FSR Integration and Calibration: Design the Sensing System Around Its Real Limits

Engineering illustration · FSRTEK technical reference

1. Defining the Realistic Capability Boundaries of FSRs

Before designing an FSR product, engineers must align application requirements with the physical capabilities of polymer thick-film technology:

2. Mechanical Stack-Up & Actuator Puck Design

The mechanical interface directly dictates measurement repeatability. Applying force directly with soft human skin produces highly variable readings due to changing contact area. Repeatable integration demands a defined mechanical stack-up:

• Rigid Backing Substrate: The FSR must be mounted on a smooth, flat, unyielding surface (aluminum plate, FR4 PCB, or rigid injection-molded chassis). Flexible or warped backing introduces parasitic bending strain that the sensor registers as false force.
• Actuator Puck (Plunger): Utilize a slightly compliant elastomer puck (e.g. 50~70 Shore A polyurethane or silicone) positioned directly over the active sensing area. The puck must be sized slightly smaller (80% ~ 90% of active diameter) than the sensing boundary to avoid edge-clamping shear forces.
• Pre-Load Management: A small mechanical pre-load (1% ~ 3% of full-scale force) holds the stack-up in intimate contact, eliminating dead-band hysteresis and ensuring instantaneous zero-delay response to dynamic loads.

3. Static Creep, Hysteresis & Dynamic Drift Compensation

Polymer thick films exhibit natural viscoelastic relaxation under sustained mechanical load, known as Static Creep (conductance slowly drifts upward under constant force over minutes). FSRTEK engineers utilize three standard compensation strategies:

1. Logarithmic Time-Decay Creep Compensation

Under static load, creep follows an empirical logarithmic trajectory: $Delta F_{creep}(t) = alpha cdot ln(1 + beta t)$. Software tracks the continuous dwell time of active force and subtracts the estimated creep offset in real time.

2. Dynamic AC Filtering for Transient Events

For athletic strikes, musical key hits, and robotic tap detection, high-pass filter the force signal ($f_c approx 0.5,text{Hz}$). This eliminates 100% of slow baseline drift while preserving crisp, high-frequency impact dynamics.

3. Periodic Zero Baseline Re-Tare

Whenever the signal drops below a defined touch threshold for more than $500,text{ms}$, automatically re-tare the zero conductance baseline $G_0$.

4. Calibration Procedures for Production & Field Deployment