The text of NHTSA's TPMS ruling is available online. Essentially, the ruling requires that TPMS gradually be phased into new U.S. light vehicles during the period between November 1, 2003, and October 31, 2006. During this phase-in, more data on functionality and reliability will be gathered. NHTSA will then publish the second part of the ruling on or before March 1, 2005, to finalize what the auto industry will be required to do starting November 1, 2006.
Direct vs. Indirect TPMS
During the phase-in period, NHTSA is allowing two compliance options: direct and indirect TPMS. A direct TPMS uses a pressure sensor inside each tire and a wireless transmitter to communicate the pressure from inside the tire to a central receiver module.
An indirect TPMS uses a vehicle's antilock braking system (ABS) to determine changes in tire pressure. In an ABS, wheel speed sensors determine whether a wheel has locked. If so, then the ABS implements the antilock mechanism. By upgrading its software, an ABS system with a wheel speed sensor at each wheel can be used to detect changes in wheel rotational speed that occur as a result of low tire pressure. When a tire's pressure decreases, the vehicle's weight causes that tire's diameter to decrease, and this causes the tire to rotate at a different rate than at full pressure. This change, after proper calculation, is used to trigger a warning to the driver.
Each type of TPMS has its advantages. Because direct systems can determine the actual instantaneous pressure inside each tire at any time, identifying a problem is easy. Indirect systems can be less costly because cars that already include a 4-channel ABS (one that has a wheel speed sensor at each tire) require only a software upgrade. Such ABSs were present in about 67% of vehicles in the year 2000, according to U.S. Department of Transportation Docket No. NHTSA 2000-8572, Federal Motor Vehicle Safety Standards; Tire Pressure Monitoring Systems; Controls and Displays. Current indirect systems, however, are not as accurate as direct systems, nor can they identify which tires are low. System calibration is also extremely complicated. In addition, this type of system simply does not work in several important circumstances, e.g., when two tires on the same axle are simultaneously low.
A hybrid TPMS, which has some of the advantages of each system, is a possibility. A hybrid system uses direct sensors in two tires located diagonally across from each other, along with a 4-channel indirect system. This arrangement not only costs less than a fully direct system but also enables sensing of multiple low tires—something that a fully indirect system cannot do. Such a hybrid system, however, still does not present real-time data of the actual pressure within all four tires. Only a direct system can do that.
Given the superior effectiveness of direct TPMS, NHTSA is likely (when it revisits its TPMS rule in 2005) to require manufacturers to replace all the indirect systems allowed during the phase-in period with direct sensing systems. Such a requirement would create a very large market for direct TPMS hardware. Another possibility is that NHTSA could mandate the phase-out of all indirect systems and introduction of either fully direct or hybrid systems. Either way, there will be a big market for direct TPMS.
Components of a Direct TPMS
Aware of the large market potential, many companies are in the process of unveiling direct TPMS solutions. Because the NHTSA mandate is currently only for light vehicles and excludes vehicles with a dual rear axle, passenger cars and light trucks are where most development is taking place.
The typical TPMS for such vehicles includes 4 or 5 tire modules (depending on whether the spare has a sensor) and one central receiver module. A bare-bones tire module consists of a pressure sensor, temperature sensor (remember the Ideal Gas Law? PV = nRT), control module (such as an ASIC or an MCU), transmitter and antenna, and battery. More exotic systems might include a low-frequency (LF) detector for initiating transmissions (which makes the module a transponder, not just a transmitter), an inertial switch or an accelerometer, and a batteryless power supply. Such features are not required to meet the NHTSA mandate, but they help a manufacturer distinguish its system from those of other manufacturers.
A simple receiver module consists of a central antenna, a receiver IC, and an interface to the rest of the vehicle. Popular interfaces include the Controller Area Network (CAN) interface commonly found in a vehicle's body controller. The data are processed by the body controller and used to warn the driver if pressure in a tire is low.
Many companies are seeking to distinguish themselves by providing more features in the receiver and/or tire module. Such features include automatic tire location (which can detect tire position after you rotate your tires, without requiring system recalibration) using LF signal initiation to each tire and distributed antennas. If the right transmission carrier frequency and communication protocols are implemented, the receiver system can be integrated with the remote keyless entry (RKE) system. The display to the driver can be a simple telltale or data display integrated into the dashboard, an enhanced rear-view mirror with an integrated display, or an independent dashboard-mounted screen. As long as the driver is warned that a tire or tires have fallen 25% below normal inflation pressure, any of these systems could meet the NHTSA requirements for a direct TPMS.
Implementing a Minimal NHTSA-Compliant TPMS
With such a large market in the very near future, companies really need to get up to speed now. Because the time before TPMS must be put into production is so brief, many companies are implementing a simple system that just meets the NHTSA requirements. The simple, low-component-count direct TPMS system described here could meet all requirements of the NHTSA rule while facilitating quick product development. This minimal system consists of one sensor/transmitter module in each of the four tires and one central receiver.
Inside the Tire. The tire module consists of both hardware and software components (see Figure 1).
Figure 1. This block diagram shows the interconnections between the MPXY8020A sensor and the MC68HC908RF2 MCU/transmitter IC. The sensor has 8 pins: its power supply, ground, and 6 digital I/O pins. The MCU controls the sensor, obtaining 8-bit values for temperature and pressure. It then processes the data and transmits the information over an RF link.
The sensor has a digital output for pressure and temperature and requires an external digital controller. The controller (Motorola part no. MC68HC908RF2) combines an MCU with an ultrahigh frequency (UHF) transmitter in its package. This Flash-programmable controller has a low-power stop mode—very useful for a battery-powered application like a TPMS.
With these two components plus a crystal and a battery, the entire module (aside from a few resistors and capacitors) is complete. If the antenna is carefully designed, sufficient radio frequency (RF) power to reliably receive the signal from inside the tire can be achieved, even with the antenna printed on the circuit board. An omnidirectional antenna (with equal power radiated in all directions) would be ideal. A 1/4 wavelength stub antenna, on the other hand, is not only too large but also increases the cost. The signal must reach the receiver equally well, whatever the tire's orientation during rotation and turning.
The rather simple software for a TPMS has three major tasks: measure, process data, and transmit. Because the tire module will be powered by a lithium coin cell battery (a typical coin cell has a capacity of 250–300 mAh), an extremely efficient algorithm is required to provide the 7–10 yr. lifetime adopted by many TPMS providers. The efficiency of the algorithm is related to the timing, and a system designer has to ask the following questions:
- Will the receiver display each tire's pressure, or will it simply indicate a low pressure warning? The tire modules require much less energy to check a low pressure threshold than to obtain a full 8-bit value.
- How often are pressure and temperature measured?
- Does the system always measure both pressure and temperature, or is one measured more often than the other?
- Is temperature compensation performed (i.e., does the low-pressure warning threshold change as a function of temperature)? At the tire or at the receiver? The more calculation the tire module has to perform, the shorter will be the battery lifetime.
- How often are data transmitted?
- How many bits of data are in each data frame? The transmitter consumes the battery energy faster than any other mechanism, so a shorter data frame transmitted less often is better.
- What happens when the pressure gets low? To ensure that these data get to the receiver in a noisy environment, a tire module could transmit a warning signal several times.
In addition to answering these questions, a smart algorithm does as much as possible to avoid data collisions—when more than one tire module sends its data at the same time. To avoid data loss due to collision, a module could transmit several data frames (rather than one), with a random time interval (longer than the time required to transmit one data frame) between each data frame. This design would greatly reduce data loss but would reduce the battery lifetime and increase the complexity of the receiver's software. A good compromise is transmitting only 3–4 data frames with 10–100 ms of pause between.
Aside from these standard concerns, most module suppliers won't want to deliver a minimally compliant solution in the future. Additions such as motion detection, LF initiation, automatic localization of all tires (including the spare), elimination of the battery, and other features can be used to make a supplier stand out from the crowd.
Outside the Tire. Like the TPMS components inside the tire, the external components consist of both hardware and software. The most cost-effective receiver module would consist of a UHF receiver, a central antenna, and an interface to the rest of the car, and would function as the receiver for both the TPMS and RKE systems. Optimally, such a module would require only a software upgrade to the existing RKE system in most vehicles.
Higher-end systems might include distributed receiver antennas at each wheel well. The tire modules could then transmit at a lower power, increasing their battery life. Some companies are also devising simple systems to automatically locate the problem tire by comparing the signal strength at each antenna.
The highest-end systems include an LF signal initiator in each wheel well along with an LF receiver on the tire module. Such a design allows the central body controller to send a signal to a single tire, thus asking for a transmission from that tire only and eliminating data collision issues. Automatic tire location is also efficiently managed in such systems. The system's usefulness could also be enhanced by using the LF initiator to send data to the tire module—anything from new low-pressure thresholds to instructions for completely reprogramming the MCU.
If the central receiver acts for both the TPMS and RKE systems, communication protocol compatibility is a priority. Most RKE systems use amplitude shift keying (ASK) to modulate their signal. Although ASK works well for a stationary transmitter such as a key fob, the data coming from a rotating tire are not as reliable. Most TPMS use frequency shift keying (FSK) to increase data reliability. For this reason, the best practice is to use a receiver that can receive and demodulate both ASK and FSK. Luckily, many receivers already on the market for RKE can do just that.
With respect to software, many automobiles with RKEs should require only a software upgrade at the body controller to enable them to accommodate a TPMS. The receiver should be reconfigured to alternate between ASK and FSK modulation so that it can receive signals from both the TPMS and RKE systems. One option is to always default to ASK so that existing RKE transmitters don't have to be modified. The TPMS modules transmit a wake-up tone to the receiver, which the receiver takes as a cue to reconfigure itself for FSK modulation. Once the TPMS data are received, the receiver goes back to ASK.
Although power consumption is not as critical for the receiver as for the transmitter, it is still important to program an efficient algorithm so that the car battery isn't drained during long periods of non-use. Some receivers periodically oscillate between a sleep mode and a receive mode, in which case the transmitter must send a signal when the receiver is guaranteed to be in its receive mode.
Because the time before the NHTSA ruling will take effect is so short and the market is so large, companies will want to consider a methodology for creating a simple, NHTSA-compliant TPMS. The main system component is a pressure sensor that can withstand the harsh environment inside a tire. Suppliers can easily upgrade such a system to distinguish their product from others. Many possible upgrades have been mentioned here, but the flexibility of an MCU-based system is practically limitless.