TPMS... DIY and Design Anatomy

[srt8-in-largo]

If I were designing my own TPMS, these are the useability features I'd want:

- System… must read pressure and temperature and have user selectable units for both
- System… must be able to obtain live readings when the bike is parked
- Display… must be weatherproof
- Display… must be backlit for night viewing
- Display… must be hard wired to the bike instead of being battery powered only
- Display… must show pressure and temperature by default and not require a button press to see this
- Sensors… must report frequently; at least once every 1 minute
- Sensors… must have a “fast pressure loss” mode that reports immediately when this happens
- Sensors… must have reasonably long battery life; at least two or three years… ideally 5 to 7 years

- Nice to have... low sensor battery indicator (maybe this should be a requirement)
- Nice to have... since the display is hardwired to the battery, battery voltage can be displayed
- Nice to have... battery option for the display instead of requiring hard-wiring


I don't like dash clutter so I'd see about getting a panel-mount display that installs in the panel near the shelter keyhole. This is an out of the way location but is at least somewhat visible at a glance. Since the panel isn't in the riders field of view, you could install a small but BRIGHT dash mounted LED to alert you if any pressure or temperature thresholds have been crossed.

https://lh3.googleusercontent.com/-gIn9maVdFlg/VGgSSCQy_TI/AAAAAAAACR0/OJGdwtK05lk/s800/DSC00598_TPMS.JPG

[srt8-in-largo]

The OEM Goldwing TPMS is what I would call a simplified system that is typical of those made specifically for motorcycles. The OEM system is illustrated below and consists of three parts... two sensors/transmitters in the tires and a controller that monitors the transmitted signal and produces a low pressure warning when appropriate.

TPMS sensors have three function blocks: 1) a sensor that measures pressure and temperature, 2) an RF transmitter that sends the measured data to the controller, and 3) an LF receiver that accepts commands and issues them to the RF transmitter. A "simplified" system is one that doesn't have a built-in way to issue commands to the LF receiver in the sensors.

Why would you want to issue commands to the sensor? Read on...

The second picture below is an exploded view of the Goldwing sensor; this is typical of any other sensor on the market. The three function blocks mentioned above are implemented with electronics on a tiny PCB housed in an enclosure with a battery. Function 1, the sensor, is a tiny analog IC that continuously reads pressure and temperature. Function 2, the RF transmitter, is a programmable microcontroller that reads the analog pressure and temperature data, converts it to digital, and transmits it to the controller on a 315 or 433 MHz carrier. And function 3, the LF receiver, is an LC tank circuit tuned to pick up 125 kHz signals and send them to the microcontroller.

The OEM sensor uses built-in accelerometers to send motion data to the microcontroller. When the bike is parked and the microcontroller does not sense motion for 7 minutes, it will not transmit data and therefore you cannot read tire pressure after the bike has been parked. This is done to save battery life of the sensor. When you get on and ride faster than 9 mph for 20 seconds, the microcontroller senses this and begins transmitting data every 1 minute. This is a cheap usability feature IMO because the most important time to check pressures is in the morning BEFORE you ride.

If the bike had an LF command module it could be configured to detect a key on, and when it sees the key is on it can send a single transmit command to the sensor... and then allow the sensor to go back into sleep mode until the accelerometers take over. With this setup you could read pressure whenever you wanted and not really affect sensor battery life.

So where can you get an LF command module that will do this? That's the complicated part... do-able but complicated.

https://lh3.googleusercontent.com/-aFy2Z0V3jkc/VGlRrk7j7wI/AAAAAAAACTQ/nSU-1KrosYs/s800/tpms%2520-%2520oem%2520goldwing%2520-%2520system.jpg

https://lh4.googleusercontent.com/-S9O-3Mp_qKs/VGlPObXdp2I/AAAAAAAACS0/hjxkXgmMHBc/s800/tpms%2520-%2520oem%2520goldwing%2520-%2520sensor%25201a.jpg

[srt8-in-largo]

After taking a good hard look into all of this... I've decided to move forward and make my own TPMS system.

I'm taking a slightly different route than what I pictured above and I'm going with this setup pictured below. The location of the Honda plaque is just big enough to be replaced with a 2-line 8-character display that is enough to display pressure and temps for both tires simultaneously... and this location is easier to glance at than the side panel. I've already found an LCD that matches the Honda OEM display and so it'll look pretty good, almost OEM. I still plan to wire a glaringly noticeable LED that will alert the rider of pressure warnings or rapid decompression events and have this mounted near the gauge cluster.

I've already prototyped some hardware for the pressure sensor using Freescale's FXTH87 TPMS chip; proto board is shown below. I'm currently getting the programming sorted and then will move onto developing the receiver/display module.

https://lh3.googleusercontent.com/-Qg5MnSEtkEE/VISqcDOez1I/AAAAAAAACWI/C7ID-WHPzM0/s800/mount.png

https://lh6.googleusercontent.com/-jj7iQI_Nclg/VISnI72SFvI/AAAAAAAACV8/nYB8Pxj74HA/s800/tx%2520sensor%25201.JPG

[Steve 0080]

Impressive..... we have a lot of talent on this board !!!!!

[Spanky]

sitting on a million bucks! great job! I can see a Hellcat in your future! now don't forget all those on the board that support and encourage you ( who support you enough to pay cost+ 10%)! remember make it easy to install! :-)

[srt8-in-largo]

Thanks guys!

The hardest part of the install will be pulling the shelter cover to get to the underside of the Honda plaque and routing the alert LED. I'm hoping to develop the low frequency (LF) communication so that riders can get a fresh reading just by turning the key to ACC. These LF communication modules will need to be mounted near the tires... and I haven't yet worked out how all that will come together.

I plan to use an in-tire sensor that will mount to the rim on the opposite side of the air valve. This creates a better tire balance and, more importantly, allows you to use tire sealer like Rideon and not worry about that stuff clogging the sensor. Like this picture from the Mobiletron manual.

https://lh5.googleusercontent.com/-JcGK94mnkH4/VIS45ZpfZKI/AAAAAAAACXw/HW5wVL0RhwQ/s800/mobiletron_strap_sensor.jpg

[Phantom]

:Saweeet smilie:

[srt8-in-largo]

Thanks Phantom!

I built a test chamber with a pressure gauge, thermocouple, and a schrader valve to let me test out the sensor board. PVC in general is NOT a good material to pressurize, especially the type that says "not for pressure", but it did ok to 40 psi. The sensors seem to be working fine.

The part of the system that I've been concerned with was the antenna circuitry; currently it has no filtering and a first pass design of the matching network. To test how good (or bad) it is I downloaded HDSDR and bought a $20 antenna to go with it. The video below shows the sensor transmitting at a distance of about 4ft from the antenna. From what I can see, I think Freescale designed a helluva RF transmitter on the chip! Even with this non-optimized proto-board signal strength is fantastic.

For test purposes I have the transmitter configured to send 16 consecutive packets... these are the pulses and beeps you see. Each packet contains pressure and temperature data measured by the chip. I plan to have the software setup where it'll take measurements every 0.5 or 1 second but only send the packets every 20 or 30 seconds in order to conserve battery power. I might even extend the transmission to 60 seconds or more... but I have yet to create a power budget. Like I said above, I want a sensor with long battery life.

Sending consecutive packets is done for redundancy; if one packet gets lost there are multiple others available and therefore the data monitoring can be consistent and continuous. When I look at the power budget and do some link testing I'll probably reduce the number of packets to 2 or 4 to, again, optimize battery life.

In addition to the above software configuration, I plan to create a comparison loop where if a significant pressure drop is detected the chip will transmit immediately and more frequently.

I've also been working on the receiver module. Next steps are to get the transmit-receive link operational and then work on incorporating the LCD display. Here's a picture from the vendor website; I think this is with the white backlight turned on.

https://lh3.googleusercontent.com/-7mYPzaFL4Yk/VJOBeYTGYfI/AAAAAAAACZA/wveqkucDgko/s800/lcd%2520display.png


https://lh3.googleusercontent.com/-HdfxLKPBx_4/VJN0oajqMdI/AAAAAAAACYA/Hj0BloylfcA/s800/test%2520chamber.JPG


https://www.youtube.com/watch?v=4HcjpWnG6m0&feature=youtu.be

[hiflyer]

Thanks Phantom!

I've also been working ......



srt8, I think that's an understatement!!! You are serious about this thing!! Get er done!!:yes:

[srt8-in-largo]

Haha thanks Flyer; I hope I can get it all together before long.

To illustrate the transmission options, I reduced the packet count to 4 in this video and maxed out the time between each of the four packets. I have the program compiler in debug mode and you can hear me clicking the keyboard to initiate each transmission; when the chip is programmed it'll do this automatically.


https://www.youtube.com/watch?v=ANBdtaNRjVU&feature=youtu.be

[Fla_rider]

This looks great! I'm sure you will have many board members wanting this! I'm pretty lazy so having this tell me the tire pressures is the cat in pajamas! :clap2:

[srt8-in-largo]

Thanks John. I didn't start this project with the intent of selling them but if people like it...

[Limoles]

From engineering point of view , this is very nobel project . In automotive business , such a conveniences are noted and practiced already . If you are so skilled and have desire to see those improvements in Goldwing's future production , I'd suggest to present your ideas to Mother Honda , which might provide obvious benefits for the owners of incoming models . Until now the only few motorcycle companies offer such a displays . Kudos for sharing . I might follow your improvements , so please do not hide any future and so "bright" enlightenments ...

[srt8-in-largo]

I think Honda could have very easily made a better TPMS but that they have their reasons for deciding not to do so. It's a shame really, considering that tire pressure is so important on bikes.

I'm working on the RF link between the sensor transmitter and the display receiver. Once I'm past this, things should move pretty quickly.

[Phantom]

I think Honda could have very easily made a better TPMS but that they have their reasons for deciding not to do so. It's a shame really, considering that tire pressure is so important on bikes.

I'm working on the RF link between the sensor transmitter and the display receiver. Once I'm past this, things should move pretty quickly.
Some trivia .....Honda has had TPMS sensors on their Full Goldwings for a few years.

[srt8-in-largo]

I understand... but the information display is pretty much just a dummy light.

I think they could have done better than this but maybe they think just a light is good enough; I want to see the numbers :icon_biggrin:

[Limoles]

Since such a valuable gadgets are so popular , idea of "beefing up" newly purchased bike would be more attractive , if buyer had option to chose and order any of desired and already available accessories , like : shields , seats , lightings , combination of colors , foot positioning , etc . It could attract the product , increase sale and add the value . But producers have different mind set , so it's never ending struggle . Such a "packages" exist in car's business and successfully help to make a choice .

[srt8-in-largo]

This is looking pretty good guys... I've made some major progress.

I've finalized the software for the sensor/transmitter and have been running bench tests to find a good balance between how it operates and battery life. What I DON'T want is a system like what is currently on the market that only updates pressure info every 5 minutes, yet still can't make their battery last more than a year or two. Whoever is putting systems like this on the market is either using really old technology or they don't know how to write good software.

I have this sensor programmed to measure pressure and temperature every 3 seconds and then transmit every 30 seconds. This is a pretty busy schedule for a sensor but I'm only drawing 12.5 uA (microAmps) to do this. Figuring a coin cell battery rated at 550 mAh, my calculations are EASILY exceeding a 5 year battery life. If I lengthen the transmit interval to 60 seconds or go to a 1,000 mAh battery, I think I can get 10 years out of this sensor.

With that said, long battery life is just the icing on the cake. The actual cake is having a system that'll keep you safe. Gradual pressure loss in a tire is one thing, but a blowout at highway speeds is something different altogether... and this is what I'm focused on.

At the end of every measurement cycle (3 seconds) the sensor compares the new pressure with the last pressure measurement and if there's more than 6 psi difference it'll send an alert to the receiver to light up the dashboard LED. For this to work, the LED has to be bright enough to be noticed in all riding conditions, it has to be mounted within your field of view, and you better be ready to immediately get off the gas if you ever see it light up.

What do you guys think?

Is 3 seconds short enough to catch a catastrophic tire blowout?

Is 6 psi a reasonable pressure difference to check for?

Any other thoughts?

https://lh6.googleusercontent.com/-lUybyvSdInI/VM7XZEpji5I/AAAAAAAACaQ/ds5JjBAQnAI/s800/DSC00663.JPG


Here's the LCD display wired to the receiver; I think it's a fairly good match to the OEM LCD. There's no pretty pictures, cute graphics, or any other extraneous distraction. It's there to give you tire information clearly and quickly, nothing more.

I currently have it configured to show front tire pressure and temperature on the top row and rear on the bottom row. Psi and Fahrenheit are shown; kPa and Celsius will be available for our metric friends.

The LCD is shown with the backlight off. The little blue box with white wires coming out of it is a potentiometer connected to the backlight control. I'm currently looking into an ambient light sensor that will automatically adjust the backlight when it gets dark. I'll bet some people will want it lighter or darker than others so I plan to have a manual control present also.

I was initially concerned that the characters would be too small to see well but I don't think that's the case. If you see good enough to ride, you'll be able to hold this at arms length and read it quite easily.

https://lh5.googleusercontent.com/-yNb40tUh7Zo/VM7XY_5zSBI/AAAAAAAACaM/mV5lmlpx8NI/s800/DSC00667.JPG


After seeing how well this little sensor does at drawing small battery current, I'm ditching the idea of adding an LF controller to the system. I initially planned to add an LF controller for two purposes: 1) to shut down the sensor when the bike is parked and 2) to allow the rider to request a pressure reading before getting on the bike.

Reason number 1 is unnecessary; as it's turning out, the sensor should run continuously for more than 5 years and not need to be turned off.

Reason number 2 will be baked in; since the sensor will run continuously, I'll make the receiver continuously capture data even when the bike is turned off. When you turn the bike off, the display will shutdown but data will continue to be captured in the background. When you get up in the morning and turn the key on, that will turn the display on and show the most recent tire pressure.

https://lh4.googleusercontent.com/-ikjkev5MUKU/VM7Y6yeOp0I/AAAAAAAACac/02YoTWTaRfQ/s800/system.jpg

[hiflyer]

This is looking pretty good guys... I've made some major progress.



At the end of every measurement cycle (3 seconds) the sensor compares the new pressure with the last pressure measurement and if there's more than 6 psi difference it'll send an alert to the receiver to light up the dashboard LED. For this to work, the LED has to be bright enough to be noticed in all riding conditions, it has to be mounted within your field of view, and you better be ready to immediately get off the gas if you ever see it light up.

What do you guys think?

Is 3 seconds short enough to catch a catastrophic tire blowout?

Is 6 psi a reasonable pressure difference to check for?

Any other thoughts?




Yes you have made some major progress. Two questions.

1. Is there any way to have an audible warning in conjunction with the warning light?

2. Are you prepared to make a bunch of these? We will all want it.

[srt8-in-largo]

Interesting; I haven't considered an audible alert. I'm assuming you mean an audible alert in the bike's sound system.

When the receiver detects an alert status from the sensor, all it can do is turn on a signal. With an LED the process is simple; all that needs to be done is to use this signal as the power source for the LED. Receiver detects an alert... turns on a signal... the LED gets power and lights up.

However, even my digital multimeter generates little beeps when I press buttons on it. There has to be simple tone generators out there that can be used here. Receiver detects an alert... turns on a signal to power the tone generator... sound is created. Hmmm... yes I think that's a possibility! The output of the tone generator would just need to tie into the sound system.

Now that I'm past the hard part, these will be easy to make, and the more I make the cheaper I can offer them. I currently have these built on generic prototype PCB's from Amazon but for ruggedization for automotive use these will need proper PCB's made which are cheaper in higher volumes.

[hiflyer]

I'm not concerned about it coming thru the sound system, half the time I'm listening to the wind only. You could mount a small warning horn out of sight under the dash somewhere. I've seen these small horns in electronic stores.

[srt8-in-largo]

I'm gonna look into this. Thanks for the idea; it's a good one.

[terrydj]

Audible warning through a 3.5 jack at set below normal tyre pressure would be excellent :icon_cool:

[Fla_rider]

SRT8 - your doing great, your project is going to be a hit! Positive enforcement always speeds up production!:clap2:

[srt8-in-largo]

Audible warning through a 3.5 jack at set below normal tyre pressure would be excellent :icon_cool:

With the design completely in my control, anything is a possibility!



SRT8 - your doing great, your project is going to be a hit! Positive enforcement always speeds up production!:clap2:

I really appreciate that FLA; I won't say how many all-nighters I've pulled trying to get this going. I have parts for the sensor in hand and ordered custom PCB's last week.

[hiflyer]

With the design completely in my control, anything is a possibility!
.

I still say a small warning horn separate from the sound system. But who am I to say? You're the one staying up all night! Thx for working on this.

[srt8-in-largo]

I still say a small warning horn separate from the sound system. But who am I to say? You're the one staying up all night! Thx for working on this.

You're now the 3rd person to cast that vote... so I think a horn is "sounding" good :icon_biggrin:

[Tedubya]

Would it be simple enough to run the LED trigger to a relay to activate the factory horn?
Might scare the hell out of you but it would get your attention quick enough for a catastrophic tire failure.

[srt8-in-largo]

Would it be simple enough to run the LED trigger to a relay to activate the factory horn?
Might scare the hell out of you but it would get your attention quick enough for a catastrophic tire failure.

That's actually a brilliant idea Ted.

[srt8-in-largo]

Someone recently asked me in another thread how this is coming, but I only saw the question briefly in-passing. I thought I'd be able to come back to the question and reply but I can't find it! So I apologize for not responding in that thread.

:icon_redface:

I have custom PCBs for the sensors in hand and I'm patting myself on the back for getting the sizing as good as it seems; the boards drop nice and snugly into the sensor housing that I want to use. The only tasks left for the sensor is to assemble the loose parts onto the board, connect the battery, and seal it back up with potting material.

I have a couple of technical issues to figure out on the receiver... and then need to figure out how to package it up and mount it on the bike.

https://lh5.googleusercontent.com/-gIA6w5CpETM/VRy24BpUajI/AAAAAAAACns/9bNnqFaYVHs/s800/pcb%25202.JPG

[Fla_rider]

Wow, George your really exceeding my expectations. Keep up the good work, I'm first in line!:yes:

[stroguy]

The alarm could be a simple AND gate tied to a PC relay then on to a piezoelectric buzzer. With the AND you don't complicate it with a separate circuit for each tire. It alarms and you look at your display. Great work Largo!

[MAYHEM01]

this is FANTASTIC!!! I cannot wait to order mine...... Great job.....:yes:

[srt8-in-largo]

Wow, George your really exceeding my expectations. Keep up the good work, I'm first in line!:yes:

Thanks John; your encouragement is giving me a 2nd wind.



this is FANTASTIC!!! I cannot wait to order mine...... Great job.....:yes:

Thanks Mayhem... I appreciate that :icon_biggrin:

[srt8-in-largo]

The alarm could be a simple AND gate tied to a PC relay then on to a piezoelectric buzzer. With the AND you don't complicate it with a separate circuit for each tire. It alarms and you look at your display. Great work Largo!

Thanks Stro, I like the way you think; the differentiation is handled in software though. Since I decided to not implement LF communication modules to keep things simple, the receiver won't know if temp\pressure data is from the front or rear. So... to get around this I added a tire location byte to the data packet sent by the transmitters.

The sensors transmit using essentially the same technique as FM radio, using an additional encoding scheme called FSK (frequency shift keying). In FSK, a base frequency is chosen and an excursion above or below base are recognized as zero or one. For example, my base frequency is 325 Mhz and my excursion is 38 kHz. Signals received at a frequency of 325 MHz - 38 kHz is decoded as zero and signals received at a frequency of 325 MHz + 38 kHz is a one.

If the videos above were zoomed in you'd see this... the pulses on the left are below baseband (zero's) and the pulses on the right are above baseband (one's).

https://lh3.googleusercontent.com/-NHkuczlbtgc/VR3vPPUF-MI/AAAAAAAACoQ/gbeDMEjr8J0/s800/encoded%2520signal%25201.jpg

These pulses are picked up at the receiver and decoded into a binary stream of one's and zero's which then makes it possible to process the information with digital circuitry. I've gone through a few iterations of exactly what information needs to be transmitted and I think I've firmed up on the below. The less information to be transmitted, the better, because it takes valuable battery power to send every one and zero. Data transmission is by far the largest power draw used by TPMS sensors.

The first 3 data bytes are a preamble; when the receiver detects a preamble being sent, it initializes its' receive circuitry for possible reception of data. The next 3 bytes are sync words which act like an encryption key; if this sequence matches the same sequence programmed in memory, it accepts the remaining bytes of the data packet... otherwise the packet is discarded as invalid. This is what will keep the TPMS system from picking up pressure data from the car in the next lane over.

The length byte tells the receiver how many more data bytes to accept.

And, finally, we get to the tire location byte. This is a unique sequence used to inform the receiver of where the data is coming from, front or rear. If front tire data is detected, for example, it will display that data on the top line of the LCD. If rear tire data is detected, it'll display that data on the bottom line of the LCD. The system is currently setup with just one alert LED, and an alert status sent from either tire will light it up. However, this doesn't tell the rider which tire is having problems. So... I wrote some code to inform the rider on the LCD. After seeing the alert LED and pulling over, the rider will see a message on the LCD telling which tire is the problem.

I'm thinking the alarm buzzer should work the same way; either tire can set it off and then the LCD will tell you which one is the problem.

https://lh4.googleusercontent.com/-x6Nz4hdr8Q8/VR3qgbHN-VI/AAAAAAAACoE/OXn91lIPAaI/s800/decoded%2520signal%25203.jpg

[maxrider]

Way to go, reread the entire post twice, awesome, I hope you fully succeed.

You got me as a customer :icon_biggrin:

Good luck on the prototype.

Max

[valkmc]

I think Honda could have very easily made a better TPMS but that they have their reasons for deciding not to do so. It's a shame really, considering that tire pressure is so important on bikes.

I'm working on the RF link between the sensor transmitter and the display receiver. Once I'm past this, things should move pretty quickly.

You are right there, all Honda had to do is use the same one that has been on C-14's since 2008 or so. Worked great and after checking many times with different pressure gauges it was very accurate!!! I hated the one on my 12 Wing. What's the since of doing it without a read out of the pressure. The one you are making looks like the real deal!!

[srt8-in-largo]

Thanks guys. This is still on my radar, I've just been overloaded with a slew of summer time tasks around the house that I've put off for far too many summers already :icon_biggrin: