A Simple Flexible Plate for Usage as Force Sensor Element With Constant Stiffness

Control of pressure is one of the most decisive parameters influencing the quality and reproducibility of products when dealing with plastics producing techniques such as injection molding, extrusion, and related techniques. For that purpose different types of pressure sensors for specific tasks come into action. Different tasks can be different working pressure ranges, static or dynamic pressure, working temperatures and requirements concerning the environment. Therefore different types of pressure sensors are commonly used which most often transform mechanical deformation into electrical signals and these in turn are converted into the value of pressure. This in turn can mean extensive effort for installation and insulation of the pressure sensors. An approach to design a pressure sensor based solely on its mechanical deformation is basic idea of the present work. The pressure sensor consists of a flexible element which is in fact a thin plate with a ram on its outer side and can be clamped easily at any desired area on a melt channel. When being applied to pressure the working pressure causes a deflection of the flexible element. Measuring this deflection and knowing the mechanical behavior of the flexible element allows the calculation of the value of pressure. To get to know to the mechanical behavior, especially the stiffness, of the flexible element a test rig was designed and refined. In the test rig as substitution of the pressure variable but defined load can be applied using a piezo-electrical actuator with integrated position sensor. Measuring the applied force using a force sensor and measuring the resulting deflection of the flexible element using a length gauge allows to calculate the stiffness of the flexible element. Regarding the flexible element as Kirchhoff plate a constant stiffness is expected and this could be proven taking measurements. When taking a look at the deflection of the piezo-electrical actuator and the flexible element there is a difference due to the elasticity of the test rig itself. When determining the stiffness of the test rig a non-constant stiffness was detected which was not expected. Reason for that is the non-constant stiffness of the used force sensor though stated other by the manufacturer. This could be proven substituting the force sensor with the flexible element. With that the flexible element showed to be appropriate to be used as force sensor with a constant stiffness.