Small Mouth: Models A, B, C – 8″ sq stand x 26-31″ high or Model D – 10″ sq stand x 42″ high

Time to do something, when Batch Weighing Accuracy Performance does not meet Accuracy Requirements. Understanding the problem should the first thing on the to do list.

Sometimes accuracy problems are design issues and sometimes maintenance issues. If it worked before, you are lucky you only have a maintenance issue. If your system has trouble maintaining itself within tolerance, you may not be so lucky after all.

Closed Loop Control and every component all affect Closed Loop Control Performance.

1) Scale accuracy and

2) the response time it takes for the control system to sense the change accurately

3) and the response time it takes to control the flow to the scale all affect performance

Material is being fed between controlling devices and scale. The shorter the distance between the two improves Closed Loop Control response time. Controlling devices can be gates, valves, rotary valves, augers, conveyors, etc. A consistent head of material feeding these is important to maintain. The system is adjusted for a given amount of material to be delivered in a certain amount of time. When that material is not there at the scale, your system has to adjust on the fly. Production rates and product quality suffers.

An unknown factor affecting accuracy is the amount of material (in the air) between the controlling device and the scale. The material in the air the flow controlling device and the scale is always questionable when flow rate is not consistent. This problem can be minimized at the cost of production rates.  So the question is; At what cost do you fix the inconsistency or live with lower productions rates or trade off to less accuracy?

Production Rates and Quality Control often seems at odds with each other. Tie that in with inconsistent feed to the weighing device and meeting set point becomes more difficult to achieve.

Most readers already have a n electro-mechanical system in place. Usually, mechanical fixes are harder to implement than electrical systems.

Batching systems compare setpoint to the scale readings. Material can be weighed in one of two ways.

• Continuous Weighing (Dynamic Weighing) devices such as Bin Weigh Loss or Impact Scales or Belts Scales weigh material on the fly. Weigh Loss Systems have an advantage because material is not free falling to it but diminishes when the system takes some time to average out over time what the flow rate is. Belt scales have the same advantage but the advantage diminishes when a belt scale reaches set point and the belt is left with a half belt full of material. This half belt full is an unknown about to be fed to the next batch. Consistent feed rate helps to minimize that problem The impact scale is your best choice because it weighs at the end of a conveying device. Continuous Weighing devices take less vertical] space and are easier than Batch weighing to fit into a process. Some continuous weighing devices only take 4” of vertical space.
• Batch Weighing (Static Weighing) receives material in a container, weigh hopper. The hopper issuspended from load cells and has means to empty the material out of the bottom. When set point is reached, the filling process is stopped and the discharge device is activated. If the flow rate remains consistent from batch to batch, a Preliminary Cutoff Factor can be entered into the Batch Control to shut the filling process early to compensate for the material in air not that is yet weighed.

Whether you are weighing through the scale or to the scale, material will not be accounted for in free fall. This becomes an assumption. This assumption can be most accurate the more we keep our flow constant. A constant head of material helps.

When flow is not constant and when slower production rates are acceptable, the batch control can be set to creep up to setpoint at slower feed rates  by adding an additional set point or two. Auger, rotary valve and conveyor speeds can be slowed, gates can go into a chatter mode, gates can be positioned from completely open to half open to 10% open to closed.

The human element is always a factor. Everyone seems to have their recipe for the best operating procedure. Develop your best operating procedure taking all tradeoffs you have into consideration. Train your operator to use it consistently.

Net Weights vary…

• Check for varying feed rates. Minimize varying rates.
• Check that scale holds zero rate before and after weighing material.
• Check sensitivity of batch control and actuator system. Adjust for best performance
• Check for leaking feed and discharge gates.
• Check for consistent controlling device activation-deactivation. Loose linkage is often a problem. Operate the system in Manual Mode. This puts your problem in slow motion which it makes it easier to understand the problem.
• Check that all material weighed is being captured in the container being filled.
• Check reference scale for consistency.
• Develop your best operating procedure taking all tradeoffs you have into consideration.
• Train your operator to use operating procedure consistently.

Life is all about trade-offs. Educate yourself on the trade-offs and navigate to the best set of them for the job.

Clarence Richard, Instructor

Plant Operation Workshop Web-Based Training Provider http://clarencerichard.adobeconnect.com/workshopdemo

clarence@clarencerichard.com www.clarencerichard.com 952-939-6000