Wireless connections have not been widely adopted for automotive command and control applications, for some very good reasons. The relatively short distances and fixed nature of the communicating sub-systems make wireless links unnecessary for many applications. Also, the reliability required for safety-critical functions excludes wireless systems as an option for these tasks.
The introduction of Control Area Network buses to reduce the number of wires in a vehicle has eliminated the need for wireless connections to save weight. However, in some areas, such as Remote Keyless Entry (RKE) or Tyre Pressure Sensing (TPS), a wireless connection can offer convenience.
Since they were introduced several years ago, keyless entry systems have improved considerably. Early systems had a serial number of just a few bits that was transmitted from the key to the controller in the vehicle. The controller would simply verify that the serial number was correct and unlock the door.
These systems were very insecure as the signal could easily be intercepted. This led to a very elegant way for thieves to gain access to a car without damaging it or looking suspicious. The thief would either just transmit all potential combinations of the serial number or capture the code when the owner unlocked the door and retransmit it at a later stage.
Today's RKE systems solve these problems by first making the serial number or code so long that it would take several decades to scan through all the potential numbers. To prevent capture and transmit attacks, RKE systems today change the code every time a button on the remote is pressed. This is done with a method known as code hopping, which is implemented by incrementing and encrypting a counter value before the code is transmitted. The receiver decrypts the code and ensures the counter did increment from a previously stored value before unlocking the car. These high-security systems are easy to implement today with the Microchip Keeloq family of code hopping products.
In the future, access control for vehicles will use Passive Keyless Entry (PKE) systems, which are activated when the owner approaches the vehicle with the key in his pocket. The vehicle will verify that the key is close to the door and unlock it when the owner pulls the door handle. This requires a bi-directional wireless communication system that sends a challenge from the vehicle to the key and the key responding back to the vehicle by encrypting the challenge, as shown in figure 1.
Fig 1: Passive keyless entry system
Driving availability
A few higher-end vehicles already have tyre pressure sensing systems, but the recent reports of accidents attributed to incorrect tyre pressure are driving manufacturers to make tyre pressure sensing available on all vehicles. A basic system consists of a sensor that measures the temperature and pressure and periodically transmits this data to the receiver unit in the vehicle through an RF link. The receiver is linked to a warning display inside the vehicle to warn the driver when one of the tyres is over or under inflated.
One issue with uni-directional sensors is how to determine which tyre is transmitting. This can be solved in several ways, such as identifying the tyre based on the serial number, using directional receivers or detecting the wheel rotation speed when the vehicle is turning and then calculating which wheel is transmitting. None of these methods are optimal. The serial number method runs into problems when a tyre needs to be replaced or when the tyre positions are rotated, which then requires a manual initialisation process to assign the serial number to a position. The directional receivers and rotation speed methods are complex and can be expensive.
Power consumption is another challenge in TPS systems. The uni-directional sensors have to wake up periodically and transmit the pressure, even if the vehicle is not being used, because it does not know when the vehicle is running. To solve this a longer-life battery or an additional accelerometer is required in each wheel to detect when the wheel is turning. Neither of these are optimal solutions, because the battery adds weight or the accelerometer makes the system more expensive.
While it is possible to design a uni-directional system that will last the required 10 years on a battery, the ideal solution is to have a bi-directional wireless link to each wheel as shown in figure 2. This will solve the identification issue because the vehicle can poll the individual wheels and request the pressure measurement. It also helps with the power consumption because it will only wake-up the sensor when the vehicle is in use.
Fig 2: Tyre pressure measurement system with bi-directional communication.
Using a magnetic field for communication
The challenge for both PKE and TPS systems is to have a bi-directional wireless link that meets the specified power consumption, range, cost and physical size constraints. Communication from the key or tyre to the car can be done with the low power RF transmitter circuits already widely implemented in existing RKE and TPS systems.
However, converting the transmitter into a transceiver will not be viable because the transceiver will have to be switched on all the time, draining the battery very quickly. What is required is a very low power receiver for the communications to the key or tyre.
This can be done using the same technology found in various identification systems such as those commonly found in clothes shop tags (also known as transponders) to detect when an item is stolen. These systems use a resonant inductor (antenna coil) and capacitor circuit to generate a magnetic field at 125kHz or 13.56MHz and are comparable to a very low efficiency transformer.
Communication is done by switching the field on and off to create the digital signal. This allows the receiver in the key or tyre to be implemented with just an inductor and capacitor.
In the clothes shop tag, the field needs to be strong enough to power the tag itself, but in this case, both the wheel and the key require batteries to power the sensor circuitry or if the key is used as a normal RKE transmitter.
While the magnetic communication to the key solves the power consumption and size constraints, it has limitations in the range, data rate and power required on the vehicle side to achieve a reliable wireless link. For both PKE and TPS a range of between 1 to 2 metres is required, which requires a sensitive input detection circuit that uses very little power.
Another limitation is that the magnetic field is directional, which makes the physical orientation of the receiver important. This is not a problem in TPS, because the orientation of the wheel on the axle does not change significantly.
In the case of PKE however, the key can be orientated in many ways in somebody's pocket or wallet. To solve this, three inductors/capacitors can be positioned orthogonally, with three sensitive input circuits to provide three-dimensional reception.
Microchip's range of Keeloq products, a standard in traditional RKE, also provides PKE solutions with up to three sensitive inputs integrated so that the designer just has to add the inductors and capacitors.
Low-power RF connections
As previously stated, the communication from the key or tyre to the vehicle uses existing RF circuit designs for the 315 and 433MHz bands. The low-power RF designs today are mostly based on Surface Acoustic Wave (SAW) resonators, which require very few components to implement a very cost-effective Amplitude Shift Key (ASK) RF transmitter.
With increasingly stricter regulations on RF emissions, especially in Europe and Japan, these designs are moving towards Phase Lock Loop (PLL)-based designs using a crystal for better frequency stability. This allows the use of a narrow-band receiver to provide better range when compared to the SAW-based designs. Some designs are also moving to 868 or 915 MHz, due to saturation and interference in the lower frequency bands.
The PLL-based designs are also available integrated with a microcontroller in a single package. The rfPIC devices are an example of a microcontroller with the RF circuitry integrated to fit these wireless control applications, where physical size is a critical design constraint.
The integrated solutions require even fewer components than the SAW-based circuits and permit Frequency Shift Key (FSK) designs, which are more noise immune and can provide another 3dB of gain, depending on local regulations.
Safety and security
While the TPS system is implemented as a safety feature, it acts merely as a warning to indicate when the tyre has lost pressure, and the driver still has to react to that warning. However, it is clear from recent events that a failure in this system can have large potential impact on the companies that develop these systems.
The electronics in the tyre pressure sensor will have to withstand high temperature and acceleration over an extended period of time. The amount of metal in the wheel and rotation makes the placement of antennas important for reliable wireless links.
One aspect of TPS systems that is often overlooked is security. Code capture and re-transmit attacks are possible with insecure TPS systems. A car thief can pull up next to a target vehicle, capture a pressure measurement transmission, and then transmit a false low-pressure signal. This will create the ideal opportunity for the thief to hijack the car when the driver pulls over to change the wheel.
The bi-directional communication system will solve part of this, because the receiver will know when to expect a transmission from a specific wheel and the range of the magnetic communication will not be sufficient for the thief to detect the command reliably. Further security enhancements include using an encryption scheme for tyre pressure sensing as well as checking the tyre pressure at random times and also in random order.
Combining keyless entry and tyre pressure sensing
With so many similarities between the PKE and TPS systems, it is clear that they can share many components. At a system level, they can share the RF receiver to receive transmissions from a key and the tyre pressure measurements. The antenna coil used to create the magnetic field for activating the pressure sensors can also be used to activate the PKE key, although the range achieved with magnetic communication could be a limiting factor. The combined system is shown in figure 3.
Fig 3: Combined PKE and IPS system
At the component level, both systems can share the same antenna coils on the vehicle side as well as on the wheel or key. The same microcontroller can be used to interface to the pressure sensor and key.
This microcontroller will require low standby current and 2V operation for low power consumption. It will also require a reliable onboard EEPROM to store the security information for the key and calibration values for the pressure sensor interface. Future integration of the transponder input circuitry and the RF transmitter would provide a clean solution with minimal external components.
Conclusion
While wireless connectivity for vehicles is required to connect to the Internet, phone use and other personal uses, it is not immediately evident where wireless can be useful in other automotive systems. However, TPS and PKE are good examples of useful and practical wireless applications in the vehicle of the near future. The similarities of the systems provide the designer with an excellent opportunity to combine the systems to provide a reliable and cost effective solution.
Kit simplifies LIN-based design
Microchip has produced a PICDEM LIN development kit which eases the development of PICmicro microcontroller-based systems using the LIN bus protocol.
The LIN (Local Interconnect Network) protocol was created as the standard for vehicle networks. The protocol enhances communication among vehicle subsystems, lowers system cost and improves reliability. Typical automotive applications include wiper controls, door locks, and sunroof controls.
The kit includes a set of boards, three PICmicro microcontrollers and example programs to demonstrate the LIN protocol in a simple distribution network.
The demonstration boards accept 18-, 28- and 40-pin DIP devices and feature a prototyping area, an on-board +12V regulator and a jumper to disconnect the on-board RC oscillator.
A control panel interface for the LIN bus master module, LIN bus slave modules, seat control module and an instrumentation panel module is included.
Additional features include an RF stage for remote keyless entry (RKE) function, RS-232 interface capability, in-circuit serial programming technology for each individual Flash module and a LIN-Bus master featuring an MCP2510 CAN-Bus interface.
LIN protocol supports bi-directional communication on a single wire, while using inexpensive MCUs driven by RC oscillators, to avoid the cost of crystals or ceramic resonators. The protocol includes an autobaud step on every message. Transfer rates of up to 20Kbaud are supported along with a low power sleep mode, where the bus is shut down to prevent draining the battery, but the bus can be powered up by any node on the bus.
The bus itself is a cross between I2C and RS232. It is pulled high via a resistor and each node pulls it low, via an open collector driver like I2C. However instead of having a clock line, each byte is marked via start and stop bits and the individual bits are asynchronously timed like RS232.
Additional applications for the LIN bus are assembly units such as doors, steering wheel, seats, climate regulation, lighting, rain sensor, or alternator.
The cost sensitive nature of LIN technology enables the introduction of mechatronic elements such as smart sensors, actuators, or illumination. They can be easily connected to the car network and become accessible to all types of diagnostics and services.
For more information on LIN bus see: www.lin-subbus.org/
Published by Embedded Systems (Europe) June 2002