Dust Collection bin full sensor

Thread Starter

Jcpilot

Joined Jan 27, 2012
5
Hello,

I studied electronics in high school 23 years ago and my dad is a retired electrical engineer. Between the two of us we can figure out a schematic so any help here is appreciated. I am building a dust collection cyclone for my wood shop and assembled a sawdust bin level shutoff circuit but I have a concern. A normally open 110 vac photoelectric switch energizes a dpdt relay and shuts off power to a 40amp motor relay when the sensor see's an object within four inches. My concern is the sawdust in the bin will be swirling around and might trigger the sensor before the bin is full. I dropped a handful of sawdust in front of the photoelectric switch and it rapidly switched the 40amp relay on and off. I do not want this happening to a 5hp compressor duty motor. My thoughts are to integrate a time delay relay that only shuts off the 40amp relay after it has a steady object sensed for at least :30 seconds and ignore intermittent object sensing.

Thanks,
John
 

CDRIVE

Joined Jul 1, 2008
2,219
Maybe you could test the weight of the bin? Something as simple as a spring and a microswitch could do it.
I have a wood shop and two dust collectors. On the surface this sounds doable but I wouldn't go that rout because the weight of wood varies drastically over species and how green it is.

Chris
 

CDRIVE

Joined Jul 1, 2008
2,219
Hi John,
Because of the dusty environment in there I would think that any type of photo sensing device would be a crap shoot, as the lens and or reflective surface would become obscured quickly, making the system erratic.

I believe the capacitive sensor that GDI posted works by virtue of comparing the relative 'Permittivity' (Dielectric Constant) of air to the that of wood. As the permittivity increases so does the effective capacitance.

You may find the information in this link very useful to you and novices on the forum. It includes a short video and very clear theory of operation. The author uses a microcontroller but this should not scare anyone away because he also provided the code and schematic. For certain, just about any family of uC's can replicate the functions of the chip he used. I work with the Picaxe line of uC's. They're very inexpensive and the editor/compiler is free. This circuit can be replicated using a Picaxe 08M 8 pin DIP.

BTW, I looked up the dielectric constant of air and wood. Air is ~ 1.0 while wood is ~ 2.0 to 6.0.

http://www.nerdkits.com/videos/halloween_capacitive_touch_sensor/


Chris
 

John P

Joined Oct 14, 2008
1,808
This is like a project that someone asked about a couple of months ago, where a guy wanted to detect the level of sewage in a plastic tank. Some of us suggested a capacitive sensor working through the tank wall. I don't know if he ever tried it, but it seemed like a workable idea. The most plausible way to do it seemed to be to have two electrodes, either a bullseye shape with one encircling another, or a parallel pair of strips. Then you'd set up an oscillator or timing circuit using the electrodes as a capacitor, and monitor the resulting characteristics. In this case you'd need to tell the difference between sawdust swirling around, and sawdust that had settled. Or does the whole bin fill with swirling sawdust when the machine is running? That would be a problem, if you basically had to measure a mix of air and dust which just got more heavily loaded with dust as the bin fills. And there is a question of how much the capacitance would really change because of the level of the measured material, versus the effect of the container itself.
 

CDRIVE

Joined Jul 1, 2008
2,219
This is like a project that someone asked about a couple of months ago, .....
Dust collection systems can be either all metal, plastic (PVC) or a combination of both. When using plastics, in any of the lines and containers, the user must take static electricity precautions by running ground wire within the pipes and bins. This is because wood dust can be explosive. If his bin is metal he would have to measure capacitance from the bin walls to some other point, possibly a suspended vertical plate within the bin.

Chris
 

John P

Joined Oct 14, 2008
1,808
Hey, that's got some promise. You could just monitor the current it takes to turn the motor, where the denser the air/dust mix is, the harder it is to churn it. It sounds like making ice cream!
 

CDRIVE

Joined Jul 1, 2008
2,219
Paddles in the dust bin? Sensing motor loading? Guys, y'all need to check out how a cyclone dust collector system works. The collection bin is located in a low turbulence area of the system at the bottom of the cyclone. There isn't much air movement down there.

A cyclone system has two collection points. The first is the drop bin (input side of the turbine) that collects the heavier particles before it enters the dust collector turbine. The turbine's output blows the remaining lighter dust into large bags.

The motor will not load because the bin is full and it's doubtful that it would load if the bags were full. These systems don't produce the inches of vacuum that a shop vac does but they move an enormous amount of air (CFM), far greater than a shop vac. For them to remain effective, any air restrictions and friction must be kept to a minimum. The line diameters are typically 4 to 6 inches.

For sure, I'm not saying that my idea is an better ... but maybe? :D
 

CDRIVE

Joined Jul 1, 2008
2,219
Maybe you could test the weight of the bin? Something as simple as a spring and a microswitch could do it.
I commented on this earlier but dismissed it because of the variables in weight over various species of wood. I was also concerned about green and dry wood variables. However, your idea is sounding more feasible by the second.

I don't know what the OP is going to do but If I elect use this approach I'd experiment by setting the trip weight for the lightest wood that I mill.

Chris
 

THE_RB

Joined Feb 11, 2008
5,438
I commented on this earlier but dismissed it because of the variables in weight over various species of wood. I was also concerned about green and dry wood variables. However, your idea is sounding more feasible by the second.

I don't know what the OP is going to do but If I elect use this approach I'd experiment by setting the trip weight for the lightest wood that I mill.
...
Yep you made some good points. It's not just the density of the wood but probably the chip size too, which would also affect weight:volume results.

But there's a lot of reliability in a weight system even if the "full" accuracy will vary and needs to be set for a worst case wood type.

That capacitive sensor idea sounded interesting. I wonder how hard is is to do, and how reliable that is?

Actually, a real bright laser sensor (interruptor) across through the top of the container might do it. Because incoming dust is less dense and swirling etc there would still be enough light getting across or reflected/diverted bouncing around in the top of the container. But when "full" of dust there is no way the laser will get across to activate the sensor, it would be absorbed into the solid mass of the dust.
 

CDRIVE

Joined Jul 1, 2008
2,219
a relatively common name in industry;
http://www.bindicator.com/products/point/roto.asp

I've worked with plenty of material conveyance systems, and the bindicator keeps popping up. Majority of gravometric (wieght) I've come across are at the feedstock side of processes, but I don't see why not the tail end.
Sorry about that GDI. I thought you were referring to paddles driven via the system's air flow. Yes, this concept is viable but I would guess quite expensive, especially since there are no prices listed. On the up side, a scaled down home brew version looks very doable.
;)

If I were building this I would make every attempt to mount it on the cyclone/bin coupling and extend the paddle shaft down near the top of the bin. This would eliminate having the motor, mechanicals, and electronics connected to the bin, which must be removed for emptying.

Hey, has anyone noticed that the OP is AWOL? :D

Chris
 

CDRIVE

Joined Jul 1, 2008
2,219
Actually, a real bright laser sensor (interruptor) across through the top of the container might do it. Because incoming dust is less dense and swirling etc there would still be enough light getting across or reflected/diverted bouncing around in the top of the container. But when "full" of dust there is no way the laser will get across to activate the sensor, it would be absorbed into the solid mass of the dust.
Ha, that brings us full circle to the OP's first post. Yes, I don't think it would be a big deal to design the circuitry to ignore the snow while it's filling the bin but I'm still concerned about the optics getting totally obscured by the nasty environment in there. The optics could be wiped each time the bin is emptied but I'm still concerned about green wood.

I'm primarily a wood turner. I harvest most of my wood from local tree trimmings and the bonanza following hurricanes and thunder storms. Green wood particles turn into a paste in short order. I have brushes on the upper and lower wheels of my band saw but they are next to useless when dealing with green wood. When my band saw starts making thump, thump sound I pop the wheel covers and scrape the wheels with a stick while it's running. One of my "To Do" projects is to mount spring loaded levers on the outside of wheel covers. This way I could scrape the wheels without removing the covers.

BTW, if a wood worker is working with southern yellow pine the situation isn't any better, even if it's bone dry.

Chris
 
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