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Feeder accuracy

It is Based on the principles of mathematical statistics using standard deviation and averaging, we usually describe the accuracy of screw feeders in terms of repeatability and linearity

Repeatability is an indicator of consistent flow of screw feeders.

Linearity accuracy is a measure of the accuracy of the screw feeder's operating flow range.


Repeatability


The repeatability is based on the standard sample discrepancy, which describes the flow rate of the screw feeder in a particular time, and the discrete situation of several flow samples in each sampling period. 

It is one of the important indicators to describe the repeatability error of the screw feeder. 

The repetition error can be quantified based on the standard deviation.

Its calculation formula is as follows:

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n is the number of samples.

x is the weight of the samples in the sampling period.

We usually describe repeatability as 95.5% of the sample weight within 2STDEV (2 sigma):

Repeatability%=x (average weight) / 2STDEV


Linear accuracy

Linear accuracy describes how accurate each operating point is over the feeder's operating range from minimum feed to maximum feed.

That is the error between the actual feeding amount and the set amount in the whole range. The smaller the error higher is the linear accuracy of that feeder.

The characterization formula is as follows:

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X is the average weight of the sample weight.

F is set to the set standard weight value

To obtain linear accuracy, take the array sample weight with respect to time, as you do to measure the repeatability of the feeder.

Under normal circumstances, we take 5%, 25%, 50%, 75%, 100% flow rate and continuously take 4 groups, each group has 10 sample weights.

Then calculate the average sample weight of each group.

The difference between the linearity error rates for each set point is calculated according to the above formula.

These errors are the linearity accuracy and characterize the accuracy of the feeder within its operating range.


How to properly test a screw feeder?

How to correctly sample the standard deviation obtained is the key to accurately describe the repeatability error of the screw feeder. Due to the lack of professional measurement methods in most factories, traditional sampling is obtained by manual methods.

The tools used are a chronograph(Stop Watch), two standard containers, electronic scales for sampling, recording paper, test materials and screw feeders to be tested.

Place one of the containers on the electronic scale, under the screw feeder outlet, prepare the stopwatch, put the test material into the feeder silo, start Run the screw feeder at a certain set flow. After it is stable, turn on the stopwatch. The sampling period is generally 1min. When the sampling time is up, read the weight on the electronic scale and record it. Generally, 30 such samples are taken. When the container is full it can be replaced with another container to continue sampling.

The main factors affecting the accuracy of manual measurement are the weighing accuracy of the electronic scale and the error of manual sampling timing. If the measured sample weight is smaller, the accuracy of the electronic scale is required to be higher, and the accuracy is usually 0.1% of the sample weight. The sampling timing should be appropriately increased to reduce the human error of the sample weight. At the same time, the same person is required to time the operation, and the sample collection should be carried out continuously.

For professional feeder manufacturers, automatic sampling procedures are usually used to ensure accurate sampling, reduce human errors in manual sampling, and simplify the data processing process.

The standard automatic procedure sampling tool is a high-precision electronic scale, which is connected to a computer for data processing. The connection is done through a standard communication port, such as an RS232 or Ethernet port. A standard data processing program is used to quickly process data, calculate and give the accuracy indicator.

Sonner uses a self-developed sampling program to achieve differential dynamic sampling. This sampling method can arbitrarily set the output sample weight at intervals of 0.1 to 10 seconds to meet the needs of subsequent 1min sample weight data processing.

This process obtains any group of sample weights without systematic errors, and correctly evaluate the repeatability and linearity of the feeder. . 


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