Contributions regarding force differences usage in weighting devices.

Hadar, Anton ; Szabo, Adam ; Bordeasu, Ilare 等

Abstract: The cylinder with differential piston is a measurement sensor, which works based on the force difference (pressure). In this paper the description and functioning of the measurement sensor is presented. For the first time this sensor is used in an installation for mass and weight measurement. With the use of microelectronics, a low weight, modern weighting device, precise, easy to use, employed for weighting fixed and mobile objectives, but also to protect civil constructions against overcharges that could affect their functional safety, is built. From an electrical point of view, the installation is made from two distinct functional blocks which are discussed in this paper.

Keywords: weighting devices, cylinder with differential piston, transceiver, measurement sensor

1. INTRODUCTION

In the literature, the cylinder with differential piston can be found in command and hydraulic drive diagrams (Vasiliu, N., Vasiliu, D., 2005), (Balasoiu, V., Padureanu, I, 2002), (Bordeasu, I, 2007). The authors have extended its usage in the mass and weight measurement domain, thus building a prototype for a weighting installation in which the cylinder with differential piston is the primary element of the installation. Very big masses (1-100 tones), so weights of [10.sup.4]-[10.sup.6] N, can be measured with mobile devices, light and precise; this being possible because the measurement sensor has the role of a pressure regulator. Basically, a 10 tones mass can be weighted and monitored by a helicoidally arc subjected to a force of 200 N, 250 times smaller that the external load.

Another technical problem that can be solved with the help of the weighting installation is the possibility to measure and weight very big loads, anywhere is needed. For example, a truck with a mass of 100 t can be weighted using these devices anywhere on the road, without the need to guide it to a special scale that could be located hundreds of kilometres from the place where the weighting is imposed. Of course these things are also true for an engine, a wagon on the railway, a car, camion, trailer, etc. The filling level of fix objectives such as bunkers, silos or receivers can be monitored all the time and during their entire lifecycle, this parameter being very important information for the administrators of this kind of storages. These devices can also be easily used to protect civil constructions against overcharges that could affect their functional safety (resistance structure). For example the access on a bridge for trucks with a maximum weight can be correctly established with the help of the above mentioned installation. When exceeding the reference weight (limit) a warning signal (light and/or acoustic) is immediately activated.

2. DESCRIPTION AND FUNCTIONING OF THE MEASUREMENT SENSOR

From the assembly drawing shown in figure 1, one can see that the measurement sensor is made of a cylinder body 1, continued on the superior part with body 6, which also has the role to separate the lower part under hydrostatical pressure from the upper part, where is created an insulated space, with no pressure; in this space one can find the displayed elements for the measured load. On the lower part of the cylinder body 1, there is a piston 3 for load reception, which with the help of elastic membrane 4 produces a hydrostatic pressure in the working fluid.

[FIGURE 1 OMITTED]

According to the load subjected to the instrument, the pressure is range in the interval 0-10 MPa.

Because of the communication holes "a" in the differential piston 2, the hydrostatical pressure will be the same on both faces of the piston, but the forces will be different. The difference of the two forces is due to the conversion core bar for translating movement in the rotation of the disc 7, static neutral, which makes a difference between the two surfaces of the piston with 0.2 [cm.sup.2]. This way is possible for the spring 5 to work only at 1/250 from the force value taken by the reception piston and transmitted to the liquid.

For the same reason, when a force is exercised on the device, thus creating a hydrostatical pressure in the fluid, the differential piston 2 will immediately start an upward movement, until the progressive force which appears in spring 5, along with force F' from the internal face of the piston, will re-establish balance (Bordeasu, I., et al., 2005), (Bordeasu, I., et al., 2002).

The gauge spring 5 from the general assembly takes 1/250 from the load value applied to the reception piston.

When weighting objects with masses up to 10 t, the nominal dimensions for the active components of the cylinder, obtained after designing and dimensioning (Vasiliu, N., Vasiliu, D., 2005), (Bordeasu, I., et al., 2002), are:

D = 80 mm--nominal diameter of differential piston (active face);

d = 5.05 mm--core bar piston diameter;

f = 30 mm--piston stroke;

[D.sub.arc] = 60 mm--spring diameter;

[d.sub.arc] = 4.3 mm--spring wire diameter;

N = 5 x [10.sup.4] N--maximum charging load for a cylinder;

p = 100 bar--maximum fluid pressure.

[FIGURE 2 OMITTED]

Using these values the following results are obtain:

S = [pi][D.sup.2]/4 = 5026 [cm.suip.2], F = S x p = 50,265 x 100 [approximately equal to] 5026 daN;

S' = [pi]([D.sup.2] - [s.sup.2])/4 = 50,06 [cm.sup.2];

F' = S' x p = 50,06 x 100 = 5006 daN,

As a result F - F' = 5006 = 20 daN

And the reduction ratio: i = N/F - F' = 50000 N/200 N = 250

Regarding the spring, the elastic constant is:

k = G x [s.sup.4]/8 x n x [D.sup.3.sub.arc] = 8,4 x [10.sup.4] x [4.3.sup.4]/8 x 2,5 x [60.sup.3] = 6,648 N/mm

and, as verification, spring force:

P = k x f = 6,648 x 30 = 199,44N [approximately equal to] F - F'.

The maximum stroke of the differential piston 2 is 30 mm. There is a correlation between this stroke and the trapezoidal fillet step of the nut 8 and disc 7, so that at the end of the piston stroke and its core bar, the indicator disc 7 will rotate with 360o, thus resulting a pretty good accuracy in indicating the measured load, close to 140 N/grad.

We considered two possible solutions for reading the measured values: mechanical display and electronically display, for the same 30 mm stroke of the differential piston.

3. DESCRIPTION OF THE ELECTRICAL DIAGRAM OF WEIGHTING INSTALLATION

From an electrical point of view, the installation contains two distinct functional blocks: "Measurement block" and "Display and control block". The two blocks communicate between them through radio waves so that the functional blocks are independent, regarding the position, one from the other. The measurement block will determine, indirectly, the motor vehicle mass. When the frame is loaded ss, piston 2 will move with a distance '[DELTA]x'.

Display and control block is portable and fed from an incorporated accumulator Cd-Ni. It is responsible with displaying the ma measurement block.

A potentiometrical transducer measures this displacement and then gives a stress proportional with the piston displacement '[DELTA]x'. The stress is then converted into a digital measure by a 12 bites analogy/digital converter, commanded by a micro-controller.

[FIGURE 3 OMITTED]

The information from the measurement block reaches the transceiver, then it is delivered to the micro-controller, where is transformed and then transmitted to the digital display (alphanumerical display with liquid crystals). A serial interface type RS232 can also transmit the mass information to a PC. In a PC information can be deposited or listed on a printer.

From the keyboard of the display and control block, the operator can select the following functioning menus: measurement menu, serial transmitter of the data to a PC menu, menu for memorizing the displayed data, menu for inserting the calendar date (day, month, year) and time (hour, minute), menu for introducing the identification number of the motor vehicle for which the mass is measured, menu for introducing the identification number of the operator, menu for selecting the mass measurement units (kg, tone).

A weighting installation made by the authors is showed in figure 3.

4. CONCLUSIONS

The weighting devices made by the authors are designed for mass and weight measurement for both static and mobile objectives, the last ones being stopped for a short period of time, the weighting time.

The cylinders with differential piston are proved to be regular, robust, tight sensors (they are not affected by the hostile environment where they work temporally or permanently), of good sensitivity and precision.

The use of microelectronics led to the achievement of a modern product, easy to use and competitive to a global level.

4. REFERENCES

Balasoiu, V., Padurean, I. (2002), Basics of Hydraulic. Applications, Editura Orizonturi Universitare, Timisoara

Bordeasu, I., Balasoiu, V., Baciu, I.D. (2007), About Braking of Big Masses, Acting by Linear Hidrostatic Motors, Annals of the University of Petros ani, Mechanical Engineering, Vol. 9, pp.59-64

Bordeasu, I., Doband a, E., Velescu, C., Galeriu, D., Baciu, D., Manea, A., Sucitu, L., Badarau, R. (2005), Hydrodynamics Theory and Problems. Tubes and Hydraulic systems, Editura Politehnica

Bordeasu, I., Baciu, I.D. (2002), Hydraulics. Theory and hydrostatic problems, Editura Politehnica, Timisoara

Vasiliu, N., Vasiliu, D. (2005), Hydraulic and Pneumatic Actuators, Vol. I, Editura Tehnica, Bucuresti