Research On The Variable Lever Ratio Lever-type Precision Force Source Technique

Force Standard Machine which is equipped with standard force source,can achieve thecalibration of force measuring apparatus. loadcell performance index is calibrated by forcestandard machine in load cell production industry. force standard which is an important linkof force transfer system is widely used in important departments of our country,such asmetrological service, research department and dynamometer production and usedepartment.According to dynamic effect and based on the lever magnifying principle, Lever-typeForce Standard Machine produces standard force by amplifying gravity weight accurately.traditional lever-type force standard machine with complicated structure and high cost is alarge lever scale in essence,it can also be consist of two parts: weight loading or unloadingsystem of dead weight force standard machine and lever amplifier. In recent years, with thedevelopment of economy and science technology, traditional lever-type force standardmachine can't meet the demand because of some flaws and deficiencies such as loadingspeed lowering, low efficiency, inconvenient operation and high cost of manufacture anduse. Aiming at these problems to development new force standard machine, especially newlever-type force standard machine solves the existing problems of traditional force standardmachine so as to satisfy the market demand.According to the principle of lever amplification, using the first lever principle,we amplifygravity weights that have different sizes but accurate mass accurately in order to generatestandard force, Hereby the theoretical part of the lever amp lification force generatingtechnology with changing the lever ratio is established,and we study and test that how toimprove contact properties of edge-support.1.Mechanism of the variable-arm lever-type force standard machinesHere's a beam AB with a fulcrum C, let,be the gravity produced by the masson left side and right side of C, let,be the distance between the centroid and thefulcrum, see figure 1. A weight with a mass of m moves on the lever beam AB, let bethe distance between the weight and the fulcrum at a certain time; the gravity of the weightis set at . B is the action point of the imposed force, which is set at .Let be thedistance between B and C. The friction of the fulcrum, the force point, and the deformationof the beam are all dismissed. According to the principles of statics, we have It is clear that when the structure of the lever is defined, namely, and when m,,,,,are all fixed, the weight with a mass of m produces a downward force, and is amplified by the lever into , which falls on the dynamometer, then isin proportion to , the position of weight m.Unlike traditional lever-type force standard machines, variable-arm lever-type forcestandard machines have adopted a fixed weight mass and a variable lever ratio. Also, therider is driven by the ball bearing screw, and directed by the guide installed alongside thelever body; it moves along the lever with the servo motor providing the motive, thusproducing forces at various scale and in various amount, that is, the novel machine hastransformed the control of the force value into the control of precision displacement. Thenew lever-type standard force resource standard device is feasible, both theoretically andtechnically.2.Sources of error in variable-arm lever-type force standard machines and thecompensation mechanismThe physical factors affecting the accuracy of force values in the lever-type forcestandard machines, apart from the four factors in dead weight force standard machine(mass of the weight, acceleration of gravity, the material density of the weight and the airdensity), are mainly the lever ratio i,as well as errors from edge-support. We can see fromthe working mechanism that errors of the variable-arm lever-type force standard machinesmainly come from the effect of displacement resolution, the friction torque of theedge-support and the variation of the length of the lever arms. Since the rider's position canbe precisely controlled during its movement, this feature can compensate for errors causedby other factors ( such as deformation of the lever, friction at the edge and change in environment temperature), which is impossible in traditional lever-type force standard machines.3.construction and implementation of variable lever ratio lever-type force standard machineTraditional lever-type force calibrating machines is composed of the main engine, lever system, the gravity weights adjusting system, monitoring system and others, while the new variable-arm lever-type force calibrating machines substitute the single constant-mass weight for the expensive gravity weights and the complex adjusting system in its system structure design. Also, a set of knife-bearing device is removed( namely, use double-knife-bearing). Since the length of the arm is variable, no accurate positioning for the knife-bearing device is required, hence the bearing system is simplified. The centroid(质心) adjusting systems in traditional machines are also absent in the novel machines, because the centroid-adjusting can be achieved by varying the position of the rider to change the balance status of the lever. Since an accurate lever ratio can be obtained by adjusting the position of the rider, the distance between the force point and the fulcrum need only keep at a certain value, unlike in traditional mechanic levers where the force point, the fulcrum and the gravity point have to keep a fixed and accurate relation. Therefore, the novel machines spares the adjusting mechanism and process, In sum, compared with the traditional ones, the new variable-arm lever-type force calibrating machines can greatly reduce costs and improve efficiency in working.4.The working efficacy of variable-arm lever-type force standard machinesSince variable-arm lever-type force standard machines have avoided periodic changes of the force value caused by the vibration of the weight, they can reach stability and balance faster than the traditional machines. The time of load-adding depends on the speed of the rider, therefore we only need to move the rider fast to achieve a high working efficiency. The loading and balancing in variable-arm lever-type force standard machines are essentially precision controls of two single-degree-of-freedom systematic displacement. Consequently, we can achieve the goal of precision and high-efficiency by using precision displacement control technology to precisely control the displacement, to achieve fast and automatic loading and stabilizing, to reduce, eliminate and curb vibration. However, similar with traditional lever-type force standard machines, the novel machines still have vibration in its lever system. In order to reduce this vibration, we should maximize the stiffness and damping of the components. We can impose damping on the lever to reduce vibration, in this case the ideal value for the resistance torque of the imposed damping would be M C= 1.6JK,in which J representing the moment of inertia of the lever system, while K the stiffness of the lever system.5.Edge-supportThe supporting of the lever is one of the main factors affecting the force-imposing precision in lever-type force source devices (including the balances). This is because the fulcrum part produces resistance torque, and such an error is the main error in lever-type force standard machines. Increasing the contact strength and wear resistance at the edge-support can significantly improve the precision of lever-type force standard machines. To do so, we adopted a multi-arc ion plating method, specifically, to coat the cutting-edge cylinder with a tier of Tin, so as to greatly increase the contact strength and wear resistance when at the edge-support. The experiment result shows no significant change at the surface topography before and after coating, indicating that the method is feasible within the precision range of the design.6.The load characteristics experiment of the load cellLoad characteristics experiments of the load cell were conducted using the variable-arm lever-type force standard machine, with repeated adjustment on the variable-arm lever-type force standard machine, a great deal of experiments and some data selected and analyzed. Data analysis shows that the variable-arm lever-type force standard machine has an ideal repeatability, with an error usually no greater than 0.006%. The loading time and balancing time can be controlled within 10s, therefore it has a high working-efficiency that meets the State Standard.In sum, the variable-arm lever-type force standard machine is a working device with a high precision, high efficiency, high reliability and low cost. It totally meets both international and domestic requirements for standard force source devices, and has a great practical value and realistic significance.