Scale control

The control of scales is a highly complex topic. Even if basically only valves are opened and closed and the measurement signal is processed, the boundary conditions and in particular the differences between continuous and discontinuous scale control are significant.

Basically, the weighing signal is recorded via sensors (load cells), usually using Siemens technologies, amplified via a measuring amplifier and converted into a digital value, thus creating the basis for processing. Nowadays, however, the electronics not only control the loss-in-weight feeder but also provide important parameters for controlling the entire process.

Mr. Opdenhoff: Contact person for mixer systems

We can answer all your questions about scale control. Book a free, no-obligation consultation with Jürgen Opdenhoff.

To the date overview

Analog vs. digital

There are analog measuring cells (whose measuring signal must then be converted) and native digital measuring cells. Which is preferable depends on the requirements: If there is a low dynamic range and the requirements for scale control are rather low, analog measurement can be used for cost reasons. The more agile you need to be, the more likely you are to prefer a digital measuring cell.

Centralized vs. decentralized

There are three ways to decide whether to install the control unit centrally or decentrally:
The scale control can be implemented on site directly on the measuring amplifier. The advantages here are
- Less cabling work
- self-contained, robust units
- Standard interfaces such as Ethernet/Profiner
- the control algorithm is centralized and therefore easily interchangeable

However, this solution is usually not calibratable and requires external setpoints, which results in a high communication load. In addition, it is usually not calibratable.

With a "real" central control system, the measurement signal is processed in the central control system. This cannot be calibrated, is far less agile and is therefore practically no longer used

Instead, a decentralized, autonomous control system is usually used, which only receives setpoint values. The advantages here are
- Available "out-of-the-box" from Siemes
- calibratable or non-calibratable executable

On the other hand, there are disadvantages:

The control algorithm is not interchangeable
and you have little influence on the scale control.

Continuous vs. discontinuous

While continuous dosing controls continuous mass flows, filling process scales for batching are controlled discontinuously in accordance with the calibration law. The two processes have very different requirements for scale control and weighing electronics.

Up to now, continuous and discontinuous scale control have usually been carried out separately in order to implement legal-for-trade requirements and for economic reasons. The shift towards more software-based solutions will allow more flexibility here in the future.

Discontinuous scale control

Masses and batches are dosed here
A control system is used
A single-stage or multi-stage adjustable dosing unit is used (coarse flow/fine flow)
Many products are often dosed via a scale. The scale therefore often has many inputs and outputs
Discontinuous control is also used for filling scales / checkweighers

Continuous scale control

These are used for dosing continuous mass flows (mass flow control)
A control system is used
A continuously variable dosing unit is usually used here
Usually only one product or a few products are dosed
Continuous scale control is not subject to the Weights and Measures Act

High weighing accuracy is important for continuous processes. With a good control algorithm, you can come close to the static weighing accuracy of the hardware. The algorithm must compensate for disturbing influences such as refilling, aspiration or vibrations from the environment (e.g. from forklift trucks). In addition to the weighing signal, the actual speed of the dosing drive is usually also measured and controlled here.

Test of scales

A scale control system should always be tested under production conditions, because the individual influences are only present in the respective production environment. Nowadays, solutions are often only tested in the laboratory with test weights.

If you take a predetermined, calibrated quantity in production and check the measured values, it is more time-consuming but more honest.

The decisive factor here is to implement discharges reliably and reproducibly using discharge aids so that what has been weighed is actually discharged. Control of the scale control, tare and input conditions are also decisive factors here.

OPDENHOFF also provides you with the design of the load cells so that you can measure all factors influencing the measuring accuracy under production conditions: Dosing accuracy, tare weight, payload of the body

Selection of load cells

Selecting the right load cell is essential for the control result. Factors such as resolution, tare load and boundary conditions play a role.

Scale control with OPD. Pro Care Opdenhoff

Retrofit

Is your existing scale still from the Stone Age? An "analog treasure" whose accuracy leaves a lot to be desired, whose load cell or measuring amplifier is no longer available as a spare part and for which integration into process chains or control systems will remain a pipe dream?

The solution: OPDENHOFF Retrofit. We

analyze your needs
select the right scales together with you
Replace the existing hardware and integrate the new scale into your processes

Your advantages:

Spare parts supply

A new component can be delivered promptly in the future.

Increased requirements

Communication with automation and visualization components is now standard.

Avoid production downtime

What did each non-productive hour cost you?

Avoid loss of know-how

What if your expert who is still familiar with the old scales retires?

Mr. Brunetti: Contact person for dosing systems

in matters of scale control Antonio Brunetti likes free of charge and without obligation at a personal consultation appointment.