How do wireless tyre pressure/temperature sensing systems work?

[teliocide]

After looking at different systems it appears most operate at a Transmission Frequency of 433.92Mhz.
My experience of "things "operating on this frequency is limited to ON/OFF systems like gate and door opening remote controls.
So my question specifically focuses on how the temperature and pressure values are encoded into the 433.92 Mhz transmission and how this is decoded and displayed at the receiver.?

 

[Pommie]

There are many ways to encode data. Even your on/off systems have some kind of encoding, if not then every door would open with your remote.

Mike.

 

[teliocide]

True, but in the cheap systems it a constant code.

 

[Pommie]

An often used encoding method is Manchester Encoding.

Mike.

 

[Diver300]

True, but in the cheap systems it a constant code.

There is still a code, consisting of 1s and 0s. As Mike said, those 1s and 0s are probably encoded using Manchester encoding.

Even the cheap systems will have a processor to send out a code that is unique. It's not difficult to add more data to what is sent, so that something that changes is sent along with the fixed code.

Tyre pressure systems have to encode a few bytes of information about the pressure and temperature, that will change and probably a few bytes that don't change, so that there is an identity for the signals and the receiver on the car can work out which wheel is which, and probably also not get confused by other transmitters on nearby cars.

Car keys also use the same frequencies, nowadays with more elaborate coding.

All digital systems are based on encoding data as 1s and 0s. The transmission system, in this case, bursts of power at 433.92 MHz, is used because it's cheap, low power, doesn't need a licence, and can get the very small amount of data sent adequately quickly. For tyre pressure systems, that is very slowly. Most of those systems will only transmit very rarely if the car isn't moving and the tyre pressure hasn't changed.

433.92 MHz systems are generally not allowed to be used continually. Even when tyre pressure is changing, the transmitters will be sending data for just a few milliseconds, and repeating less often than once a second.

 

[rjenkinsgb]

For the concept, think of it a bit more like a TV remote control - that sends an address, that ties it that that particular make or series of TVs, and data to identify the button that was pressed.

There is no difference in the data stream or "packet" in how the address is sent to how the data part is sent; after decoding at the receiver it's just a string of binary bits.

It's down to the way the system designer defined how long, how many bits, are used for the address part of the packet.

The receiver compares that number of bits to the address it is configured for, and if they match it can use the rest of the packet and decode that to a button press or any other kind of number or data, depending on how the packet format for that particular system is defined.

(Whether IR, RF or such as Ethernet, there is also some "framing" data before the packet payload, that allows the receiving system to recognise the start of a data packet from random noise, typically a fixed sequence that's searched as received data is shifted through the receiver; it starts accumulating the bits of the packet content after seeing that sequence).

This is an example of a TPMS packet data format; the first eight bits have the address and three bits to indicate which data is included in the packet, then ten bits of data for each included sensor reading.
Plus a four bit checksum, that the receiver can use to detect some errors for eg. interference, and only use the data of that matches the content.
(Not sure what the frame type bit is used for).

As soon as the receiver has the first eight bits, it knows how many more bits there are in that packet from which sensor bitflags are set, so how much to use and where the checksum will be in the data that follows.

 

[teliocide]

Thanks to everyone that responded.
These devices sparked the idea of having wireless instrumentation in vehicles so there was a real need to get an Idea how these things work.
If you have ever run wires from one end of a car to the other you know very well what a joyful activity it is. (Not)
I have installed some wireless switching (433 MHz door/gate systems) in my Van, for lights horn and fans.
Lights and horn are fine, you get visual and audial feedback of their operation state, but for the fans I had to use a second 433 MHz system to transmit ON/OFF status of the fan back to the cabin.
There does exist wireless instrument kits from the US but hideously expensive, there are also some wireless ODB instrument kits as well, but these are no good for after market add-ons.
So it would appear to make such a system oneself would be a task limited to experts.

 

[rjenkinsgb]

All normal vehicles now use CANBUS for communications - two network wires plus power and ground, and you can control dozens of devices without any extra wiring.

Vehicles actually use two or more separate CAN systems, to keep critical things like braking and engine controls separate from important things like lighting, then non critical like power windows and entertainment stuff.

A large of the bulkiness of wiring harnesses is down to the need to fuse every power feed, to prevent overloads and fire risk in case of a short, without disabling too much if a non critical device develops a fault and fuse blows.


Adding on to that for monitoring is easy - software such as "Torque" that works on an Android phone or tablet & is free, just needing a cheap interface in the OBDII socket.
Adding new controls, unless the manufacturer allowed for those in some version of the vehicle, is not easy.


To control accessories wirelessly, you really need a two-way radio link with built in status or acknowledgement. That's easy enough, but you do need to do some parts assembly and programming.

Wireless links should never be used for time-critical or safety-of-life applications as it's always possible they can be blocked by external signals any time and without warning, for unpredictable durations. Systems such as radio controls for industrial cranes simply stop everything, if there is any interference, then the operator can restart movements once the controls are working again. You cannot do that with road vehicle controls!

And be extremely wary of changing how any standard equipment such as lights or horn works, in any way - it will void your insurance in many parts of the world and you can be prosecuted for it.

 

[Amit Bajpayee]

All normal vehicles now use CANBUS for communications - two network wires plus power and ground, and you can control dozens of devices without any extra wiring.

Vehicles actually use two or more separate CAN systems, to keep critical things like braking and engine controls separate from important things like lighting, then non critical like power windows and entertainment stuff.

A large of the bulkiness of wiring harnesses is down to the need to fuse every power feed, to prevent overloads and fire risk in case of a short, without disabling too much if a non critical device develops a fault and fuse blows.


Adding on to that for monitoring is easy - software such as "Torque" that works on an Android phone or tablet & is free, just needing a cheap interface in the OBDII socket.
Adding new controls, unless the manufacturer allowed for those in some version of the vehicle, is not easy.


To control accessories wirelessly, you really need a two-way radio link with built in status or acknowledgement. That's easy enough, but you do need to do some parts assembly and programming.

Wireless links should never be used for time-critical or safety-of-life applications as it's always possible they can be blocked by external signals any time and without warning, for unpredictable durations. Systems such as radio controls for industrial cranes simply stop everything, if there is any interference, then the operator can restart movements once the controls are working again. You cannot do that with road vehicle controls!

And be extremely wary of changing how any standard equipment such as lights or horn works, in any way - it will void your insurance in many parts of the world and you can be prosecuted for it.

Nice Information , thanks for this information

 

[For The Popcorn]

I believe some car had this feature, but with CANBUS, it should be simple to implement....

Each switch is a stand-alone module with a CANBUS controller. Any switch can be plugged into any position, so you can position the switches in the position most convenient for you. The controls you use often can be close at hand, the seldom-used ones positioned a little farther away.

 

[Tony Stewart]

There were several different types developed

What is a tire pressure sensor (TPMS) and how does it work?

Tire air pressure is of particular importance for the active safety of the car as well as fuel consumption. If the pressure in the tires is below the limit

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