Indirect TPMS
Indirect TPMS do not use physical pressure sensors but measure air pressures by monitoring individual wheel rotational speeds and other signals available outside of the tire itself. First generation iTPMS systems utilize the effect that an under-inflated tire has a slightly smaller diameter (and hence higher tangential velocity) than a correctly inflated one. These differences are measurable through the wheel speed sensors of ABS/ESC systems. Second generation iTPMS can also detect simultaneous under-inflation in up to all four tires using spectrum analysis of individual wheels, which can be realized in software using advanced signal processing techniques. The spectrum analysis is based on the principle that certain eigenforms and frequencies of the tire/wheel assembly are highly sensitive to the inflation pressure. These oscillations can hence be monitored through advanced signal processing of the wheel speed signals. Current[when?] iTPMS consist of software modules being integrated into the ABS/ESC units.
iTPMS cannot measure or display absolute pressure values, they are relative by nature and have to be reset by the driver once the tires are checked and all pressures adjusted correctly. The reset is normally done either by a physical button or in a menu of the on-board computer. iTPMS are, compared to dTPMS, more sensitive to the influences of different tires and external influences like road surfaces and driving speed or style. The reset procedure, followed by an automatic learning phase of typically 20 to 60 minutes of driving under which the iTPMS learns and stores the reference parameters before it becomes fully active, cancels out many, but not all of these. As iTPMS do not involve any additional hardware, spare parts, electronic or toxic waste as well as service whatsoever (beyond the regular reset), they are regarded as easy to handle and very customer friendly.[2]
According to Nira, based on their request to TÜV SÜD to do a pre-test according to similar requirements of the EU legislation, the iTPMS system passed that pre-test.[3] However, the full test procedure as required by the EU regulation, completed by the regulatory body assigned to make the homologation, has not yet been done. Manufacturers like Dunlop Tech also claim their products to fulfill the regulations.[4]
iTPMS are regarded as inaccurate by some stances due to their nature. As such, most TPMS units now on the market are of the Direct type.
Direct TPMS
Direct TPMS employ pressure sensors on each tire, either internal or external. The sensors physically measure the tire pressure in each tire and report it to the vehicle's instrument cluster or a corresponding monitor. Some units also measure and alert temperatures of the tire as well. These systems can identify under-inflation in any combination, be it one tire or all, simultaneously. Although the systems vary in transmitting options, many TPMS products (both OEM and aftermarket solutions) can display real time tire pressures at each location monitored whether the vehicle is moving or parked. There are many different solutions but all of them have to face the problems of exposure to tough environments and the majority are powered by batteries which limit their useful life. Some sensors utilise a wireless power system similar to that used in RFID tag reading which solves the problem of limited battery life by electromagnetic induction. This also increases the frequency of data transmission up to 40hz and reduces the sensor weight which can be important in motorsport applications. If the sensors are mounted on the outside of the wheel, which is the case for some aftermarket systems, they are in danger of mechanical damage, aggressive fluids and other substances as well as theft. If they are mounted on the inside of the rim, they are no longer easily accessible for service like battery change and additionally, the RF communication has to overcome the damping effects of the tire which additionally increases the need for energy.
A direct TPMS sensor consists of following main functions requiring only a few external components — e.g., battery, housing, PCB — to get the sensor module that is mounted to the valve stem inside the tire:
- pressure sensor;
- analog-digital converter;
- microcontroller;
- system controller;
- oscillator;
- radio frequency transmitter;
- low frequency receiver, and
- voltage regulator (battery management).
Most originally fitted dTPMS have the sensors mounted on the inside of the rims and the batteries are not exchangeable. With a battery change then meaning that the whole sensor will have to be replaced and the exchange being possible only with the tires dismounted, the lifetime of the battery becomes a crucial parameter. To save energy and prolong battery life, many dTPMS sensors hence do not transmit information when not rotating (which also keeps the spare tire from being monitored) or apply a complex and expensive two-way communication which enables an active wake-up of the sensor by the vehicle. For OEM auto dTPMS units to work properly, they need to recognize the sensor positions and have to ignore the signals from other vehicles' sensors. There are hence numerous tools and procedures to make the dTPMS "learn" or "re-learn" this information, some of them can be carried out by the driver, others need to be done by the workshops or even require special electronic tools. The cost and variety of spare parts, procedures and tools has led to much trouble and confusion both for customers and workshops.
Aftermarket dTPMS units not only transmit while vehicles are moving or parked, but also provide users with numerous advanced monitoring options including data logging, remote monitoring options and more. They are available for all types of vehicles, from motorcycles to heavy equipment, and can monitor up to 64 tires at a time, which is important for the commercial vehicle markets. Many aftermarket dTPMS units do not require specialized tools to program or reset, making them much simpler to use.
