How a Package is Weighed

Thompson Scale checkweighers are the only checkweighers on the market that do not require the use of photo-eyes to detect the presence of a package crossing the weigh station conveyor. Each package is weighed based upon changes to the load cell output alone. To determine an accurate and repeatable weight, the checkweigher constantly monitors the load cell, determines an operating "Zero," and then compares current load cell output to that zero.

Production rates and weigh station conveyor speeds are based upon several factors. These are package length, weigh station conveyor length, and distance between packages. As a package enters the weigh station conveyor, the load cell output increases. This value continues to increase until the package is completely on the weigh station, at which time the output plateaus or levels off. The package must be completely on the weigh station conveyor for a minimum of 0.6 second for a valid and accurate weighment. Speed charts found in this section depict the maximum production rates and conveyor speeds attainable on a variety of different conveyor sizes. Use these charts to select the conveyor length, production rate, and speed best suited to your application.

Since we determine the weight of a package by capturing the weight at the plateau, we must also have a means of terminating the weighment cycle for that package. This is done by monitoring the load cell output and searching for a set number of consecutive declining weight samples. Once this criteria has been met, the weighment is terminated, and the package's weight is displayed. Typically, this takes place when a small portion of the package enters the conveyor downstream of the checkweigher. At this instant, another package can enter onto the weigh station conveyor without interfering with the current weighment.

A good rule of thumb is that the distance between the head of one package to the head of the next should be equal to the weigh station conveyor length, plus 2 inches.

Weight Data Calculations

As soon as the set number of consecutive decreasing weight samples is met on a package crossing the weigh station, the 4693 Controller terminates the weighment. The controller's CPU then goes back to the samples collected from the plateau and averages them to obtain a weight value. This is then compared to the zero stored directly before the weighment began. The Container Weight value stored in the current product setup is then subtracted from the remainder. If the Container Weight is set to Zero, then GROSS weight is displayed on the 4693 Controller. If this value is a positive number, then NET is displayed.

The final weight is compared to the Target Weight and zone tolerances entered into the current product setup. If the final weight is within acceptable tolerance, it is classified as a "Good" weight. If it falls out of acceptable, but is still not out of tolerance, it is classified as either "Low Pass" or "High Pass." All other comparisons fall into the Reject categories. The appropriate zone box on the 4693 Controller's display will change to reverse video for that weighment, and its associated counter will increment by one.

A corresponding 110 VAC relay output is actuated for the particular zone of this weighment. Zone outputs (outputs #1 through #5) will stay on continuously until the next weighment occurs. These outputs are provided for interface to Thompson's RZDP remote zone light tower. A reject output is also actuated if the weighment is out of tolerance. The reject output is operated by two timers within the 4693 Controller. The first timer initiates a delay from the moment the package was weighed, allowing the package to travel to the reject station. The second timer is the duration the reject output will be held on. The 4693 Controller can store up to seven (7) rejects in memory, running separate sets of timers for each.