Load weighing systems are generally known in the art. They have been used, in one application, in the weighing of loads carried by waste trucks. The need for accurate weighing systems in this industry are known and include the fact that waste carriers are charged an amount based on the weight dumped in landfill sites. The waste carriers in turn would also like to charge their customers on a per weight basis as this would be more fair and cost-effective.
It is also important to know the load that a waste truck carries because of concerns of weight restrictions on many roadways and fines resulting from trucks being overweight.
The impending development of a charge imposed on residential users for the pick-up of their domestic garbage, again on a per weight basis, has focused the need to develop an accurate and efficient load weighing system.
The load weighing systems of the prior art typically use load cells or they measure indirect forces in these applications. A load is placed, for example, on a fork lift assembly of a waste truck and measurements are taken. The load cells in the prior art systems generally isolate and measure forces acting in one axis only, being typically the vertical axis, and attempt to compensate, in their measurements, for forces acting in other axes. Prior art systems have also been used to measure indirect forces such as forces acting within the hydraulic cylinders used to lift the fork and forces on the arm of waste trucks.
Many problems have been encountered with the use of load cell systems which attempt to isolate forces acting in a single axis. When the waste truck is emptying a container, problems such as uneven placement of material in the container, containers moving during the lift cycle and the rough operation of the hydraulic system with acceleration and deceleration forces make weighing-in-motion very difficult. These factors must be compensated for through the use of additional devices such as inclinometers to locate the preferred angles for measurement; accelerometers or devices which allow for smooth acceleration; complicated mathematical algorithms involving calculations of centers of gravity; software extrapolations; and calibration curves. These additional methods or devices possess inherent error factors and therefore contribute greater error to single axis weighing systems.
There have been many solutions disclosed in the prior art directed to load weighing systems, but all suffer from drawbacks. Waste trucks typically have arms or forks which engage, lift, empty and return containers or bins to the ground in a cycle. The preferred time to measure the weight of the load is usually during this cycle. Typically, these waste trucks contain powerful machinery and hydraulic cylinders in order to be able to lift large loads. Weighing systems which require the stopping or slowing down of the arms or forks in order to measure the load of the weight during the cycle suffer from several problems. There is firstly an unacceptable time loss with these systems; this type of static weighing reduces the productivity of the waste truck. There are secondly large vibrations generated by this type of system which cause wear and tear and premature breakdown of the parts associated with these systems. These drawbacks add costs to the systems which have been found to be unacceptable. U.S. Pat. No. 4,645,018 to Garbade is an example of such a system.
Other weighing systems which have been proposed involve measuring fluid pressure or other variable forces in the hydraulic cylinders used to lift the load. These systems measure variables remote from the load and therefore larger inaccuracies will result from external factors such as acceleration of the fork and fluctuation of the hydraulic pump. This type of system typically includes calculations and algorithms based on assumed centers of gravity or methods of measuring centers of gravity. Due to the remoteness of the measurements, the influence of external forces, and factors such as off-center loads, these types of weighing systems are not as accurate and efficient as desirable. Examples of these systems are illustrated in U.S. Pat. No. 4,771,837 and U.S. Pat. No. 5,178,226.
There have also been weighing systems proposed which utilize a vertical load cell. The load cells are located such that the load is placed upon these cells. However, these cells tend to suffer from unacceptable wear and tear because of the large loads which are placed upon them. Examples of these devices are illustrated in U.S. Pat. Nos. 4,645,018 and 4,714,122.
Other systems have also been disclosed, as in U.S. Pat. No. 5,245,137 which utilize strain gauges mounted on areas of the waste truck remote of the load. These systems also involve frequent calibrations of sensors. The remote location of the strain gauges means that the strain must travel a great distance to be sensed by the strain gauges. Consequently, the measurements are indirect and their accuracy will be affected by other factors. One such factor is the physical properties of the lift arms where the strain gauges are typically mounted. This will have a great impact on accuracy as the lift arms possess individual characteristics which vary from arm to arm and truck to truck. These types of weighing systems have led to inconsistent weight results with errors in the range of +/-1 to 10% per lift. Any change in the characteristic of the lift arms, for instance, cracks on the arms, welds on the arm, plates welded to the arm, and temperature defined changes affect the strain profile and lead to a wider range of inaccuracies which are not acceptable. These systems also rely on frequent calibration of the load cells in order to compensate for many of these factors and to achieve some degree of accuracy. The frequent recalibration of the systems have been found to be inefficient and impractical.
Weighing systems used in the prior art also have had problems with cable noise, signal distortion and generally with communication of the signal from the load cell to the computer where the data is processed and the weight calculated. Typically, analog signals are generated by the load cell and transmitted to the computer along long cables which run the length of the entire truck and sometimes twice the length of the truck. The integrity of the analog signals are generally affected by factors such as vibration, bends in the cable and engine noise. These factors directly affect the accuracy of the weighing system.
When using single axis systems in order to determine the weight of loads, some but not all of the external forces can be compensated for with software extrapolations, angle measuring devices and accelerometers. There remains, however, components of error due to the external forces, due to the load cells themselves and due to the devices and extrapolations used to compensate for the external forces.
Errors in these prior art devices can range from +/-1 to 10% per lift which is not acceptable. As a waste truck is generally rated to lift up to 9000 lbs of refuse, it can readily be seen that on a per pound basis large ranges of error can result in waste truck operators or their clients losing a lot of money. Therefore, an accurate load weighing system would be desirable.
According to one aspect of the present invention there is provided a method for determining a weight of a load using a lift assembly comprising a weight bearing portion, at least one loadcell sensor, located on or adjacent to the weight bearing portion, wherein each loadcell sensor comprises at least one weigh post and a plurality of strain gauges operatively connected to each weigh post to take measurements of forces acting on the weight posts, the method comprising:
pre-calibrating the loadcell sensors of the lift assembly and generating a calibration table;
lifting the load with the lift assembly;
continuously taking strain gauge measurements of the forces acting on the weigh posts in at least two different axes while the load is being lifted;
taking multi-axis force measurements at at least one pre-determined position while the load is being lifted; and
calculating the weight of the load using the multi-axis force measurements and the calibration table
wherein accurate weights of loads can be repeatedly determined without pre-calibrating the loadcell sensors and generating a new calibration table before each subsequent weight determination.
According to a second aspect of the present invention, there is provided a method for determining a weight of a container, engaged by a lift assembly that travels through a lift-return cycle wherein the lift assembly comprises a weight bearing portion, at least one loadcell sensor located on or adjacent to the weight bearing portion and comprising at least one weigh post and a plurality of strain gauges operatively connected to each weigh post to take measurements of forces acting on the weigh posts, the method comprising:
pre-calibrating the loadcell sensors of the lift assembly and generating a calibration table;
causing the lift assembly to travel through the lift-return cycle which comprises lifting the container and returning the container;
continuously taking strain gauge measurements of the forces acting on the weigh posts in at least two different axes throughout the lift-return cycle;
taking multi-axis force measurements at at least one predetermined position while the container is lifted; and
calculating the weight of the material using the multi-axis force measurements and the calibration table,
wherein accurate weights of containers can be repeatedly determined without precalibrating the loadcell sensors and generating a new calibration table before each subsequent weight determination.
According to a third aspect of the present invention there is provided a method for determining a weight of a container engaged by a lift assembly that travels through a lift-return cycle and of material in the container, wherein the lift assembly comprises a load bearing portion, at least one loadcell sensor located on or adjacent to the weight bearing portion, each loadcell sensor having at least one weigh post, and a plurality of strain gauges operatively connected to each weigh post to take measurements of forces acting on the weigh posts, the method comprising:
pre-calibrating the loadcell sensors of the lift assembly and generating a calibration table;
causing the lift assembly to travel through the lift-return cycle which comprises lifting the container, emptying the material from the container and returning the container;
continuously taking strain gauge measurements of the forces acting on the weigh posts in at least two different axes throughout the lift-return cycle;
taking first multi-axis force measurements of at least one predetermined first position while the container and material are lifted;
taking second multi-axis force measurements at at least one pre-determined second position while returning the container, wherein the second position correlates to the first position; and
calculating the weight of the material and of the container using the first and second multi-axis force measurements and the calibration table,
wherein accurate weights of material and containers can be repeatedly determined without pre-calibrating the loadcell sensors and generating a new calibration table before each subsequent weight determination.
According to a fourth aspect of the invention, there is provided an apparatus for weighing a load being lifted by a lift assembly having a weight bearing comprising:
at least one loadcell sensor position on or adjacent to the weight bearing portion of the lifting assembly wherein each loadcell sensor comprises at least one weigh post;
a plurality of strain gauges operatively connected to each weigh post to take measurements of forces acting on the weigh post in at least two different axes; and
calculating means for calculating the weight of the load using the strain gauge measurements of the forces and for pre-calibrating the loadcell sensors to generate a calibration table,
wherein accurate weights of loads can be repeatedly determined without pre-calibrating the loadcell sensors and generating a new calibration table before each subsequent weight determination.