S220 Application Notes
The SMD S220 Load Cell Sensor is a family of monolithic stainless steel-bodied structures with a proprietary thin-film technology strain gauge applied, capable of producing a linear, analog voltage output in response to loadings of 6 or 12 lbs depending on the model chosen.
Mounting: Two 0.150” diameter mounting holes are provided through the termination end of the sensor body. This end of the sensor should be rigidly mounted to a “mechanical ground” the surface contact of which should not extend beyond the mid‐point of the termination area. Care should be taken when routing the wires. A generous bend radius to prevent local bending at the bonding pad is preferred.
Loading: Load forces may be applied in either direction to the free end of the sensor, perpendicular to its body and on the center axis of the 0.171” diameter hole. For accuracy and repeatability, a single point of contact is best and often achieved by contacting a spherical element with a flat surface. The configuration of the S220 sensor provides for no inherent overload protection (loading in excess of 200% of rated capacity) therefore provisions should be designed into the application hardware to preclude excessive deflection and damage to the sensor.
Wiring: Color-coded lead wires are provided for electrical connections however when extended, adequate forms of strain relieving should be provided in the assembly design.
As seen in the schematic drawing, 4 resistive elements are created by proprietary thin film technology in a Wheatstone Bridge configuration and adjoining areas of strain concentration. The application of a load to the sensor causes an electrical imbalance between the resistive elements and produces an output voltage (typically in millivolts) that is linear and proportionate to the load applied. Less than the maximum excitation voltage may be used however the resulting output will be reduced proportionately.
Environment Concerns & Precautions
While the thin-film technology is itself immune to normal humidity levels, care must be taken to avoid condensing moisture and direct water exposure. The S220 is very stable over time and temperature. Due to the nature of the thin film technology, care in handling should be observed and no damage be allowed to occur especially in the area of the thin film strain gauges. During handling and assembly procedures antistatic discharge practices should be observed.
The sensor moves with the applied load. When mounting the sensor, take care not to interfere with the movement of the sensor.
*IDEALLY, LOAD INTERFACE SHOULD BE VIA A SINGLE POINT OF CONTACT, SUCH AS THE INTERSECTION OF A SPHERE TO A PLATE.