Introduction
Weighing machines have long been used for measuring weights of various objects. The basic principle behind weight measurement is balancing of force due to gravity of an object with some known standard. You could easily find weight of small objects by using standard weighing balances having pair of pans i.e. one carrying the object to be weighed and the other is used to put loads of known weights until machine is balanced. This method cannot be applied on large scales involving heavy machinery, vehicles, or even humans.
Another traditional method of weighing involves balancing force of an object due to gravity by spring force acting in opposite direction. The object is placed on a platform that exerts force on the spring. Tension develops in spring and that produces a force to counter the weight of the object and to restore the spring to its original position. The measured weights are displayed in digital or analog format by calibrating them against the balanced spring force. Instead of springs, you could also employ load cells that could be calibrated for weight measurement by calculating the deflection induced by object on them . However, equipment used in these methods of weight measurement is susceptible to fatigue and wearing. As a result, springs and load cells will lose their precision after some amount of time . Moreover, weight measurement may also be affected by the external disturbances and environmental conditions.
In order to overcome the disadvantages presented above, hydraulic weighing system was introduced in 19th century. Plenty of arrangements have been proposed in the literature for weight measurement on small and large scales for a variety of objects. This study proposes a novel method of weight measurement of trucks and heavy loads. This effort finds plenty of industrial scale applications where fork-lifters are used to carry heavy weights. Our proposed system will determine whether the load lies within the capacity of fork-lifters or not.
Problem Description
In this section, we state the objective of this research study and project. Our mission is to design a small scale hydraulic weighing machine for measurement of weight of objects. The weighing machine design could be extended for large scale applications as well with slight modifications only. This project could be utilized for weighing trucks and other heavy loads. As mentioned earlier, this idea is applied in designing fork-lifters for transporting heavy objects. By measuring weight, we will have idea whether the heavy object lies within loading capacity of fork-lifter or not.
We will first provide the literature review of the hydraulic weighing methods proposed in previous studies. Then, we will explain our weighing machine design in detail, stating equipment required and their specifications. The proposed design will be further elaborated in next section in which we will derive our results. All required formulations and calculations will also be discussed. At the end, we will summarize our results, discussion, and findings in the section of conclusions.
Literature Review
The notion of hydraulic weight measurement is not novel and literature is saturated in this aspect. Plethora of arrangements has been reported involving the simplest to intricate designs. We will discuss some examples of hydraulic weighing arrangements that have been employed in the past.
Other ideas of hydraulic weight measurement follow the traditional mercury tube temperature measurement in one way or the other. Consider the arrangement proposed in a patent containing an expandable bag pressing against weight to be measured . The bag is pushed by the object placed in the platform. The bag contains liquid of know density. It will be pressed against the force applied by the weight of the object. The liquid contained in the bag is forced into a transparent tube, whose level is calibrated to show the desired weight. Hence, the graduations of the scale provide the mass or weight of the object on the platform.
Another hydraulic weighing package used for refuse collection vehicles in discussed in a patent . This system dynamically weighs the garbage collected in vehicles. The weighing apparatus includes a hydraulic actuator for moving a lifting arm. It takes two measurements of the pressure applied by the hydraulic fluid as the hydraulic arm lifts the garbage basket to be weighed. The measurements of pressure need to be adjusted by multiplying them with a known factor derived from standard weights. The results are averaged to have more accurate values of weight. The system also contains arrangements for dealing with changes in hydraulic flow characteristics of the fluid. The inclination of the vehicle is also adjusted to provide true representation of weights.
The aforementioned hydraulic weighing systems are just a few examples in literature. They have their own advantages and disadvantages. In this research study, we will propose another hydraulic weighing system for small scale applications that could be easily extended for large scale applications for weighing trucks and vehicles. Our arrangement will be simple and effective, taking into account external disturbances, dynamic load variations due to density changes, and pressure surges. The complete design features are discussed in the upcoming sections.
Design Considerations
In this section, we will describe our hydraulic weighing system design features. We will consider small scale applications like measuring weights of waste bins, machinery used in industries, and domestic items. The design will be flexible to accommodate large scale applications also.
The following apparatus will be used in our implementation of small scale hydraulic weighing machine.
Lifting platform
Cylinder containing water
Measurement tube graduated in Kg
Movable piston
Level sensor
Analog to digital converter
8051 Microcontroller
16×2 LCD
The object to be weighed is placed on the lifting platform. Such a platform could be attached to vehicles or fork-lifters for carrying various objects. Weight measurement will guarantee that the platform is not heavily loaded.
As the object puts its weight on the platform, there should be some mechanism to convert its weight to calibrated format that could be easily displayed. We place the piston beneath the platform and it will get pushed into the water filled cylinder. The hydraulic drag force exerted by piston onto water is proportional to the force applied by the object due to gravity. The water-filled cylinder is hydraulically connected to the measurement tube. The displaced water by the piston moves into the measurement tube that is initially empty. The column of the water inside tube will be representative of the weight of the object. Therefore, we need to calibrate it in suitable units like kg.
Apart from analog measurement of weight of the object using graduated scale on the measurement tube, we also display it in digital format. For this purpose, we use level sensor that measures level in tube with the help of static head applied on its bottom. This level measurement is converted to a digital format using analog to digital converter. Then, we input this digital reading to an 8051 microcontroller. It is interfaced with an LCD with 2 rows and 16 columns. The digital input will be displayed after applying conversion factor into LCD. We can use any of the four input ports of microcontroller depending upon the byte size of the digital data. The programming of LCD to display the data available on Microcontroller ports is also straightforward as standard codes are available for this purpose.
The design of our proposed hydraulic weighing machine looks simple and effective. However, we should cater for sources of error in this weight measurement method. Load variations due to absorption of liquid presence in object, pressure surges, ambient conditions of temperature and humidity affecting density of object and also of water in cylinder are some disturbances that could lead to erroneous weight assessment. We have taken the following measures to cope with these situations:
Density compensation is provided in converting the level of water in measurement tube to corresponding weight. This action will compensate changes in density of water due to changes in ambient conditions.
In order to cope with pressure surges, we have incorporated the system in microcontroller that it averages out readings over the duration of 10 seconds. Therefore, the object needs to be kept for at least this duration for reliable weight measurement.
While measuring weight of the object containing liquid, there are chances that evaporation might occur depending upon ambient conditions. As a result, the weight of the object might vary and we need to counter this situation. We will calculate the vapor pressure of the liquid with the help of some portable gage and then input it to microcontroller for required adjustment.
Dynamic load variations due to its movement on the platform are avoided by clamping it from its corners. Hence, accurate weight measurement is possible in this manner.
Although the design perfectly suits small scale applications, yet it is easily extendable for weight measurement of heavy loads such as trucks. The extension is done by enhancing the designs of platform, cylinder, and measurement tube. The rest of the equipment will stay the same. Depending upon the anticipated largest weight, we need to design a suitable platform that has required load bearing capacity. The volume of cylinder, size of piston, and measurement tube need to be scaled accordingly. The piston should be strong enough to move linearly as the weight is put on the platform. The swaying movements inside water-filled cylinder should be avoided. The precision of the graduation scale on measurement tube will also vary. The minimum count should be adjusted to have required accuracy. The size of the tube will be large enough to accommodate the displaced fluid as piston forces it inside cylinder. The design of microcontroller for weighing huge objects will stay the same. However, we need to choose a suitable analog to digital converter that acts in the suitable range of analog data of static head created by water in the measurement tube. The programming of LCD to display data in the required range will be modified.
Overall, the design of hydraulic weighing machine is quite flexible and can be maneuvered depending upon situation and load requirements. The design is scalable to a great extent. The formulations, equations, and calculations will be presented in the next section.
Formulations, Calculations, and Verifications
In this section, we will provide the dimensions, calculations, and other formulations required in the design of our proposed hydraulic weighing system. First of the all, the design of platform is under consideration. The maximum load must be estimated for the current design. On small scale, we assume that the maximum weight will be 20kgs. The dimensions of the load are assumed to lie within 4 square feet. Therefore, we need to design square shaped platform with 2 feet length on each side.
Next, the size of the piston needs to be estimated. The size of the cylinder should correspond to the entire length of the water-filled cylinder so that it could move inside it freely. The height of cylinder and length of piston should roughly be same with some safety margin for cylinder’s height. Assume the piston size to be 1 foot and height of the cylinder to be slightly more than that to avoid piston touching the bottom of the cylinder when it is inserted to its entire length.
Our next design consideration is the volume of the cylinder. The required parameters are height and radius. The height has already been estimated to be slightly more than 1 foot (say 1.2 feet). The radius should be approximately 0.25 feet. The volume of the cylinder is calculated as follows:
Volume=πr2h=3.141×0.252×1.2=0.24 ft3
Now, we come towards the design of measurement tube that displaces the liquid contained inside cylinder. The water volume inside cylinder must be less than its entire volume (say 0.2ft3). At maximum, the measurement tube will have to displace the whole water inside it if the maximum weight is put on the platform. Therefore, the volume of the measurement tube should roughly correspond to the volume of the cylinder. Since, radius of the measurement tube is normally very small, so its height should be sufficient to carry the required volume. The height is estimated to be around 5ft.
The pressure sensor attached at the bottom of the measurement tube will be representative of the static head developed by the displaced water inside tube. The static head will be additional to the atmospheric pressure acting on the measurement tube. Since the density of water is roughly 1000 kg/m3, so the formula for finding the height of the measurement tube will be:
P=ρgh
Where P is the applied pressure by the water column at the bottom of the tube, g is the gravitational acceleration, ρ is the density of water, and h is the unknown height that needs to be measured. As discussed in the previous section, density compensation is incorporated while calculating the height of the measurement tube. This provision will ensure that weight measurement is reliable under all ambient conditions.
We need to calibrate height of the water column inside tube with weights put on the platform. For this purpose, we put some standard weights of known values on platform and then measure the heights accordingly. In this way, height of the tube is calibrated against weight. The value of height is provided to analog to digital converter that outputs to the 8051 microcontroller. The LCD is interfaced with microcontroller and it is programmed to display the calibrated weight. The conversion of height of tube into weight is carried out in programming of microcontroller.
The design of our hydraulic weighing machine is complete now. You may fabricate it with simple equipment available in market and use it for small scale applications as well as large scale applications.
Conclusions
This research project is related to designing a small scale hydraulic weighing machine. The design should be flexible so that it could be easily extended for large scale applications also. First of all, we introduced the subject and research scope in problem description. Then, we discussed the previous efforts in the same domain. We discussed the design of hydraulic weighing machines proposed in some patents. Then, we provided the detailed description of our design. Configuration of equipment and their interfaces are stated. Briefly stating again, it consists of a platform for carrying weight to be measured. The platform pushes a piston that forces the water contained inside a cylinder and it gets displaced in a measurement tube. The level of water is measured using pressure head developed at its bottom and it is then input to the microcontroller using analog to digital converter. The weight is displayed on LCD. The calculations, formulations, and dimensions of equipment are discussed in the last section of the report.
Overall, the design of our hydraulic weighing machine is simple and effective without involving too many active components. The flexibility of design allows it to be used for weighing heavy loads such as trucks. You will find many designs of hydraulic weighing systems, but this design covers virtually all applications discussed in literature. It is easy to implement and fabricate by using simple and cheap components.
References
Chang, J.-L. (2003). Patent No. US Patent 6,667,445. USA.
Creswick, R. (2003). Patent No. US Patent 6,627,825. USA.
Morris, A. (2001). Measurement and Instrumentation Principles. Elsevier Science.
Philadelphia, P. (1828). Journal of the Franklin Institute. Pergamon Press.
Smith, R. (1991). Patent No. US Patent 5,038,876. USA.
