The BST1712 “TPMS” valve operates outside the tyre and transmits pressure and temperature data every 4 seconds. Power is supplied by a 3V lithium battery.
The “MEM” pressure sensor is a Wheatstone bridge with a resistance of 3 kΩ and a current of 1 mA when active.
An operational amplifier inside the microcontroller (Texas MSP430F2012) is active during temperature and battery voltage measurements. The analogue-digital conversion process draws about 1.8 mA, but such measurements take just 60 µs.
The duty-cycle is 60 µs/6s= 1/100,000 for a total mean current of only 18 µA per measurement.
In RF terms, the valve transmitter is more powerful than the SPT17S3 module, sends data once a minute and draws 500 µA during transmission (two 10 ms messages per transmission). The duty-cycle is 20 ms/1 min = 1/3000, drawing on average 167 µA. The two current levels, 18 µA and 167 µA, must then be summed with the microcontroller’s standby current and component leakage current (about 300 µA).
In the microcontroller, just the VLO clock remains active on standby. In each case, the average total current over 24 hours is typically less than 0.5 µA.
The fact that sudden parameter changes are transmitted immediately has little influence.
A sudden drop in tyre pressure is fortunately a rare event, as is a parameter change due to maintenance or pressure checks. A CR1225 lithium battery (48 mA/h) with a mean 0.5 µA discharge current has an expected discharge life of 7 to 10 years, depending on environmental conditions.
Operating in a very tough and competitive market, the Micro.sp® technology and the BST1712 are protected by patents: no. MI2005A002225, filed 22th November 2005, and European Patent no. EP1787831A2, issued 29th August 2006. The patents concern the intellectual industrial property regarding the Micro.sp® tyre valve for TPMS use (intended as a means of transmission), and the principles regulating the mechanical layout regarding the PM17 intelligent tyre valve. Ste Industries has recently filed a 3rd patent for the simple and cost-effective method in which the Micro.sp® generates the particular pulses.
The patent is intended to cover the original concepts and contains a full explanation of how to design simple RF hardware to generate the RF pulses - the Micro.sp® pulses - using standard components normally available on the market.
The basic concept of the 3rd patent (not yet issued) is the generation, in the temperature range of -40° - +125°C (automotive range), of an RF pulse of the required stability, with a rising and falling edge of the required precision. It also concerns, for industrial reasons, a method of generating these pulses repeatedly and using just a few components, avoiding dedicated ASIC architectures: this concept has an extensive range of industrial applications, and reduces manufacturing costs.
Using just two RF Transistors, the Micro.sp® hardware can generate particular RF pulses with a rising edge of less than 100 ns, at the common frequencies used for SRD systems (315, 434, 868 and 915 MHz).
In the RF power stage, time constants are appropriately overcome allowing a correct pulse shape and the required rising and falling edge, so that the pulse-to-pulse time is repeatable with the required precision, to reduce bandwidth and mean current, while maintaining a peak power of up to 40 mW (at 3VDC). The accuracy and precision of the rising and falling edges is pivotal for the “Rx interface” to definitively detect their exact dislocation and timing.
A complete data message includes a preamble, the ID code; this is followed by the pressure, temperature, and battery voltage levels, and finally the “control bits”; the data message is composed of 34 pulses, and has a total length of 10 ms. Because of the low pulse “Duty-Cycle”, the mean current drawn by the battery during the transmission of the 10 ms message is less than 200 µA. This makes it possible to use a small Lithium cell (just 12.5 mm in diameter, like the standard CR1225 used in the PM17). The whole device, including antennas, fits inside a standard ISO-TS-compliant tyre valve stem (either “Snap-in” or “Clamp-in”), rather than attaching it to the back of the standard plastic box, as in traditional TPMS applications.
This innovative approach to intelligent tyre valve stems permits the use of the compliant tyre valve stem accepted by car manufacturers, leading to huge savings and freeing funds for the Tier’s supply chain processes.
Direct TPMS systems utilizing traditional standard ASK or FSK modulation have several limitations, the most important being the high energy consumption required by these systems to transmit with acceptable efficiency. In battery-powered devices, this means that either batteries need to be frequently replaced, or energy consumption must be lowered, thus decreasing system performance.
Micro.sp® technology, patented by Ste Industries, is based on short RF pulses transmitted using Pulse Position Modulation (PPM), which guarantees high RF peak power, required for radio link reliability, but very low average power (Power Density); the current draw is consequently low (in the order of 200 µA) during the transmission period. This allows the use of very small primary batteries and also the possibility of using autonomous energy sources (Energy Scavengers).
Benefits of Micro.sp® technology are: