Simple circuit to indicate rotation has stopped 1

[PRW]

I need a simple circuit that will indicate when a gear has stopped rotatong. This is a polypropylene gear that has one or more short metal rods imbedded in it.

In the past I have used the metal with a proximity switch to count the gear speed by an output to a PLC.

On the current project, there will be no output to a PLC, everything will be self contained. So I need a circuit that would run on 12 VDC and light a LED or sound a horn when the gear stopped.

Any ideas?

 

[PHovnanian]

PRW, if you still have a proximity switch, consider one of the watchdog timers available for such purposes. Maxim makes several.

http://www.maxim-ic.com/view_press_release.cfm/release_id/143

One thing to consider is that the watchdog must be edge-triggered. Otherwise, if the gear stops on one of the prox. switch targets, a simple 555 reset scheme will be held off indefinitely. The app notes for these parts may have some other insights into possible design issues.

 

[MacGyverS2000]

Power can still flow in a motor winding even when it's not moving...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[PRW]

I must offer my apologies for not looking at this post for a long time. I was on another project and could not take the time. I do value your inputs, now I just need to figure out how to use them.

I have investigated the 555 circuits ( missing pulse detector ) that were mentioned and also looked into hall effect sensors. I would like to use both, but have no experience with actual circuit design.

Terms like when triggered, the output goes low, what does that mean? Would I need a relay at the output to turn on a light of the relay is de-energized?

Then there is the problem one of you mentioned about where or not the gear stopped next to the sensor. Since the repetition stops, will the missing pulse detector not perform as designed?

I think I need to find a "Circuits for Dummies" Book!

 

[PHovnanian]

PRW, there are a couple of issues to address. What are the consequences of not detecting the cessation of rotation? Life safety, damage to equipment, cost of having an operator run over and punch the reset button occasionally? The more serious the consequences, the more I'd suggest an off-the shelf solution.

Then, there's the issue of latency. How many pulses per second (RPM, whatever) does this device run at? What is the minimum speed under which a 'no rotation' signal must be generated and how soon after it drops below this threshold must a signal be produced?

The behavior I mentioned in a previous post applies to some of the simplest 555 circuits, which are timers that will run to completion if not periodically reset. If the pulse which resets the timer is based on a sensor level (high or low), what happens if the gear stops directly under the sensor, holding the 555 'off' and incapable of generating its output indefinitely? These are all things that have been addressed by designers of things like watchdog circuits. You'll still have to do some soldering, but the application notes will probably cover a case close enough to yours that re-inventing the wheel will be unnecessary.

 

[brlewis]

If the motor stops when the gear stops then you could use a current switch. see ACS150 Series from automationdirect.

 

[LionelHutz]

Look at an Omron H3CR-A AC24-48/DC12-48 timer set to mode E - interval. Use a proxy switch as the start signal. It will keep the output contacts energized when there is a pulsing start signal. If the start signal goes away or stays on it will time out and de-energize the output contacts.

Nice thing is that one will run on 24VDC and the NPN from the proxy switch can connect directly to the start input. Y?You should just require the proxy sensor, 24VDC power supply and timer to make the circuit. It also has contact outputs which make it easy to use the output.

 

[LionelHutz]

Here's the pdf file for the relay.

http://www.omron247.com/doc/pdfcatal.nsf/5003B91A047F523986256B8100742AEA/$FILE/D19H3CRA0302.pdf

Page 20 shows how to connect the proxy switch.

 

[PRW]

Lionel,

This looks like any excellent solution, except for the cost. I was hoping to do this for a lot less money, but I don't know how. I design mechanical stuff, and although I can put together some simple electronic parts, I have no idea of how they connect.

I will keep your suggestion in mind, but I am sure the "counters of beans" will shoot it down.

Thanks,

Paul

 

[LionelHutz]

Well, I really don't see any cheaper way to do it. If you were to ask me to design something for 15 units I could not spend the time to design and build the circuit cheaper than those timers would cost. But then, I might get a price for that timer which is a lot less than you found.

If you can change to magnets instead of steel in the gear then you could find some cheaper sensors.

 

[PRW]

Lionel, I can use magnets, in fact I have some round neodimium magnets that I have used with a reed switch. The problem is the reed switch fails, maybe the contacts are welding, who knows.
In any earlier post I mentioned looking a hall effect sensors, and was thinking of using the Allegro switch on this page:

http://www.allegromicro.com/en/Products/Part_Numbers/3245/

Based on my very limited knowledge, I thought this would work.
Someone mentioned earlier in the post that there would be a problem if the input stayed high. How do I overcome that? Would that be a problem if I used the timer you mentioned?