How to tell a sensor to stop counting

Thread Starter

McNuggies

Joined Feb 26, 2019
11
Hi, so I'm currently doing a project to count the length of a string. The project consists of using magnets embedded into a spool with a hall effect sensor in proximity to it that counts pulses as a motor turns the spool. As the string gets wrapped around the spool, knowing how many magnets there are in one rotation gives you the premise to work off calculating the length from there from the number of pulses. The Hall effect sensor is attached to a pcb with the output of the hall effect sensor connected to a MSP430 microcontroller that is coded to count the pulses. However a problem arises when it comes to how to tell the system to stop counting. We have a laser sensor and a receiver but this could be rife with problems. What would be the best way to tell the system to stop counting once the string is fully wrapped around the string?
 

Alec_t

Joined Sep 17, 2013
10,355
Welcome to AAC!
What would be the best way to tell the system to stop counting once the string is fully wrapped around the string?
How does the micro know that the string is fully wrapped?
Does the spool motor stop automatically?
 

Thread Starter

McNuggies

Joined Feb 26, 2019
11
Welcome to AAC!

How does the micro know that the string is fully wrapped?
Does the spool motor stop automatically?
The micro has no way of knowing when the string is fully wrapped at current.
The micro will stop counting when whatever method of stopping it has been applied and store the value.
hi,
Do you mean string is fully wrapped around the spool?
E
Yes, at the end of the cycle the string will be wrapped around the spool, basically as soon as the end of the length of string reaches the spool, the micro should then stop counting
 

ericgibbs

Joined Jan 29, 2010
8,723
hi,
Is it possible to feed the string over a spring loaded jockey wheel, so when the string end leaves the jockey wheel the spring operates a micro-switch.?
E
 

MaxHeadRoom

Joined Jul 18, 2013
19,014
If the pulses are regular, could anything past the time between counts, turn off the counter?
Max.
 

Thread Starter

McNuggies

Joined Feb 26, 2019
11
hi,
Is it possible to feed the string over a spring loaded jockey wheel, so when the string end leaves the jockey wheel the spring operates a micro-switch.?
E
It could be possible to feed the string through some kind of spring operated sensor before it meets the spool but how you would integrate the jockey wheel into the spool design where the hall sensor counts the pulses might be tricky
If the pulses are regular, could anything past the time between counts, turn off the counter?
Max.
They likely would not be regular since the speed of the motor is not constant and can fluctuate leading to different times between rising clock edges
 

Thread Starter

McNuggies

Joined Feb 26, 2019
11
I wonder if it would be possible to put in some kind of pressure sensor/weight sensor and when the string is no longer on it and the micro simply takes the distance between the pressure / weight sensor and the spool into the final measurement but whether a sensor like that could detect a light piece of string on it is another issue
 

MaxHeadRoom

Joined Jul 18, 2013
19,014
If counting off the reel.
Don't you also get error when the dia of the reeled in string changes as the end becomes closer?
Max.
 

ericgibbs

Joined Jan 29, 2010
8,723
hi,
Is it possible to interrupt the Hall effect output pulse, ie: open circuit the connection with the micro switch.?
 

MaxHeadRoom

Joined Jul 18, 2013
19,014
Systems like this generally have the metering device on the material itself, not the reel.
This could take the form of a simple low res digital encoder with 1/rev marker pulse, the marker resolution could replace the hall sensors, and the encoder measuring pulses would be a fine enough resolution to detect a stop condition.
Exact details of the mechanics etc would also enable an enlightened solution.
Max.
 

Thread Starter

McNuggies

Joined Feb 26, 2019
11
Systems like this generally have the metering device on the material itself, not the reel.
This could take the form of a simple low res digital encoder with 1/rev marker pulse, the marker resolution could replace the hall sensors, and the encoder measuring pulses would be a fine enough resolution to detect a stop condition.
Exact details of the mechanics etc would also enable an enlightened solution.
Max.
Could you expand on this? We have to have a high enough resolution to count in mm.
 

MaxHeadRoom

Joined Jul 18, 2013
19,014
Instead of counting the reel, a digital encoder could be used on the string , these come in various resolutions (pulses/rev) and also an option of a one/rev marker pulse to replace the hall sensors.
The encoder would monitor the string as it traveled over a couple of rollers that had the encoder attached, the resolution could be tailored by way of roller dia etc.
The (high resolution) counting pulses could be used to detect any stoppage of the encoder, as their resolution would be much high than the marker counter.
Without knowing exact details, just an option to work with.
Max.
 

Yaakov

Joined Jan 27, 2019
1,536
It seems a rotary encoder with a pulley that the string passes over possibly including a spring loaded tensioner and an idler to maintain the string in the pulley would be the best bet. If the total length down to a millimeter is required, an optical sensor that was normally interrupted by the string could follow directly behind (or be directly in front of) the encoder pulley and when it detected the absence of the string, it would be the end,

The curent method seems very hard to make reliable and precise.
 

Alec_t

Joined Sep 17, 2013
10,355
We have to have a high enough resolution to count in mm.
That's going to be a challenge if the string doesn't wind evenly on to the spool, or if the string is compressible or has any elasticity.
Is there any back tension on the string?
How thick is the string? What's it made of?
 

Yaakov

Joined Jan 27, 2019
1,536
Instead of counting the reel, a digital encoder could be used on the string , these come in various resolutions (pulses/rev) and also an option of a one/rev marker pulse to replace the hall sensors.
The encoder would monitor the string as it traveled over a couple of rollers that had the encoder attached, the resolution could be tailored by way of roller dia etc.
The (high resolution) counting pulses could be used to detect any stoppage of the encoder, as their resolution would be much high than the marker counter.
Without knowing exact details, just an option to work with.
Max.
Ha, looks like we have the same idea. I think we were typing it at about the same time.
 

MaxHeadRoom

Joined Jul 18, 2013
19,014
There is also devices out there where the spool mechanism resembles a spring return measuring tape, but in place of the tape, a steel cable is used, as the steel wire is pulled out and spring returned, an encoder in the case records the wire movement precisely.
These may be an option, depending on what the rest consists of.
Max.
 

iONic

Joined Nov 16, 2007
1,650
In reality, does one really need to know when the spool is "full?" Or should one be more concerned about "how much" the spool SHOULD get.
Manufacturers don't want to be giving extra string to customers, do they? Is there going to be an "absence of string?" There will be string until one says "no more string please." Time can dictate when the spool is full. There are a lo of possible vriables here!

Just thinking here. I may be a wet sponge in the rain with these ideas...:D
 

MisterBill2

Joined Jan 23, 2018
3,863
If the length of the sting is unknown then there is no choice but to have some mechanism to detect the end of the string. That could be as simple as an LED and phototransistor set up with a narrow beam that the string would block until it ended. There are many models of a device with that arrangement available from many suppliers, so the hard part would be holding the srting so that it blocks the light beam.

Another way, which is mechanically easier, is to first pass the string over a roller before being wound on the spool. Then the roller can have some scheme to generate a string of pulses as long as it is turning, and the pulses stop when the string runs out. There will need to be a second roller pressing the string against the first one so that it will always turn as the string passes.The pulses can keep resetting a short timer that enables counting the spool pulses until it times out.. Easy, simple, and cheap, AND it does not cause any wear on the string.
 
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