High Forces? High Temperatures? 1.5 MM Force Actuations? No Problem for FlexiForce

The following is an independent 3rd-party study of High-Temperature FlexiForce sensor performance in their most extreme durability test to date. The complete citation can be found at the end of this article. 

FlexiForce™ sensors have shown the ability to maintain linearity and performance after over one million force actuations, and under rigorous testing parameters. These results were consistent across all three FlexiForce sensor technology types, including standard, enhanced stability series (ESS), and high temperature (HT).

However, one independent research group[1] went even further in their own durability evaluation, specifically examining the durability of the HT FlexiForce sensor format.

Disclaimer: Because this was an independent, third-party, proprietary study, Tekscan cannot provide access to the raw data from this experiment.


Summary of Test Setup

  • Three (3) HT201 sensors were separately mounted into hydraulically-loaded testing fixtures (Figure 1). The force load was concentrated onto the sensor’s sensing region.
  • The testing fixtures were placed inside an environmental-control chamber (Figure 2)
Figure 1: Hydraulically-loaded brake calipers, each inserted with a FlexiForce HT201 sensor.Figure 1: Hydraulically-loaded brake calipers, each inserted with a FlexiForce HT201 sensor. Figure 2: Testing fixtures placed within a temperature-controlled environmental chamber.Figure 2: Testing fixtures placed within a temperature-controlled environmental chamber.

Summary of Test Parameters

  1. Sensor evaluation was conducted over a 26-day period (or, to a total of 1.5 million load cycles) according to a 24-hour schedule (illustrated in Figure 3)
  2. Ambient temperature was raised from -30°C (-22°F) to 121°C (250°F) over a 5-hour period.
  3. Temperature was maintained at 121°C (250°F) for 14 hours, then reduced to -5°C (22°F)
  4. A cyclic load of 2,224 N (500 lbf) was applied at 1 Hz frequency
  5. A load sweep was periodically conducted at various temperature levels throughout the experiment
  6. Also shown in Figure 3, a load level was varied between 0 and 2,269 N (600 lbf) over 100 seconds
  7. Measured variables included apply load, sensor output voltage, ambient temperature, and test block temperature
  8. The test chamber was opened periodically to visually inspect sensors for any obvious damages

Figure 3: Summary of daily test cycles.Figure 3: Summary of daily test cycles.

​Evaluation Results

  1. All three sensors survived and maintained performance within Tekscan’s published typical performance over the 26-day period, or a total of 1.5 million load cycles, without a permanent failure
  2. Although sensor output voltage did not exhibit significant cycle-to-cycle variability, it diminished significantly over time
    1. It was observed that output voltage was highly sensitive to temperature. However, all three sensors remained functional over the complete testing period
  3. All sensors maintained good linearity, producing output voltage in proportion to a load level during sweep cycles (Figure 5). Both hysteresis and output variability remained below 10% except in late stage evaluation
  4. It is speculated that an elastic property change in both load-sensing elastomer and protective polyimide film affects pressure distribution over the sensing area, contributing to reduced voltage over time and at low temperatures (Figure 6)

Figure 4: Normalized sensor output voltage at different temperatures over the 26-day testing period.Figure 4: Normalized sensor output voltage at different temperatures over the 26-day testing period.

Figure 5: Sensor linearity, hysteresis, and variability over the 26-day testing period.Figure 5: Sensor linearity, hysteresis, and variability over the 26-day testing period.

Figure 6: Sensor output voltage profiles at different temperatures throughout the 26-day testing period.Figure 6: Sensor output voltage profiles at different temperatures throughout the 26-day testing period.


Implications for your Design

Based on the results of this independent study, design engineers can be confident that HT FlexiForce sensors will perform within the published specifications for linearity and repeatability error in high-volume actuation applications, including ones with significant swings in operating temperature.

While this performance should not be expected absolutely across every possible FlexiForce sensor that Tekscan manufacturers, these trial’s results, combined with the results from our internal study, help to support the fact that FlexiForce sensors are manufactured to perform favorably in the most extreme conditions.

Download More FlexiForce Sensor Integration Resources

[1] Fujii, Yuji (2009) “Characterization of Pressure Resistive Elastomer for Low-Cost Load Sensing” Powertrain Research & Advanced Engineering (Ford Motor Company); Presented at the 2009 CTI Symposium & Exhibition (May 12-14, 2009; Detroit, Mich. USA).