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TPMS -Tire-pressure monitoring system
A Tire-pressure monitoring system (TPMS) is an electronic system designed to monitor the air pressure inside all the pneumatic tires on automobiles, aeroplane undercarriage, straddle-lift carriers, forklifts and other vehicles. The system is also sometimes referred to as a tire-pressure indication system (TPIS). These systems report real-time tire-pressure information to the driver of the vehicle, either via a gauge, a pictogram display, or a simple low-pressure warning light.
Types: Direct and Indirect
Direct-sensor TPMSes employ physical pressure sensors inside each tire and a means of processing and sending that information from inside the tire to the vehicle’s instrument cluster. These systems can identify simultaneous under-inflation in all four tires in any combination.
Direct-sensor TPMS are specifically designed to cope with ambient and road-to-tire friction-based temperature changes, both of which heat up the tire, and increasing its pressure. The alarm-activation threshold pressures are usually set according to the manufacturers recommended “cold placard inflation pressures”.
In order to transfer data from a rotating wheel, a direct-sensor system may use a radio-frequency (RF) communication channel or an electromagnetic coupling means to overcome the tire/chassis rotational boundary.
The pressure sensor devices used in direct-sensor TPMS may be either battery-powered or battery-less.
Indirect TPMS do not use physical pressure sensors. Indirect TPMS measure the “apparent” air pressure, by monitoring individual wheel rotational speeds, and other signals available outside the tire itself. Most indirect TPMS use the fact that an under-inflated tire has a slightly smaller diameter than a correctly inflated tire and therefore has to rotate at a higher angular velocity to cover the same distance as a correctly inflated tire. Newer developments of indirect TPMS can also detect simultaneous under-inflation in up to all four tires using vibration analysis of individual wheels or analysis of load shift effects during acceleration and/or cornering, which can be realized in software using advanced signal processing techniques. However, the vibration analysis technique requires the use of additional suspension sensors which result in increased complexity and cost of the overall system as long as vertical chassis movements are concerned. That is why most current advanced indirect systems use the spectral content of the wheel speed sensor signals so no additional sensors are needed and the computations can also be carried out by usual processors for example in usual ABS or ESC control units.
Indirect TPMS are realized in software in combination with wheel-speed sensors for anti-lock braking systems, and electronic stability control systems. A disadvantage of indirect TPMS is that the driver must calibrate the system by pushing a reset button on the dashboard or through the on-board computer and if this is performed when any tire is in an under inflated condition then the system will not report correctly.