In most food processing facilities, the typical weighing system is comprised of a weight controller, a load cell-based platform scale, and a programmable logic controller (PLC). In the early 1980s, process weighing systems and control instruments found significant advancements after the introduction of strain gauge-based loading cells that allowed for batch weighments in containers as large as 6 feet high and 8 feet wide.
In recent years, however, modern weighing systems have since been equipped with programmable logic controllers (PLCs) that offer more robust industrial capabilities. These modern weighing system rely upon load cells, also known as transducers. The load cells transfer weight, a mechanical force, into electrical signals, receiving a specific excitation voltage of 515 VDC to generate a tiny analog signal in proportion to its applied weight or force.
The excitation voltage runs through the weight converter, which also converts the load cell’s information into a digital signal after receiving and conditioning it. Finally, the weight converter transmits the weight value to the PLC. This, in turn, logs the weight and controls the process by releasing and closing the appropriate gates, valves, and feeders to maintain the correct programmable measurements.
Let’s take a look at a few common process weighing applications and how PLCs are integral to these systems:
Mass Flow Measurement
Process weighing systems use the difference between the weight of a given material at two distinct points in time to arrive at a measurement of mass flow. Material flows on and off a scale through a screw auger at a different rate than it would a pump or a vibrating conveyor belt. And the interference noise impacts the readings, which influence the calculated flow rate.
Plant and manufacturing noise can, therefore, affect mass flow readings as well, particularly in the absence of instrumentation that filters out environmental noise. Not unlike blending and filling, which we’ll discuss below, accuracy and speed are vital to this weighing system.
Blending
Blending is among the most reliable weighing methods to reduce batch time because it allows for a simultaneous dispersal of ingredients into one mixer. To accomplish this, the weighing system should come equipped with multiple scales with the capacity to control feed rate so that it doesn’t exceed the programmable cutoff weight.
Since speed and accuracy are paramount to this method as well, PLCs are crucial to blending weigh system applications. PLCs equipped with the appropriate weight sampling rates and scan times ensure precision process inputs for complete control.
Batch Weighing, Dispensing, and Filling
Batch weighing, dispensing, and filling are independently distinct procedures, but their speed requirements are the same. Hence, they tend to be categorized together as weight processing systems. Each system accurately and continuously detects weight changes at speed to provide the appropriate weight cutoffs. They monitor weight at a rate of at least 20 times per second to gather information about small variances. Likewise, these systems block out plant process and mechanical noise during filling and dispensing cycles for accurate flow rate readings.
They provide even system response times since variable response times contribute to incorrectly measured materials. Lastly, these systems rely upon PLCs to seek out potential faults that signal malfunctioning gates, valves, or ingredient bridging.
For these systems, a local display and some degree of manual operation of weighing functions and controls are helpful. PLCs do assist in making weight cut-off decisions in these applications. However, paying attention to the means by which you transfer weight data to the PLC is crucial. Be sure to dedicate the PLC to the respective weighing operation exclusively.
Binary Coded Decimal (BCD)
BCD-based communication is the most efficient means of transferring weight data into an I/O module contained within a PLC. Not only is BCD communication among the quickest, but it is, likewise, tried and trusted as one of the oldest programmable single-scale systems.
Nonetheless, while BCD remains one of the fastest and most dependable industrial automation solutions for scaling, is not without its faults. The weight converter must be in close physical proximity to the PLC whenever using BCD. This allows the PLC to run the numerous wires needed to establish an interface with the system. Another downside exists in the fact that the BCD interface is a one-way communication, requiring a dedicated programmable logic controller to calculate weight.
How to maximize the capabilities of weight controllers and PLCs
PLC integration always benefits weight control systems. However, the best approach to maximizing the potential of weight controllers and PLCs while reducing costs is through the use of a robust bidirectional interface. While PLCs perform a pletheora of essential tasks, many of those functions are better allocated to a dedicated weight controller.
While PLCs have advanced process weighing systems considerably, the fundamentals of good process weighing can’t be overlooked. Careful attention to weight control placement and how you manage weight cutoff decisions will provide the most desirable outcomes.
Where do I find industrial automation companies near me?
Are you exploring the idea of integrating programmable logic controllers into the process weighing systems at your food processing plant? EZSoft develops software that controls complex automated machinery at food processing facilities in your region.
Call (484) 568-5040 today to learn more about EZSoft’s information and process control solutions and how PLCs will benefit the weighing systems at your food and beverage manufacturing plant.
