A, moment balance, statically indeterminate system
Gravity weighing system and most commonly used weighing apparatus. In addition to a very few weighing devices such as hanging scales, the gravity of the measured object and the devices used to measure the force, such as sensors, their forces are parallel to each other, rather than co-point forces, so the mathematical equation of force measurement is based on the balance of moments. Due to the customary "intuitive" thinking, for example, for a weighing device such as a car scale, it will directly add the force measured by four sensors to equal the weight of the object being weighed. The result of this habit is that when the truck scale is actually calibrated, the offset load is not adjusted according to the regulations, or even the offset load is not adjusted, and the result is the same weight
When the center of gravity is in different positions of the carrier, the display results will be significantly different, and the display value is out of tolerance. This phenomenon usually occurs when the calibration brick code is placed at the four corners. Why is that? Because the truck scale is a statically indeterminate system. As we all know, more than three support force measurement points of the system, are statically indeterminate systems, such as truck scale, at this time there are four support sensors, and can only get three balance equations, so it is not possible to obtain the size of the four key forces, that is, the force value of the four sensors combined force size, not equal
The gravity of the weighed object.
Type approval and verification
Weighing instrument is a measuring instrument which belongs to mandatory management. For new products and products produced by the manufacturer for the first time, the legal metrology department must approve the type of products before they can be put into production. For new products, it is necessary to verify the principle, structure and even some materials with special requirements. The second is whether there is a reasonable and feasible test method, and the test document can be formulated before type approval. For manufacturers to produce existing weighing instruments for the first time, only the type approval is required in accordance with the existing procedures. Due to the scale produced by our manufacturers, almost all are already
Some weighing instruments, often some manufacturers in the production of weighing instruments to add different parts that may affect the weighing result from the original product, but we have not reviewed these new parts. In foreign countries, the type approval of new weighing instruments is very strict, for example, in Germany as early as 1775 there was a finalized tilt lever scale, but it was not officially used until 1876 through the type approval. Due to the restriction of metrology regulations, it has affected the development of inclined pole weighers in Germany. Also strain gauge weighing
Electronic scales for sensors did not receive type approval until the sensor uncertainty was less than 1/1000 in 1966. The purpose of type approval is only to confirm that a new type of weighing instrument is reasonable in principle and structure, and the manufacturer that produces a certain weighing instrument for the first time should also confirm that it has the ability to produce this weighing instrument before it can be put into production.
In order to ensure that the measuring instrument in a long enough time (measurement stability), can expect to get the correct measurement results (accuracy of measurement), this is called the certainty of measurement in the method of measurement. In order to ensure the characteristics of measuring instruments, it is necessary to force the verification of the weighing instruments used. Verification includes first verification and subsequent verification.
The purpose of the first verification is to make the measuring instruments recognized and permitted to be used by the legal metrology department. Verify that the scale is a copy of the type approved type, and can review whether the scale is installed and used correctly and has the functions specified in the regulations. The purpose of subsequent verification is to find out whether the accuracy and performance of the measuring instrument can still maintain the requirements of the original regulations after a period of use after the last verification. The measurement department can judge whether the verification period of the weighing instrument is reasonable and put forward suggestions for improvement after evaluating the performance of the manufacturer's weighing instrument.
Third, the limit
The most basic requirement of the weighing device or weighing system is that the force of the load must pass through the force axis of the sensor, and there can be no lateral force and torque. It is also required that the state of the weighing structure cannot be changed during the whole use process, such as the bearer and the weighing bracket. It is required to keep the stress state of the sensor unchanged and the dead load of the weighing system or carrier (no-load stress state balanced by torque) unchanged. In actual use, due to the change in temperature caused by the lateral displacement of the supporting parts, the lateral force caused by the wind, and the impact caused by the lateral force or resistance, as well as the mechanical structure: such as the foundation, structural frame, carrier, container tank, etc., the role of the load stress
Down, it can be deformed. These factors can affect the measurement accuracy. In a broad sense, the loading part of the sensor (pressure head) is also a limiting part to eliminate lateral interference forces. However, we usually only refer to the device installed between the carrier and the foundation or frame to eliminate the lateral force as a stopper.
In order to prevent the horizontal force, the limit device has two different forms: Constrainers: the constraint is the use of forced means to eliminate the horizontal force, and does not allow any movement between the load structure and the carrier sensor in the constraint direction. It works throughout the braking process.
stop: a limiting device that acts as a brake. During the weighing process, when the bearer and weighing mechanism are affected by external interference force, impact force, etc., their displacement occurs and the original weighing state is destroyed. The function of the stopper is to limit the lateral displacement within control, and when the external disturbance disappears,
The weighing device or weighing device can be restored to the original weighing state. In order to avoid the damage or even damage of the weighing structure caused by excessive impact force under the action of external force. Therefore, the "gap" adjustment of the stopper is very critical. For the "limit" part of the sensor, by including a spherical indenter, a swinging support, automatic
Positioning ball bearing seat, rubber support, etc. Components, saddle shapes, and chain links belong to limiting and self-resetting limiting components. Collision bolt limit is the limit device used by almost all truck scales in our country. However, the collision gap existing in such limits causes deformation and damage during the movement of the indenter. If the gap is too small, in the summer often make the scale and the foundation contact caused by force shunt or even stuck, resulting in measurement errors. Some manufacturers, the impact limit used for dynamic truck scales is very unreasonable. In addition to this simple limiting structure, the collision type has different structures. Other crash-type stoppers have been introduced by SCHENCK.
Binding type limiter, through the use of a pull rod. This is the most common restraint for beam stilts and cylindrical bars with both ends fixed (clamped). More complex This type of stop uses ball bearings or universal bearings at the two fixed ends or more complex restraint limits using spherical column structures at both ends of the stop. Restraint limits often introduce obvious interference force due to improper use. Some people even think that the so-called rigid clamp tie rod constraint limit with fixed ends is the worst way to limit. The main reason for this is that the user's adjustment is not to the point. This type of restraint stopper requires very precise adjustment to avoid vertical interference forces. That is, the fixed ends should be kept precisely at the same level. There can be no gap between the tie rod, otherwise it will be created under impact
Permanent deformation of the scale body, and even damage. The second thing to note is that not just using a rod can achieve a good limiting effect.