Tire Pressure Sensors in wheels

I think, and stand to be corrected... But I think they use the circular motion of the wheel to generate the tiny power needed to operate them and they communicate once a revolution to a reciever near the wheel by radio.

They are basically a tiny pressure sensor with a wobbly thing to generate electricity and a small rf transmitter...

John.

not sure that is quite correct as the unit was sensing low tire pressure on one tire before we started moving the car. Thanks for the post though.

Oh, maybe they have a small battery in them then?

I can only speak of the first ones we looked at back in the late 70s...

John.

A company near here was making them for trucks. What you describe is how they did it. If you're sitting still, hit the tire with a hammer.

Sensor in on wheel hub counts wheel revolutions. As tire looses air the diameter decreases. A improperly inflated tire has to rotate more than the other tires. The CPU is comparing their rotations. The sensor is the same one that is used for the ABS brakes (anti lock brakes). Which checks for slippage they just found another use for it and add a little program to the CPU and additional idiot lights in the dash.

That is most definitely not how mine worked on my BMW motorbike as it sensed the air pressure before starting the engine and told you so in the pre check before turning the key for starting. I am not sure if it would stop you driving to a petrol station to fill it back up as that would be a horrible experience. Caught with a flat but you cannot drive it slowly to the place where you can fix the problem Typically German.

I think they must have a battery and transmit a value to a closely placed receiver. As it only does it when starting, the batteries will most likely last until the death of the vehicle.

VP got a Mini Cooper thats how they explained it was done. I know of no sensor in the wheel.

Agreed. My dash panel shows tire pressure monitor when I turn the key "on", before starting the engine. I had a recent experience where I had a tiny brad in the tire that cound not be easily seen without taking the tire off, and my tire pressure monitor would pick up the loss of pressure every few days until I had the tire repaired. This was on a 2 year old 07 Camry.

Interesting responses.... I am still not sure how it actually works. I am assuming there is a battery in the sensor/sender and that there is a receiver near each wheel so that the wheel LOCATION is sensed and displayed. Still curious if anyone really knows how the battery/transmitter works and how long the battery lasts.

Appreciate the ideas.

The actual workings of the sensor can be found here.

As to the typical application in a wheel with a tyre on it is a simple deduction. Just add battery for the power and a transmitter for the value. Battery life depends on how much it uses and how often. If only at start it will probably outlast the tyre itself.

I am not quite sure what you want from us right now, I think the question has been answered.

I don't think theres a battery. From my experance with my wife's 2006 escape I think the wheels need to turn for the system to work but when it sees loww preasure it latches the warning light on the dash. Once you inflate the tire and start to move the light will go out.

I finally found some information

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=44#sensors

This tells about both the indirect (differential rotational speed sensing) as well as the direct sensing. You can click and see several of the actual sensors.

Thanks to every one for the input.

Here

http://www.semiconductors.bosch.de/pdf/TPMS_Product_Info_200703.pdf

is a datasheet of the last BOSCH-sensor with a standby current of 500nA.

Regards Uwe

There are basically two schools of design...both use Lithium ion batteries and an RF transmitter coupleed with an antenna mounted under the car. The first idea was to band the sensor to the rim (and this is usually how OEM designs are made). The other way to go is to mount a device onto each valve stem (more aftermarket here). The banding to the rim lessens the effect of setting the wheel into imbalance which ultimately gets corrected of course. The effort to counterweight match the sensor are included in designs as well.

As noted- there are other ways- but these are the standards that have proven cost effective and reliable.

Batteries last about 3 years- and in valvestem versions- you toss the unit out when it goes dead. Check out aftermarket versions like PressurePro.

I work for Gm and they use both the hub and a in tire sensor that has two non replaceable battery's in each wheel that send a RF signal to pickups near each wheel buy the way each sensor has its own frequency so as not to interfere with each other and need to be programed each time you rotate the tires or replace a sensor.

we had a lot of problems with them as customers change there wheels for aftermarket rims and tires and even stock rims sometimes would cause head aches

Seems odd that they don't go with an RFID style system externally powered with close proximity (couple of feet) readers. Think that's how I'd do it.

What will be the next useless item. Are we so stupid that we cannot tell a tire is flat or nearly flat?? How often do we have a slow leak or tire loosing air?? It is true it is hard to find a station that has air any more. My wife just bought a Hyundai it has that feature, I can assure you that was not the deciding factor, the dealer briefly mentioned it.

Traveler Engineer:

Imagine this: you live in the North where it snows, and towards the season end, you're driving to work in the morning and you manage to hit one of the zillion potholes accumulated from the harsh season, arriving at your destination, park the car, and unaware that you developed a small leak in the front passenger side tire. Eight hours pass, its dark out and brrr its cold out, minus 10 today, you buckleup and head for home. You drive slowly to the Interstate ramp which is but a few blocks away, not noticing any handling problems at 25 MPH, enter the ramp, once up to highway speeds, the car starts handling poorly, maybe its just the high winds. You accellerate briefly for a lane change and immediately lose control of the vehicle, spinning wildly... SCENE 1A: into the path of a snowplow, and he hits you hard and launches you off the roadway, you end up near the botton of a mountain slope 300 feet down, where a tree limb has impaled your car, and your bleeding from your femoral artery, and your still 100 feet from the bottom...not good. SCENE 1B: into the path of a semi tractor trailer, carrying gasoline, he rams you head on since your spinning, your pinned in the car, and it causes a multicar pileup, with 50 gallons of leaking fuel already on the pavement... not good. SCENE 1C: somehow you avoid an accident, and pull off over to the side, and realize you have a flat, you cell phone battery is dead, or maybe your in a signal hole, but your on the tollway so there isn't an exit for miles, you unwisely decide to change it out yourself, as you don't want to be late for your one year anniversary. Unfortuneately, you haven't change a tire out yourself in a few years, and remember its bitter cold out, you are right up against the guardrail and you need to make room to work on the tire, so you are hanging out into the lane a bit, but not much choice, since there are so many overpasses, the road engineers couldn't allow for a wider emergency lane. Its slow going and finally get the tire changed, with frozen hands, fumbling and trying lift the flattened tire into the trunk, not realizing that you're blocking the one of the hazard flashers. A car coming up behind you with his dirty headlights and windshield, gets confused, and doesn't realize your car is motionles. But at the last second, sees your car, hits the brakes, its too late, he is still doing 30 mph when he slams into the back of your car, you end up inside the trunk. You are both knocked out on impact, and passerbys stop for assitance, not seeing you in the partially closed trunk and thinking yours was an unattended parked vehicle, attending only to the other driver, when you are the one in dire need of medical assistant with crushed and nearly amputated legs. Ambulances are away, and the tow truck driver, picks up the damaged tire laying on the roadside and tosses it into the back of his trunk, wondering where it came from as he tows your vehicle away...not good.

But now had you had the Tire Pressure Monitoring System in your car, before you entered the Interstate ramp, you would have been alerted to the tire condition and wisely detoured to a gas station, Sam's, Costco, WalMart, Discount Tire..etc...or changed your tire out safely in a parking lot!!! Mind you, most people have never had a flat while driving, and don't know what it feels like to have a low tire while driving or even how to react. Needless to say, I'm glad someone wised up NTSB or wherever and are making it a standard safety feature.

PS. I have many more scenes but not the time to write them, unless HBO is interested...LOL

PPS: I read somewhere the new pressure sensors under development are going to be RFID and batteryless.

Funnily enough yesterday I read an article in one of my trade mags (I think it was 'Components In Electronics') about this very topic!!

Amongst all the descriptions for mounting the devices etc... they said that at the moment they are looking at RFID type of monitoring. They also pointed out the very real problem of owners rotating or changing wheels and finding the tyre pressure sensors weren't working!!

All of this is because the USA is introducing (has introduced?) that all cars must be fitted with warning for low pressure tyres..

They never mentioned how long the batteries last though, I've had my car from new for 15 years now!!!

John.