How to detect a dc motor jam

[david119]

motor jam

Hi,

I have a small dc motor that will be powered by an h-bridge. So that it can rotate in the forward direction or in the reverse direction. The motor rotation is very slow approx. 1.5rmp.

My question is how could I detect that something is impeding or blocking the rotation of the motor?

I should say that this error detection signal will be sent to an fpga so the motor can be shut down without any damage.

Any help would be greatly appreciated.
Thanks

 

[IanP]

jam motor

once the motor starts turning a back emf is generated which reduces the current but when it stalls (mechanical jam) the current rises ..
one of options is to use the H-bridge to gnd current sensing resistor, so when the current rises above certain threshold it is an indication that the motor is in trouble and the controller will shut it down ..

rgds,
ianp

 

[david119]

motor end stop detector circuit current

Thanks Ianp

Thats kind of what I was thinking (if I could only sense the amount of current), but I've never heard of a current sensing resistor. So I quickly looked at a couple of data sheets for current sensing resistors. How exactly do they work? Most have four terminals. Two terminals to complete the circuit path from the H-bridge to ground, and two terminals to measure the voltage (maybe?).

If those last two terminals are for measuring the voltage how could that be done?
(without a multimeter)

I understand that as the current increases the voltage should increase. But how do I translate that into something the fpga will recognize. Do I tie one of those terminals to gound? Then use the other in combiation with a voltage divider circuit to inrtface to the fpga?

Thanks
david119

 

[IanP]

dc motor current detect circuit

One of options is to use a voltage comparator ..
It will compare the voltage across the current sensing resistor with a reference voltage and if the first is higher than the second it will produce a digital signal (a "1" or a "0") that can be fed directly to FPGA ..
Here are examples of comparators:
**broken link removed**

Rgds,
IanP

 

[daviddlc]

motor jam detection algorithm

You need to consider that on a motor startup the current will go up and then will stabilize, you will need to monitor the current in a time basis, so depending on your threshold level you need to add a delay before start measuring

 

[klystron]

motor jam and how to detect it

Measure the change in lad current or slope ; the fault current of a jammed motor increases very fast

 

[R00KIE]

how to detect back emf in dc motor?

I would use a circuit like IanP showed before, measure the current the motor needs when it is working properly, measure the current when it is jammed and I would put a threshold somewhere in the middle of that. Then I would check (with a comparator) if the current is above the threshold. In your code then you check that status from time to time, if the current is above the threshold for more than a couple of seconds then assume it's jammed and turn it off.

 

[david119]

end of tape sensor jammed

Thanks everyone for the good advice

I'm going to try a current sensing resistor and a voltage comparator. I'll be able to add any desired delay into the fpga program.

For anybody else looking to prevent a motor from jamming and causing damage I came across self reseting fuses. This could be used as a very simple solution. The most common are called PolySwitch (Tyco Electronics), Everfuse (Polytronics), Polyfuse (Littelfuse) and Multifuse (Bourns).

david119

 

[daviddlc]

dc motor current detect

My recommendation is to get the delay time using a datalogger, you cannot do this using a regular multimeter or oscilloscope.

Of course if you have access to do this, or maybe checking the motor specifications.

Regards.

 

[Externet]

how to detect back emf of a motor

The best way I know of is by having either perforations on the turning gear/disc/pulley/shaft that will cause an optocoupler to emit pulses while rotating; or magnets passing near a reed switch or hall effect sensor..
When pulses cease, triggers a stop signal. (missing pulse detector circuit)
That system is used in cassette mechanisms to sense end-of-tape or jam..

 

[david119]

current sensing resistor in dc motor

daviddlc

My recommendation is to get the delay time using a datalogger, you cannot do this using a regular multimeter or oscilloscope.

What exactly is a datalogger?

Externet

The best way I know of is by having either perforations on the turning gear/disc/pulley/shaft that will cause an optocoupler to emit pulses while rotating; or magnets passing near a reed switch or hall effect sensor..

The hall effect sensor is a good idea, but I can't use one at this point.

Thanks
david119