Page 398 - Understanding Automotive Electronics

2735 | CH 11 Page 385 Tuesday, March 10, 1998 1:30 PM

FUTURE AUTOMOTIVE ELECTRONIC SYSTEMS 11

A low tire pressure warn- The difficult part of this system is the link from the tire pressure sensor
ing system utilizes a tire- mounted on the rotating tire to the signal processor mounted on the body.
mounted pressure sen- Several concepts have the potential to provide this link. For example, slip rings,
sor. The pressure sensor which are similar to the brushes on a dc motor, could be used. However, this
signals a loss in tire pres- would require a major modification to the wheel-axle assembly and does not
sure. appear to be an acceptable choice at the present time.
Another concept for providing this link is to use a small radio transmitter
mounted on the tire. By using modern solid-state electronic technology, a low-
power transmitter could be constructed. The transmitter could be located in a
modified tire valve cap and could transmit to a receiver in the wheel well. The
distance from the transmitter to the receiver would be about one foot, so only
very low power would be required.
One problem with this method is that electrical power for the transmitter
would have to be provided by a self-contained battery. However, the transmitter
need only operate for a few seconds and only when the tire pressure falls below
a critical level. Therefore, a tiny battery could theoretically provide enough
power.
The scheme is illustrated schematically for a single tire in Figure 11.14.
The sensor switch is usually held open by normal tire pressure on a diaphragm
mechanically connected to the switch. Low tire pressure allows the spring-
loaded switch to close, thereby switching on the microtransmitter. The
receiver, which is directly powered by the car battery, receives the transmitted

Figure 11.14
Low-Pressure
Sensor Concept

FPO

UNDERSTANDING AUTOMOTIVE ELECTRONICS 385