How to detect electric motor slowing down?

[jerrypoo]

Hello all!

I try to build a simple system that cuts off the power from an electric motor (small, DC motor) when there is too much resistance/work/friction ...

I guss the idea is to put a small resistor and that way measure the electric current. The best would be to use a relay, that would fit some other things, too.

But ... I'm really not good designing anything so all help are welcome! I understand the idea behind this but I need help with the basic design. As simple as possible.

I'm trying to develop a system where I have several small DC motors running. Each one can be separately stopped by slowing them down enough. The plan is that I get a signal from each when they're on/off.

Something like this (see the scetch...)

Any idea or design for a simple system?
Thanks in advance, all ideas welcome!

 

[(*steve*)]

Detecting increased current may be a problem when the motor is starting up. The motor will draw more power and your circuit may prevent to motor getting up to speed.

Another option is to use a hall effect sensor or a chopper disc, or an optical sensor looking for a mark on a shaft to produce a pulse train. If you convert these into narrow pulses and integrate the voltage using a resistor and a capacitor then you will have an analog voltage that varies with motor speed.

With a little more cunning, you could detect that decreasing rather than increasing and produce some sort of signal.

 

[jerrypoo]

Detecting increased current may be a problem when the motor is starting up. ...

Not really, I plan to use a delay. It should prevent the start-porblem.

...
Another option is to use a hall effect sensor or a chopper disc, or an optical sensor looking for a mark on a shaft to produce a pulse train. ...

That is a good idea but is not possible to use in this application The thing is that is has to be "electronical" with no sensors. Long story.

Any ideas how to design this prototype? Don't be shy All help needed!

 

[KMoffett]

The thing is that is has to be "electronical" with no sensors. Long story.

Long stories are good. They often help us solve your problem rather than trying to solve your solution.

Ken

 

[jerrypoo]

Long stories are good. They are often help us solve your problem rather than trying to solve your solution.

Okay, that's a good reason.

The thing is that I made some many years ago MECHANICAL systems. Those have been in use for about 10 years in a row but no totally trashed. Already in the beginning the plan was to do totally electronical system (that's what I try now) because I think they would last longer... As simple as possible. Just the DC motor and some simple electronics. That could last for the next decade.
And the rpm-feedback, no matter if it's a hall sensor or optical fork, I count it as mechanical. They just can't last

This was the short version of the long story.

Any ideas how to design the system? I mean the electronics?

Right now I think I make something like this: Resettable fuse and that's it.

 

[KMoffett]

Again...no details! How many motors? What voltage and current? What starts the motors? What happens after motors stop? What power supplies are available.........?????? If this is going to be one of those projects where you want to keep it a secret, but you want someone to solve for you...I'm done.

ken

 

[jerrypoo]

One motor, voltage 3 - 12 VDC, current around 0,5 A, starts by switch. When motor stops, nothing happens. Power supply is separate has nothing to do with this circuit.
Nothing secret, nothing complicated either.

I thought my fine scetch told it all

 

[KMoffett]

OK, so there will be several duplicate motor circuits. Is the motor spec'd for 3-12V/0.5A, or are you supplying 3-12V for variable speeds? This is the supply I'm referring to. This also brings up what supply is available to power the electronics. Do you know, or can you measure the starting, running, near-stall, and stall currents at whatever voltage you are supplying to the motor. Per Steve's concern, are you going to shut this detector circuit off at start-up? Will all the motors start at once? At some point do you shut all the motors off together, or individually?

Basically a sense resistor, comparator, delay/latch, relay...but the devil's in the details.

Ken

 

[jerrypoo]

The plan is to make several, identical but individual systems. In each system one motor and controlling unit. All work with DC propably from a car battery or some other source.
The voltage is depending on what motors I'll find. No other criterias. One speed.
Yes, same power to the electronics and the motor.
The current used in the start I thought to skip by a delay. I mean, in the start 1 sec delay and after that this current monitor steps in. Sooner or later each motor will shut.

I have tried now find some good ideas for the electronics but it might be to tricky. Now I have started to design again - bummer - a semi-mechanical solution. Something where a micro switch controls the motor...

I hope someone comes with a good and simple idea before that

ps. I already have mechanical systems, they work great but after 10 yrs using not much longer... These works like this: all units starts when I switch on the power. Individually each machine can stop if it is breaked down (slowed down enough). This is done by a micro switch, when the torque twist the motor enough against a spring. The micro switch cuts the power from the motor by a relay that feeds itself. This way the motor do not start before the mainpower is resetted. From the relay I also get the signal if any motor is running anymore.
This system works great but it's time to build a new one. It's already 2010

 

[KMoffett]

Here' a circuit that might work for you.
Start delay is set by R1/C1/Q1.
Over-current detect is set by R3/R4/U1/R6
Over-current stop delay is set by R5/C2/Q2
RLY1 is power-on/power-off latch
No values for R2/R3/R4 because of no motor data
R1/C1 (~2 sec) and R5/C2 (~1 sec) timing values are just estimates.

Bypass capacitors would be necessary to make the circuit more noise immune.

Ken

 

[jerrypoo]

Thanks, thank you! I'll try that.
It will take me a week or two - I'll report here how I proceed. Wish me luck

All other ideas are greatly welcome!

 

[shiekh]

Are these motors with commutators?

Might be a way to monitor the chopping frequency of the commutator.