Direct TPMS - TPM Sensor Features

TPM Sensor Features

The TPM sensors currently fitted to high volume production cars worldwide are battery-powered, self-contained units which periodically measure tire pressure, and often temperature and acceleration. The sensor is equipped with an RF transmitter circuit which is used to broadcast the measured pressure etc. within the tire.

The TPM is designed to use as little power as possible to give maximum battery life. This is done by using very low power circuitry and transmitting the data as infrequently as possible and with as low power as possible. The UHF transmitter in a TPM typically transmits around 250μW (1μW is equal to one millionth (10−6) of a watt).

TPMs do not have UHF receivers built in due to the relatively high power requirements of this technology. This means that they can't tell that they are transmitting at the same time as another TPM. Most TPMs do have LF receivers as this uses little or no power.

The pressure, temperature and acceleration sensors generate analog signals which are converted to their digital equivalents using analog to digital converters. The acceleration sensor measures the centrifugal force generated when the wheel rotates. This force is proportional to the rotational speed. The acceleration sensor may be a simple switch rather than an analog transducer (accelerometer). This is usually referred to as a roll switch. The acceleration sensor allows the TPM to be placed in a low-power communication device mode, when the vehicle is stationary which can extend the battery life. The advantage of a roll switch over an accelerometer is that the switch is purely mechanical and doesn't use any power to take a measurement.

When the vehicle is stationary, the TPM may periodically transmit to the vehicle. This allows (as long as the vehicle receiver is always on) the driver or vehicle operator to be warned of low pressure as soon as the Ignition system is switched on rather than having to wait until the vehicle is moving.

All TPM units on a vehicle operate on the same RF channel frequency and each message includes pressure data, temperature data, a unique ID code, operating state data, status information and check digits. The check digit is either a checksum or a cyclic redundancy check (CRC).

The TPM does not usually have information about the tire's correct pressure as this would be very difficult and possibly dangerous to support. However it may have an algorithm contained within it which detects both slow and rapid changes in pressure. This condition may be transmitted as part of the TPM's status. It may also cause the TPM to transmit more frequently.