Many industrial feeders and weighing systems are equipped with force transducers or sensors, also known as load cells. These devices translate any kind of force such as tension, compression, pressure or torque into an electric signal which is then measured and analyzed. There are various types of load cells. Experience has shown that especially vibrating wire load cells are ideal for continuous weigh-feeding applications as they are flexible, reliable and accurate. When looking at load cell accuracy, the resolution is an important parameter since it describes the smallest increment detected by the system. The vibrating wire load cell from Coperion K-Tron has a resolution of 1:8,000,000 valid counts in 20ms over the scale range.
How to calculate and measure load scale accuracy
To determine true accuracy of a scale it is necessary to automatically and consecutively take samples from the feeder output. In this context, Coperion K-Tron offers the K-Sampler package, a test sampling system for the evaluation and validation of the mass flow performance of gravimetric feeders. The K-Sampler system has the capability to test either a feeder scale or a reference scale. This system measures and characterizes the output of a feeder in terms of repeatability and deviation from setpoint. Repeatability describes the distribution of samples around the arithmetic mean sample value and depends considerably on the characteristics of the bulk material, the sample time and the feed rate. Standard deviation is employed to quantify the consistency of the feeder’s discharge stream. Assuming a random distribution of sample weights around the mean value, the weight of 95.5% of the samples will lie within 2 sigma (2 standard deviations) from the mean.
Deviation from Setpoint describes the deviation of the mathematic mean value from setpoint, measured over a continuous feeding time of at least 30 minutes. The deviation from setpoint is provided as a relative value in % of set point and absolute value in grams. It is also expressed over a turndown range from the maximum rate of the feeder. The typical turndown range is 20:1. This also represents the linearity of the feeder.
As a critical component in dry bulk solids weighing systems, load cells may encounter many sources of interference. In order to avoid fluctuations or other accuracy-related problems, it is recommended to check the following factors.
Three Factors That May Influence Load Cell Accuracy
Environmental disturbance. Wherever there's mass movement, there's inevitably an impact on the common surfaces supporting the various pieces of equipment. For example, an extruder sitting on a frame together with a loss-in-weight (LIW) feeder is a potential source for disturbance (vibrations) to the feeder's weighing system. As the extruder may run at different fill levels and potentially different speeds, the vibration's amplitude and frequency will vary greatly. In this case, the best solution would be to measure the disturbance so that it could then be eliminated from the weight signal, leaving only the LIW feeder's actual weight. The big problem is that in order to measure the disturbance, you have to resolve a weight that's cycling at frequencies of 1,200 rpm (20 cycles per second) or more. Using a technology with a fast sampling frequency, this disturbance can't only be measured, but, in conjunction with specialized control algorithms, it can be effectively eliminated in a range from only a few hertz (a frequency equal to one cycle per second) to near the limit of the measuring frequency. Combining this with a high-resolution load cell means accurate weighing is still possible, even under extreme conditions.
Pressure. A weighing system often needs to be enclosed, particularly when harmful or dangerous materials are being handled or when the natural environment may have an effect on the bulk material. This means that if there's any pressure within the vessel, it will act as a force against any coupled nonweighed elements. As a result, pressure above the weighing equipment will press down on the scale, increasing the weight, while pressure at the outlet will press up, reducing the weight measurement. A typical example is a refill valve for a weighing tank above the LIW feeder. Even where these weight deviations are considered as very small, they can have a big impact on a continuous metering system. With advanced weighing systems coupled with pressure compensation, either electronic or mechanical, it's possible to eliminate these influences and provide a clean weight signal.
Temperature. Whatever the weighing technology, changes in temperature will affect the weighing system's mechanical properties so if temperature changes are unavoidable, compensation is required. Some sensors have a pretested temperature profile already programmed, meaning the sensor can adjust its response based on ambient conditions. As these changes are generally nonlinear outside of a certain range, it's ideal to have a programmable profile of actual measurements for this compensation.
- email: KMelton@coperionktron.com
- telephone: +41 62 885 7333