Force sensor design process should pay attention to the solution

inductance

For the resistance strain gauge type Load Cell (hereinafter referred to as "load cell"), the structural shape and related dimensions of the elastomer have a great influence on the performance of the load cell. It can be said that the performance of the load cell mainly depends on the shape and related size of its elastomer. If the elastomer design of the load cell is unreasonable, no matter how high the processing precision of the elastomer and the quality of the attached strain gauge, the load cell is difficult to achieve high force measurement performance. Therefore, in the design process of the load cell, it is important to design the elastomer properly.

The design of the elastomer is basically within the scope of the mechanical structure design, but the structure is different from the ordinary mechanical parts and components due to the need of the force measurement performance. In general, ordinary mechanical parts and components only need to meet the strength and stiffness under a sufficiently large safety factor, and the stress distribution on the parts or components under stress conditions is not strictly required. However, for the elastomer, in addition to the mechanical strength and rigidity requirements, it is necessary to ensure the stress (strain) of the strain gauge portion (hereinafter referred to as the "patch portion") attached to the elastomer and the load that the elastic body bears. The force measurement) maintains a strict correspondence; at the same time, in order to improve the sensitivity of the load cell force measurement, the patch portion should also reach a higher stress (strain) level.

It can be seen that the following two requirements must be met during the design of the elastomer:

(1) The patch site should have a high level of stress (strain).

In order to meet the above two requirements, in the design of the elastic body of the load cell, the design principle of [stress concentration" is often applied to ensure that the stress (strain) level of the patch is high and maintains a strict correspondence with the measured force. In order to improve the force sensitivity and force measurement accuracy of the designed load cell.

(2) The stress (strain) of the patch part should maintain a strict correspondence with the measured force;

The principle of "stress concentration" for improving the irregular distribution of stress (strain)

In the design process of mechanical parts or components, stress (strain) is generally considered to be regularly distributed on the part or member. If the shape of the part or member does not change, it is not necessary to consider the problem of irregular stress (strain) distribution. In fact, in the design of mechanical parts or components, the problem of irregular distribution of stress (strain) is not considered, but is included by the safety factor in the strength calculation.

For the load cell, it measures the strain of the patch on the elastomer by the strain gauge to measure the magnitude of the measured force. To ensure that the stress (strain) of the patch portion is strictly related to the measured force, it is actually to ensure that the stress (strain) of the patch on the elastomer is distributed according to a certain rule when the load cell is stressed. . In practical applications, the factors that have a greater influence on the stress (strain) distribution of the elastomer patch are mainly the changes in the stress conditions of the elastomer.

The change of the elastic force condition means that when the magnitude of the force applied by the elastic body is constant, the action point of the force changes or the contact condition of the elastic body with the adjacent loading member and the bearing member changes. If this situation is not considered in the design of the elastomer structure, it may cause irregular changes in the stress (strain) distribution on the elastomer. The most typical example of this is the barrel load cell. In order to reduce the measurement error caused by the change of the elastic force condition, some sensor designers adopt the method of increasing the number of patches on the elastic body of the barrel type Force Sensor, and as much as possible, the circumferential stress on the patch part of the elastic body. The case where the (strain) distribution is uneven is measured. Such a treatment method has a certain effect, and can reduce the measurement error caused by the change of the elastic force condition. However, this method is a passive method after all. The number of added patches is always limited, and it is difficult to measure the uneven distribution of stress (strain) on the circumferential surface of the patch on the elastomer. The degree of reduction is not significant enough.

The essence of the force error caused by the change of the elastic force condition is the irregular distribution of the stress (strain) on the circumference of the elastomer patch, if the stress (strain) distribution on the circumference of the elastomer patch is affected. The constraint of certain conditions forces the stress (strain) of the patch to be distributed according to a certain rule, so that the stress (strain) of the patch portion of the elastomer is strictly related to the measured force, thereby reducing the elasticity. The force error caused by the change of the body's stress conditions.

Stress concentration principle for increasing stress (strain) levels

In order for the load cell to achieve higher sensitivity, the strain gauge should generally have a higher strain level, ie, the patch should have a higher stress (strain) level on the elastomer.

There are two common methods for achieving a high stress (strain) level on the patch on the elastomer:

(1) The elastic body is locally weakened in the vicinity of the patch portion, so that the local stress (strain) level of the patch portion is increased, and the stress (strain) level of other portions of the elastic body is substantially unchanged.

Both of the above methods can improve the stress (strain) level of the patch portion, but for the overall performance of the elastomer, the effect of partially weakening the elastomer is much better than the overall reduction of the elastomer size. Because the partial weakening of the elastomer can not only improve the stress (strain) level of the patch portion, but also maintain the high strength and rigidity of the elastomer as a whole, which is beneficial to improving the performance and the use effect of the sensor.

The principle of partially weakening the elastomer to increase the stress (strain) level of the patch is: by locally weakening the elastomer, causing local stress concentration, so that the stress (strain) level of the stress concentration portion is significantly higher than the stress level of other parts of the elastic body. By attaching the strain gauge to the stress concentration site, a higher strain level can be measured.

Local stress (strain) concentration methods are often used in the design of load cells, especially in elastomer designs for Beam Load Cells such as curved beam and Shear Beam Load Cells. The local stress (strain) concentration method is applied to the more successful shear beam type load cell. The shear beam type load cell is used to measure the shear stress (shear strain) on the beam type elastomer.

For beam-shaped members, the bending strength is the main contradiction. In the case where one beam satisfies the bending strength, the shear strength is generally large. When a blind hole is dug near the neutral layer, the shear stress (shear strain) on the web on the section is significantly improved, but the bending stress on the section is improved little. Therefore, after applying the local stress concentration scheme to the shear beam type elastomer, the detected shear strain is greatly improved, the sensitivity of the load cell is significantly improved, and the bending strength of the entire beam is little affected, so that the entire beam is maintained. Good strength and stiffness.

(2) The overall size of the elastomer is reduced to comprehensively increase the level of stress (strain) on the elastomer.