TPMS: TIRE PRESSURE CONTROL PART THREE
The features of the direct system
Going back to the main general subject, we focus on the direct TPMS, considering in detail some significant features of its two main components: sensors and control units
Direct systems monitor tire pressures with an average resolution of 0.025 bar at regular intervals. Monitoring operations are carried out every 5 seconds in case of an warning signal, every 30 seconds with the vehicle in motion and every few minutes when stationary.
Single under-inflated tires can be detected as well as the spare tire.
Direct TPMS operating characteristics
The ECU must recognize the sensors. When replacing tires, there is a risk of damaging the sensors and having to replace them. The maintenance time of a vehicle could increase by as much as than 20 minutes when fitting VDO Redi sensors (see photo), and resetting the ECU procedure: manual or triggered.
The battery, unable to cover the entire lifespan of the vehicle, is a 3V low consumption battery and should last a few years or at least 160.000 km. Once expired the battery alone cannot be replaced, therefore a new sensor is needed.
Sensors generally possess three modes of operation, all of which greatly influence the lifespan of the battery : their descriptions are as follows;
1) Inactive or sleep mode (in store): the sensor is not installed and is inactive. Energy consumption is greatly reduced. No signals are sent. The sensor becomes operative only when installed and triggered.
2) Parking Mode: the sensor installed on the vehicle detects no movement (minimum monitoring speed: 15 km/h). Sends a signal every few minutes.
3) Running Mode: a sensor installed on a vehicle in motion sends a signal every 30 seconds, or every 5 seconds in case of a sudden pressure drop. If no movement is detected for 10 minutes, the sensor returns to parking mode.
The sensor is activated as soon as two parameters are simultaneously read: pressure and movement (speed above 15 km/h).
Once triggered ( or activated ), the sensor cannot revert to the previous inactive or sleep mode.
Programmable sensors, once active, cannot be reprogrammed for another vehicle if it has travelled for a few hours (approximately 15 hours).
1. Non-programmable sensors
The following sensors have an ID code and do not need to be programmed. This translates into high storage for multiple vehicle brands.
> Original Equipment
Equipped with a single software protocol and able to communicate with a specific vehicle or class of vehicles. Over 150 different models are currently available on the market.
> Single One-to-One (OE copy)
The identical copy of an OE model. These have the same software protocol of OE sensors but a different ID code.
Some have multiple software protocols working simultaneously, which translates in a 20% reduction for the battery’s lifespan. Since these can be installed on different vehicles or groups of vehicles, the on-board computer automatically detects and displays the correct flow of data.
2. Programmable sensors
These sensors need programming but have very limited storing requirements.
They have dozens of software protocols installed able to cover a wide range of vehicles, but have no ID.
Only one protocol is activated and linked to an ID during configuration. Programming can take up to 30 seconds.
These lack both SW protocol and ID code, and can be used on all cars, once compatibility with a vehicle is ascertained. Programming time, to install a specific software protocol for a vehicle, is slightly higher ( 30 seconds on average).
Electronic Control Unit
Possible tire localization sensor modes in the ECU.
> No localization: a tire pressure drop will be indicated by a generic “flat tire”.
> Wireless Auto Localization: a receiver recognizes the front / rear sensors by the distance from the receiver (signal strength), and right / left sensors by processing a magnetic / gravitational field (gyroscopic and electronic compass). The sensors must be WAL compatible.
> Hybrid Localization: occurs crossing data from ABS and ESP sensors already positioned in each wheel. In this case, if the wheels are replaced the vehicle’s ECU does not need to be reset.
> Fixed localization: occurs by Learning ID codes in fixed positions. In this case if the wheels are replaced the vehicle’s ECU must be reset.
The electronic initialization procedure is called Reset or Relearning.
It is necessary to store the new ID code in the ECU at each tire change (eg. Winter tires), if the vehicle is equipped with one or more new sensors, and, during tire rotation, if the vehicle uses the fixed localization type sensors.
Can be different for each vehicle or range of vehicles.
New sensors are detected following the ECU’s directions, or, occasionally, keying in the ID code of every sensor (direct manual keying).
> Sensor Trigger
Sensors are entered sequentially in the ECU via a trigger tool and manually following the directions of the on-board computer.
> EOBD (EuropeanOn Board Diagnostic) Connection
Needs diagnostic equipment connected to the vehicle to store one or more ID codes in the ECU.
The EOBD connection offers the following advantages:
- simplicity of procedure (Plug & Play);
- convenience: avoids awkward and impractical manual procedures;
- timing: ready in a few seconds;
It is estimated that about half of the vehicles currently circulating with a direct TPMS system require initialization methods like Trigger or OBD, and therefore need an electronic management tool.
The relative information can be found in the vehicle’s manual or in the most advanced diagnostic equipment.
Vehicles equipped with an EOBD, normally have the connecting plug in the lower part of the dashboard, or at times between the front seats.
> In Motion
The vehicle can detect a number of new ID codes simultaneously, without the use of any instrument. It simply requires the car to be driven for a set distance and at a set speed, according to the vehicle type.
> Car Dealer
Some vehicles currently need the intervention of a specialized dealer to store new ID codes in the ECU’s memory.
Electronic Management Systems
Trigger: this instrument forces each sensor to send its ID code and measuring units to be detected by the on-board computer.
Sensor-only diagnostics: this instrument connects to and controls only the sensors (Trigger, read, program).
ECU diagnostics: this instrument communicates with the ECU and, through it, controls all sensors, maintenance operations as well as the system’s diagnostics (indirect: no sensor programming)
All-in-one diagnostics: the previous three diagnostics systems come together in this one, which allows controlling and maintaining all sensors as well as the EOBD connection.
Compatibility and installation instruction
For the best combination with snap-in sensors, particularly when replacing tires, the best compatibility with the wheels is essential.
In the “Valve” chapter of the Standard ETRTO (European Tire and Rim Technical Organization), certain general recommendations regarding the application of TPMS systems are reported.
The bending angle must be limited, when mounting, to max 25 °, since the bending stress increases with the weight of the systems.
For vehicles exceeding 210 km / h, only metal valves must be used, or any other approved by the manufacturers of both the valves and the vehicles.
Fitting a sensor to a rim, through a qualified operator, must comply with the recommendations of the manufacturers of both the sensor and the wheel. This for obvious reasons of space in the area of the valve hole (see Figure 6) and the rim (height and shape of the rim well).
The compatibility between different types of sensors and wheels is in fact shown in a list ("positive / compatibility list") supplied by the wheel manufacturer, to ensure the integrity of the sensor and the sealing of the various parts, both when replacing tires as well as during use.
The correct measurment
The system must be reset or triggered -as previously stated- also after each technical intervention (tire replacing ), or air pressure correction, following the normal periodic check (to be done at least once a month).
This is clearly indicated in the use / maintenance manual of the vehicle, wherein it is stated that after each pressure correction the system must always be reset.
During the initialization process, the pressure data is in fact stored and set by the system as the "master" reference, that is the value compared to which any change is signaled (alert when pressure drops more than 20%).
The reliability of the instruments used is therefore essential to ensure the proper functioning of the system and therefore the safety of vehicles, both for the indirect TPMS (without sensor) system as well as the direct system (sensor certification does not yet exist).
In this regard, in 2009, with the release of the EC Regulation 661/2009 on vehicle safety, which introduced, among other things the TPMS systems, the CEN (European Committee for Standardization) was given the mandate to update legislation concerning tire pressure gauges. For example, the 1986 Directive n° 217, the application of which was not mandatory, still did not provide for digital / electronic gauges.
The European standard EN 12645 had, therefore, to undergo revision as far as the technical and metrological characteristics for tire gauges, as well as standardizing the connections between TPMS and gauges.
In addition to reviewing technical requirements, the errors allowed, inspection methods, digital and electronic pressure gauges are also included (requirements and testing), and the principle (strongly recommended) of periodic inspection of all instruments involved.
An approved gauge, compliant with tests and procedure regulations, including calibration and metrological tests ( MOT technical inspections, etc.), guarantees the necessary reliability.