Direct TPMS or direct tire pressure monitoring systems refers to the use of a pressure sensor directly mounted on the wheels or tires of a vehicle. The pressure inside the tire is measured using a pressure transducer with the pressure information being subsequently sent to the vehicle to warn the driver of under or over inflation of a tire.
The pressure information is commonly transmitted to the vehicle using radio frequency (RF) technology, through systems using mechanical, electrical or magnetic methods. A typical direct TPMS (e.g. Ford, BMW or Toyota) comprises the following components on a vehicle:
- A direct tire pressure monitoring (TPM) sensor fitted to the back of the valve stem on each wheel.
- A TPM Warning Light.
- Unique identifier (ID's) for which tire is providing the data including speed and the direction of rotation.
- A tire pressure monitor engine control unit (ECU).
- Controller for periodic measurements.
- Source of power.
- Diagnostics and wake up system.
Direct tire pressure monitoring systems There are two main types of direct tire pressure monitoring system currently in use. These are known as 'high line' and 'low line'.
High Line Direct TPMS System
If the vehicle is fitted with low frequency (LF) transmitters near each wheel, the vehicle may use these to force the sensors to transmit. In this case:
- The TPM may not transmit on its own.
- The vehicle will periodically command the sensors to send their information.
- The TPM's will be forced to transmit when the ignition is switched on.
This will give an early indication of low pressure without having to have the vehicle's receiver switched on when the vehicle is not in use. The transmitters are usually activated one at a time in sequence so that the vehicle can inform the driver of the location of the wheel with low pressure. This information can then be used later for localisation my matching the TPM's unique ID with its position discovered by this sequential activation. This method is used on some high line systems where the TPM also transmits periodically.
On some vehicles only three LF transmitters are used in order to save money. The vehicle assumes that transmissions from a nearby TPM which has not been woken up by the LF belong to the TPM located where there is no LF transmitter.
High line systems are inherently more expensive than low line systems but they have the advantage of the vehicle knowing the pressure when started without draining the vehicle's main battery and providing localisation. These systems tend to be used on higher end models.
Low Line System Direct TPMS System
In this system, the TPM units transmit on their own at fixed or random intervals. As the individual TPM's on the vehicle do not know if another TPM is transmitting at the same time, it is possible to have collisions between messages transmitted. Measures have to be taken to ensure that the message is received by the vehicle. On some systems the message is re-transmitted multiple times to reduce the effect of interference (communication). The transmission pattern can be random or pseudo random to reduce the chance of collisions between transmissions from the sensors on the vehicle. In this case:
- TPM units transmit individually at fixed or random intervals.
- It is possible to have collisions between messages transmitted.
Another method of attempting to avoid collisions is simply to transmit more frequently such as once per minute. In addition, if the TPM detects a rapid change in pressure or too high a temperature, it will start to transmit more frequently so that the vehicle has more chance of receiving the information. The low line system is used on the majority of vehicles due to its lower cost.
In most current designs of Direct TPMS, a small electronic assembly which is rugged enough to be mounted inside a tire, measures the pressure using a microelectromechanical system (MEMS) pressure sensor and then transmits this and other information to one or more vehicle receivers. Other information can include a serial number, temperature, acceleration and the status of the complete tire pressure monitoring system. The purpose of the serial number is to allow the vehicle to ignore transmissions from other vehicles and operate with a unique data field.
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