The motorized roller conveyor: in-line weight checking system

Requirement

Build an in-line weight checking system in the logistics and handling sector.

The project is designed for the transit and weighing of pallets weighing up to 3000 kg with standard dimensions (1200×800 mm) on a motorized roller conveyor, in a system where the PLC is positioned well away from the weighing system.

The roller conveyor has dimensions of 1500 mm x 850 mm and the unladen weight of just the mechanical structure (tare) is 1000 kg.

At the moment of weighing the pallet is to be placed on the roller conveyor by a lift truck, which is why it is necessary to design a static weighing system that takes into account any dynamic stresses caused by impact.

Solution

The system consists of:

The total full scale of the system is 8000 kg. This is obtained by multiplying the capacity of each individual load cell (2000 kg) by the total number of load cells employed (4).

Knowing the system’s full scale is very important. In order to avoid damage to the load cells due to accidental dynamic overloads, it is in fact good practice to have these types of systems work at a maximum of 60/70% of their capacity.

How do you know what capacity a weighing system is working at?

You have to subtract the gross weight from the total full scale, i.e. the sum of the weight of the structure alone (tare) and the maximum load to be weighed (net weight). In this specific case: (8000) – (1000 + 3000) = 4000. The system is working at 50% of its capacity.

How does it work?

motor and a photocell are also mounted on the structure.

forklift truck rests the pallet on the motorized roller conveyor, which allows the pallet to slide to the point where the photocell is located.

When the pallet passes, the photocell is obscured and stops the system. 
The pallet is thus located in the middle of the structure, right at the exact point where it is going to be weighed.
This is the moment when static weighing takes place and the measured weight is transmitted to the PLC via the Profinet fieldbus.

Once the weight has been recorded, the PLC instructs the system to restart.

Added value

With this solution and a few small precautions, the pallet is weighed in complete safety, avoiding potential damage to the weighing system caused by overloading or accidental knocks.

What makes the weighing system so safe and functional?

  • The threaded rods, placed horizontally one on each side, which act as a constraint to the structure in all 4 directions and thus protect the load cells from accidental knocks and overloads;
  • The TLM8 in a box, which avoids using the junction box for a connection in parallel of the load cells, providing advanced diagnostics
    Installed “in the field”, close to the weighing system, it receives the electrical signal from the load cells and transforms it into the more robust Profinet signal, with which it safely reaches the PLC, even if it is located remotely.
  • PV is the safest mounting kit for this type of application: it is equipped with an anti-tilt constraint and its self-centering ball-based system transmits the weight vertically with no lateral friction.
    By always bringing the force back to the middle, it helps the load cell remain constantly accurate and repeatable;
  • The PV feet, which make it possible to adjust the height of the accessory so that the structure rests perfectly on all 4 feet with no need to add any other stabilizing elements;
  • The pre-sales consulting service for the customer, which is essential so as not to experience any issues in the future.
    Our tips on how to correctly position the constraints against lateral forces to avoid mechanical friction ensure savings in terms of the time and cost of future assistance
Weighing Applications
Batching System

Program: Base, Load, Unload, 3/6/14 Material, Multiprogram

Belt Scale

Single Idler, Multi Idler, Dual Idler

Weighing Feeder

Conveyor Belt Weigh Feeder, Screw Feeder

Loss-in-Weight Feeder

Continuously precise and consistent material dispensing

Big Bag Filling

Accurately fill large bags with bulk materials

Liquid / Barrel Filling

Precise, consistent filling, minimize wastage, maintain product quality

Tank/Hopper - Silo Scale

Accurately measuring the weight of liquids, solids or bulk materials

Checkweigher

Verify that individual products or packages meet a specified weight range

Truck Scale

Measure the weight of bulk-material inside trucks and determine the transaction value