-behaving. the Emitter of a PNP and NPN are connected to the output to the motor, and then the base, and collector . blah blah, but when i started drawing out the schematic, i notice that it was actually the collector of both transistors that are connected to the outputs.
I tripple checked this and if i go by what the PDF says, pin 1 is the collector on both (http://www.datasheetcatalog.org/datasheet/fairchild/BC547B.pdf) and (http://www.datasheetcatalog.org/datasheet2/5/0qsl0963l7i0pw6dwpsgdj3890wy.pdf)
(project is still in prototype and is running a tiny motor for testing ATM)
does this make any sense?

Also, i love the idea of the halleffect sensor, only, if the motor was to hit something, and start pushing it, it would still register as a rotation, and could cause some damage.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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The suggestion for use of a hall effect sensor was for current detection, not position (or movement) detection.

But it's good that you're moving on with this.

If your current detection is outside the H-Bridge, you will detect current in the same direction for either motor direction. Depending on your needs, it may be better to do it this way, or place it inside the H-bridge to get the alternate behaviour.

When making H bridges with bipolar transistors, you have to make sure that you don't accidentally send huge currents through the bases of the transistors.
 

Gr3mlin

Jul 29, 2011
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The suggestion for use of a hall effect sensor was for current detection, not position (or movement) detection.
OOOh. i havent really looking at using a halleffect to achieve this. Ill put that on my to do list.

If your current detection is outside the H-Bridge, you will detect current in the same direction for either motor direction. Depending on your needs, it may be better to do it this way, or place it inside the H-bridge to get the alternate behaviour.
Indeed, i found it works rather well, before the H-Bridge, and functions correctly.

When making H bridges with bipolar transistors, you have to make sure that you don't accidentally send huge currents through the bases of the transistors.
bipolar transistors? i wasnt aware i was using bipolar transistors. which is the bipolar BC557 or BC547?

The H-Bridge works perfectly, so im ignoring the fact that the way i have it setup and the way the PDF's say it should be contradict each other.

thanks for the reply.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Bipolar transistors are the normal NPN and PNP transistors that many are familiar with -- and the ones you're using. It is a name which distinguishes them from other transistor types such as MOSFETs (the T is for Transistor), JFETs (T for Transistor), and IGBTs (again, the T is for transistor, and B is for bipolar).

I don't think I've seen a schematic for the H Bridge you're using, so I just gave that little warning. I presume you have diodes across your transistors to protect them.
 

Gr3mlin

Jul 29, 2011
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Steve, thanks for your help thus far. (Who speaks like that??? i've been watching to much Big Bang Theory.)

While i've been under the weather (ill) i've had a good time blowing transistors up left right and center.

Here's a question, can you give me a break down of how a transistor works. cos i honestly feel i have begun losing my mind.
i've made hundreds of circuits using transistors. and have found myself, mixing up the functions of the emitter and the collector.
I really feel like im losing it here.

anyhow.
I've temp gone to a standard DBL Pole relay to do the current switching. just cos i blew up what transistors i had that could coupe with the current.

Thanks again.
 
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