how to determine a shaft is running?

hi all
i am gathering some info for my little project and i am absolutely have no experience in electronics.

I would like to monitor a shaft(runs on AC) and if it is running then it triggers another motor(runs on AC). However I don't know what kind of sensor to use given that I cannot modify the exisiting system. All I have is a shaft.

I found I may possibly use a Hall effect sensor but I am not sure what type of Hall effect sensors is applicable as there are unipolar,bipolar,linear,latch and etc sensors.

I am thinking maybe any of them will do the job if the PLC(s7-1200) monitor the input values, say if the input changes from time to time in the last 5 seconds, then the shaft is running?

Any help is greatly appreciated.

Thanks

I don't know the size of the shaft, but if it's large enough, then you may attach this motion sensor magnetically, without having to modify the shaft.

http://www.go4b.com/usa/products/speed-switches/whirligig.asp

You're right, there is lots of ways to achieve it. Motion sensors, encoders, current monitoring etc work pretty well depending on the application and how much you want to spend. It's usually pretty easy and people usually have a few extra proxs in stock already you could pop a star wheel on one of the shafts and then pick the arms up with a proximity sensor, or if it's slow enough and the teeth are big enough you can pick up the teeth on a sprocket.

What size shaft is it and what RPM? How accurate does it need to be?

Also, why you want detection is important to this as well. Is there a chance that the shaft could be turning without the motor and conversely, the motor is turning but the shaft is not? Does the direction matter? Does the SPEED of the shaft matter, or for safety reasons do you need to detect ZERO speed? Do you need to know what position it stopped it? If so, how accurate do either of these need to be? The reason is that all of these are issues that affect what type of technology you start with, or what would be a waste of time and money.

As an example, there are now many types of motor overload relays that can provide true kW measurement, which is an indicator of actual shaft power. So if all you want is to know for sure whether or not the shaft is connected and the motor is running, disconnect the shaft, run the motor unloaded, record the kW from that and anything GREATER than that value means the shaft is connected and turning. Since yor AC motor needs an overload relay anyway, you kill two birds with one stone. If you are using an AC inverter drive on that motor, most come with that feature built in already.

You could use a photoeye and some reflective tape if you think the tape's not going to get covered by grease/oil/dirt. If the shaft has a keyway perhaps an inductive prox might work.

Have done the reflective tape thing.
If it is difficult to access the shaft from the motor output end, you can possible attach some kind of sensor disk on the fan end instead.
If there is an intelligent overload relay, VFD or other current sensing device, then monitoring the currnet as jraef suggest is a good idea.

Apart from that maybe it is better to monitor what the motor drives rather than the motor instead.
For example:
Pump -- monitor flow or pressure.
Belt conveyor -- monitor belt idler.

Depends on speed

If the shaft is down line of the motor and running slower a prox sw may work with a time on - time on circuit, so if either timer times out motion has stopped. But if you are looking at a motor shaft 1750 RPM is hard to capture a keyway with standard sensing proxs and inputs.

If you use reflective tape that covers half the circumference of the shaft, you will get approximately 15 ms pulses @ 1750 rpm.
That can be reliably sensed with regular optical sensors and PLC inputs.

JesperMP said:

If you use reflective tape that covers half the circumference of the shaft, you will get approximately 15 ms pulses @ 1750 rpm.
That can be reliably sensed with regular optical sensors and PLC inputs.

Click to expand...

Yes, with DC inputs.

What are the sizes of the motors and what kind of external forces are involved. Have you taken the safety consideration of controlling a motor based on whether another motor-driven shaft is rotating? If the shaft turns when not under power (like manual turning with your hand), is there any chance that it could cause the other motor to start up?

Any chance you could upload a sketch of your project?

Many systems use a current switch for this, set to close just a bit less than the normal operating current. This works well with devices that draw a stable current, fixed speed fans and pumps etc. Pretty common in building management systems to let you know if belts are slipping or bad on something not normally observable.

Schneider Electric sell a range of Inductive Proximity rotation detecters for example the model XSAV11801. You just need to affix some type of target to your shaft and mount the sensor to detect the target as it passes. There is potentiometer adjustment to set the minimum speed, and the device is self contained, there is minimal wiring and no programming needed. See attached datasheet.

I have just uploaded a diagram show what the "SYSTEM" is.

there are grains on the belt and if the shaft is running, it should triggers the auger to send stuff onto the belt as well.

The shaft doesn't have teeth at the edge so I think i cannot use those sprocket sensor.

And I forgot to mention the most important part, the environment is dusty so most of the light related sensors are not very ideal as I think the dust would affect those sensors.

The shaft is not big, only has a 15cm diameter and like 100rpm.

Thanks for all reply and helpful suggestions and knowledge

Well those are all good ideas. You can also just use an input to your PLC from the motor starter to determine if the starter is pulling in. The downside of this is that if your motor isn't spinning, the PLC will still think the starter is pulled in. It is simple to troubleshoot, however, if there is constantly someone monitoring the belt. The auger is dumping material but the belt isn't moving. The contactor is pulled in, so the motor should be moving. Power circuit / motor / mechanical binding.

arad said:

I have just uploaded a diagram show what the "SYSTEM" is.

there are grains on the belt and if the shaft is running, it should triggers the auger to send stuff onto the belt as well.

The shaft doesn't have teeth at the edge so I think i cannot use those sprocket sensor.

And I forgot to mention the most important part, the environment is dusty so most of the light related sensors are not very ideal as I think the dust would affect those sensors.

The shaft is not big, only has a 15cm diameter and like 100rpm.

Thanks for all reply and helpful suggestions and knowledge

Click to expand...

I'm guessing that this shaft is installed in such a manner that it not only indicates that the belt conveyor is running, but that it actually has material present on it. 15 cm is not particularly small for installing a motion sensor. The Whirligig device that I linked to can be easily installed without having to modify the existing shaft, if you select the magnetic attachment option. It will function well in dusty environments. It uses an enclosed rotating target and proximity switch to detect shaft motion. There are multiple options available for both the number of pulses per rotation generated by the enclosed target and for the type of proximity switch to be installed in the device. If you want, you may order one with a proximity switch that will provide a steadily on input, rather than a pulse. These switches have variable time delay that can be easily adjusted for different operating speeds.

I have used these switches in very hostile environments and have been pleased with their functionality, reliability and ease of installation. I am not in any way associated with the company that manufactures them, aside from having used their products.