This invention relates to weighing and feeding apparatus for fine particulate materials, such as powders, and particularly to loss-of-weight type weighing apparatus.
Fine particulate materials, such as powders, are frequently fed in industrial processes using hopper-type scales as weighing apparatus, so that a particular weight of material required for a particular process is accurately fed. In the type of scale known as a loss-of-weight scale, a hopper containing the powder is constantly weighed, and the powder is dispensed from the bottom until the amount of weight lost by the scale is the desired weight. In the type of scale known as a net-weight scale, an initially empty hopper is weighed and the powder is fed into it from another hopper (or other feed device) until the initially empty hopper has gained the desired weight. This invention is directed primarily to loss-of-weight feeders, but has application in net-weight feeders as well.
In both types of weighing apparatus, some type of conveyor feeds the powder out of its initial hopper. These conveyors are best suited to meter volume, rather than weight. Because there is always a column of powder falling between hoppers when the conveyor is stopped, the conveyor is stopped before the desired weight is reached to take into account the weight of the falling column. Therefore, it is important to know the density of the powder as it is fed. Because the powder in the hopper is mixed with air, the powder must be "conditioned" in order to assure uniform density. If the conveyor is a screw conveyor, the powder must also be conditioned to keep it flowing in order to prevent "arching"--the formation of an arched void around the screw after an initial quantity has been fed. It is known to condition powders in hoppers by vibrating the hopper, placing an agitator in the hopper, flexing the hopper walls with motor-driven paddles, or mounting an acoustic transmitter in the hopper. However, these methods of conditioning do not offer precise control of the conditioning process.
In order to precisely meter the powder, it is known to provide the conveyor with two feed rates--a higher "bulk" or "full" rate, and a lower "dribble" rate. However, known systems have provided "turn-down" ratios--ratios of bulk rate to dribble rate--of at best about 50:1. With modern weighing apparatus, it is possible to get better resolution than can be taken advantage of with such turn-down ratios.
It would be desirable to be able to provide apparatus for conditioning fine particulate materials in a hopper with better control than has been available.
It would also be desirable to be able to provide a weighing and feeding system for fine particulate materials incorporating high-resolution weighing apparatus.
It would further be desirable to be able to provide such a weighing and feeding system with high-resolution feeding apparatus.