The basic premise of your application appears to be correct. The screw operating in the FAST mode will be difficult to repeat accuracy, so the FAST/SLOW setpoint must be sufficiently long to allow the screw to coast completely to the SLOW speed. When in doubt increase the time operating in the SLOW speed.

Considering your errors are with an empty hopper, the material may be flushing out the screw when the head pressure is reduced. Conversely the reduction in head pressure may inhibit the material flow with an empty hopper. What is the material and flow properties?

One aspect you did not mention was the sequencing of the vibratory discharger motor. Remember the discharger flows material proportional to the speed, however can give a violent transient oscillation during shutdown. This phenomena will discharge a large shot of material when you want the flow to stop so may be affecting your final batch accuracy. Make sure the discharger motor is turned off at the FAST/SLOW setpoint.

Regards, Delmar Schmidt

Melfi Technologies Houston

www.melfitechnologies.com ■

If you are only feeding 10KG/minute in the slow mode then flushing should not be a problem. What is the batch size and the weighing errors? What is the diameter of the screw and tube? The errors could be attributed to a partially discharged flight if the screw is too large.

The operating frequency if the activator is not the issue, it’s the uncontrolled oscillation (wild jumping) during the shutdown. Any oscillating component will have a resonant frequency resulting is excessive amplitude. Just watch your finely balanced car engine make a jump when you shut it off.

I am curious if the bin activator is shaking the screw feeder and knocking product from the flights? Is the bin activator shaking the screw feeder and tugging on the flexible connector and upsetting the gain-in-weight hopper?

Carefully observe the system in operation and make note of the material flow and movements of the individual components. Regardless I would turnoff the activator during the slow feed to minimize movement.

Regards, Delmar Schmidt

Melfi Technologies Houston

www.melfitechnologies.com ■

Not that I'm an expert in screw feeders, but for maximum accuracy I think you should aim to stop the scroll in the same position each time. ■

Scott

It would appear to me your control system should be telling your feeder when to change from fast to slow speed.

Typically, the scale will measure the weighment until the final 10% and then tell the screw to go into dribble mode.

In doing so, the level of material in the bin will be immaterial other than the time it takes to complete the weighment, ie: when the loading of the screw drops, the time to complete the weighment extends... ■

I just wanted to let others who might be reading this post and considering what type of discharge dosing system to use that we have VERY HIGH accuracy rates with our vibrating dosing feeders.

They also cause no product degredation like screw feeders do.

There is no reduced feeding rate when the hopper is low, it is always constant, till the last drop.

They are even used in a pigment powder dosing application that colors ceramic tiles. Previously the company (Wassau Tiles) saw a great variance in the color range of produced tiles.

With the new system using JVI dosing feeders, accuracy is on target and colors are consistant.

Read a case history about the application here:

href="http://www.jvivibratoryequipment.com/dosingfeederbatchingmixingsystem.html?medium=forumreply&source=bulkonline" target="blank">http://www.jvivibratoryequipment.com...rce=bulkonline

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A good control system is severely handicapped if the feeder arrangement does not provide a reliable and consistent supply. In general, a mass flow hopper section with integrally designed feed screw will give the best stability of flow and density and good practice dictates that a make-up supply from a bin activator should include a sufficient size of screw feed hopper to be fitted with a level probe that will cut off the supply when the feed hopper is nearly full.

A system that allows the bin activator to run irrespective of the screw speed will give variable and erratic compaction conditions that are difficult to predict or accommodate as a consistent feed.

Good accuracy should be attained by discharging 95% of the required amount in 75% of the time. If the output is not sufficiently consistent to allow this, I would suggest that it needs looking at by a specialist in hopper and screw feeder design, rather than an instrumentation technician. ■

The scale control system is not measuring throughput of the feeder, it is attending only to the weight delivered. The flow rate is of no consequence other than the desired cycle time of the operator. The question was how to stabilize the measurements to a more consistent level. The only thing that can control that is the scale. ■

Hello,

If this problem is still unsolved - Batch weighing with screw feeders must have the screw flights filled consistently - else during Inflight corrrection - the forecasted estimates will not match up !

The advice of using a vertical section above the screw inlet - with a min-max fill arrangement will surely help .

To improve the inflight correction further, some program changes in the PLC can be done . If you are interested to know the concepts , please email . ■

The problem " The accuracy is worse when the hoppers are empty" - The control scheme is absolutely right. The weigh hopper is independent I guess. As the material in the storage hoppers gets lesser and since you have vibro feeders discharging the material the vibration level of the system increases. These vibrations are transferred on to the weighing system.

Isolate the weighing system either by taking supports outside of the existing structure else insert anti vibration pads between the weigh hopper and the support structure. ■