Pressure? What Pressure?

Features inherent to resistors are helping a sensor manufacturer reliably and continuously detect pressure changes—all from inside a car’s steering wheel.

The recent smart steering wheel announcement by sensor manufacturer Hoffmann + Krippner, Inc. centers not merely on the answer to “what pressure?” but on the answer to “what’s different about the pressure?” When the pressure is that of a driver’s hands on the steering wheel (see Figure 1), the answer to the latter question can be the basis for improving safety and communication, as EECatalog learned during a recent interview with Hoffmann-Krippner CEO Jens Kautzor.

EECatalog: What’s the inspiration for the idea of measuring whether the pressure the driver is applying to the steering wheel changes?

Jens_Kautzor_headshotJens Kautzor, Hoffmann+Krippner: Safety. An event such as dozing off, having a heart attack or going into diabetic shock causes the driver’s grip to loosen on the steering wheel.

EECatalog: The monitoring of the pressure would need to continue for the entire time the individual is driving.

Kautzor: Yes, and we are doing this through the use of a resistive sensor. We are taking our SensoFoil [sensor technology], which is a sensor that is based on a layer of conductive ink. And we have spacers in between that and by pressing it together we create the contact, and what’s more, the ink is also pressure-sensitive, so we can measure, to a certain extent, how much pressure is applied. We can definitely see the change in pressure and monitor that continuously. You can only do that with resistors.

Figure 1. Courtesy Hoffmann+Krippner

EECatalog: Why?

Kautzor: Capacitors cannot measure pressure—they can only detect on/off. To a capacitive sensor the loosened grip of a driver caused by micro sleep or the other events noted earlier registers the same as the firm grip of a driver not experiencing problems.

EECatalog: What are the particular challenges encountered when putting a pressure-sensitive sensor inside a steering wheel?

Kautzor: That is an extremely difficult thing to do—to put a pressure-sensitive sensor under the leather cover of a steering wheel rim—because that leather is being tightened around the rim with extreme pressure, and so the experts at [think tank] Guttersberg Consulting [with whom we cooperated on this solution] and Hoffmann + Krippner found a way to incorporate that sensor and still get a reliable signal out of it.

We can stretch our sensors 360 degrees around the end sections, 360 degrees around the steering wheel, so we can measure pressure pretty much at any point of the steering wheel. You won’t be able to do that continuously with capacitors—that is important to know.

Imagine, you have a pressure-sensitive sensor, and you are already applying a tremendous amount of pressure to this, and you still want to detect the pressure being applied to this super-stretched leather by the driver’s hand—that is critical. You need to have a really precise sensor in there.

Another challenge to overcome is doing this in a way that does not cause the steering wheel to feel to the driver as if it has thickened. This gets us back to the topic of why resistors [as opposed to capacitors]. Even when we also incorporate a heating element into our product [for winter driving], the overall thickness is only 0.8 to 0.9 millimeters. If you were to use capacitors, the [thickness] would be greater; you’d be at 1.5 to 2.0 millimeters thick, so at that point, you would feel that there is something inside the steering wheel.

EECatalog: Within the category of products that detect unusual behavior by the driver, what are the challenges?

Kautzor: One is the speed with which the [vehicle] can react to help the driver. With our technology, the change in pressure of the driver’s grip results in a signal in 200 milliseconds—that is as compared to up to nine seconds with other technologies.

Reliability under a variety of conditions is another concern. Safety systems such as the BLISS lane departure system that can sense whether another vehicle is in the adjacent lane and signal you with vibration or noise can be helpful. But that system is based on either ultrasonic or camera systems and [in the presence of] dust, dirt or moisture [the ability of the system to perform] becomes an issue.

Another system that we know of measures the movement right at the steering rod that goes from the steering wheel down into the vehicle, but the problem there is that it is very sensitive to vibration. Guttersberg has seen reports where there was a faulty signal caused by driving over a speed bump—a warning came up that said “You Drove Too Long” and it was only 15 minutes into the drive.

And then there is a system where you measure eyelid movement of the driver and try to detect whether he is falling asleep that way, but if you are wearing glasses or sunglasses or smoking in the vehicle the camera could give you a false signal.

EECatalog: What kind of interest have you had/do you anticipate from the automotive industry?

Kautzor: The automotive industry is looking for hands on/hands off detection solutions; it is a huge topic especially in regards to autonomous driving.

It is all nice and dandy if [vehicles] are driving automatically where the GPS allows it. But what happens when you come into an emergency situation? You want to find something that can quickly turn the vehicle on or [switch] the vehicle into manual drive. Now with this technology, you can do that, just put your hands on the steering wheel and the vehicle goes back into manual drive.

There are [a significant number] of OEMs looking at this technology right now. The most important thing is that we have done basic tests [to see if our solution] is temperature-resistant according to German automotive regulations, and we have passed those tests with good signals.

And the second most important thing is whether this is manufacturable? Can we make this in production [volumes]? And the answer is yes, as confirmed by a European steering wheel manufacturer with whom we tested that this is buildable. Forklift manufacturers are also among the OEMs highly interested in adopting this technology.

anne_fisherAnne Fisher is managing editor of Her experience has included opportunities to cover a wide range of embedded solutions in the PICMG ecosystem as well as other technologies. Anne enjoys bringing embedded designers and developers solutions to technology challenges as described by their peers as well as insight and analysis from industry leaders. She can be reached at

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