Hi Folks,
I'm exploring the feasibility of inline metal detection for horizontal wood recyclers. Here is a link to a photo of a typical recycler.
These are generally 500 to 1000 HP diesel units made by many several different companies. They run debris through a rotor/grating assembly and produce chips on the other end. They are used for a number of other applications (compacting roofing shingles for landfill, chipping railroad ties, etc.), but the most typical use is by landscaping suppliers who turn yard debris and slash piles into landscaping mulch.
The metal contaminations occurs when large pieces of steel get into the piles. Axes, sledgehammers, rebar, railroad spikes, loader-bucket teeth and other large, heavy objects will break the teeth off the rotor and blow out bearings. This causes several hours to several days of downtime.
I'm looking into methods of detecting this contamination before it hits the rotor. This is complicated by the fact that the hopper is steel, as is the drag chain used to pull the material into the rotor. Non-metallic drag belts are far too fragile and are not commonly used on the infeed side. Also the loader bucket which dumps material into the hopper does tend to confuse traditional metal detection technology.
Here's what's been tried:
Traditional metal detection coils. The large amount of steel on three sides, and drag chain made the output virtually worthless. This technology could not determine a difference between a tin can (which can safely pass through the rotor and be picked out with a magnet) and sledgehammer head.
Vibration sensors on the bearings: This does work to a certain extent. Sometimes (like with a railroad spike in a tie) the high frequency ticking of the rotor bits hitting the metal can be sensed and the load can be reversed and dumped out the back to be sorted. However what more commonly happens (with loose metal) is the system simply senses the hit milliseconds before the contamination gets pulled in and destroys the bits.
What I'm curious about is if anyone out there has seen this problem solved before. Perhaps in a different application which I can leverage to this one. Also, any thoughts on sensors which might work. Other ideas? I'm open to any input...
Thanks!
Christopher Ott Ott Electronics Corp. Chandler, AZ, USA