Draft:Tire pressure sensors

Tire pressure sensors monitoring systems (TPMS) are advanced safety features in vehicles that alert the driver when one or more tires are significantly under-inflated. TPMS can be categorized into two types: direct TPMS and indirect TPMS. Each operates differently but serves the same critical function of monitoring tire pressure levels. The main difference between these two systems is that direct tire pressure monitoring systems use tire pressure sensors.

Tire pressure sensor in tire

Direct TPMS

Direct tire pressure monitoring systems measure the air pressure in each tire directly. They use pressure monitoring sensors within each tire to monitor specific pressure levels. These sensors can be part of the valve stem or attached to a metal band around the wheel within the tire. Each sensor is equipped with a battery, a pressure sensor, a microchip, and a wireless transmitter.

The sensor measures the tire's pressure and temperature and sends this data wirelessly to the vehicle's onboard computer system. This information is then analyzed, and if the tire pressure falls below the manufacturer's recommended level, the system alerts the driver through a visual and/or audible warning on the dashboard. This direct measurement approach allows for accurate, real-time monitoring of each tire's pressure.

Different brand tire pressure sensors

Components of Direct TPMS

Pressure Sensor

The pressure sensor within a tire pressure monitoring system (TPMS) is a type of transducer that converts the physical pressure inside a tire into an electrical signal. Most use piezoelectric or strain gauge technology. Piezoelectric sensors rely on piezoelectric materials (materials that generate electricity when mechanically stressed) to measure changes in pressure. Strain gauge sensors measure the deformation of an elastic material to determine pressure. This deformation alters the electrical resistance of the material, which can be measured and converted into a pressure reading.

Temperature Sensor

Temperature sensors in TPMS typically use thermistors, which are resistors whose resistance changes significantly with temperature. By measuring this resistance, the sensor can determine the temperature inside the tire. Temperature data is essential because tire pressure can change with temperature fluctuations, following the ideal gas law. This ensures that the system can accurately interpret pressure readings and compensate for thermal effects.

Transmitter

The transmitter in a TPMS sensor is responsible for wirelessly sending the collected data (pressure, temperature, and sometimes battery status) to the vehicle's onboard computer or receiver. This is typically done using radio frequency (RF) communication. The transmitter activates at regular intervals or when a significant change in tire pressure is detected to conserve battery life. The system uses either a fixed or rolling code to enhance security and prevent interference or malicious data manipulation.

Battery

Tire pressure sensor batteries

The battery in a direct TPMS sensor powers the pressure sensor, temperature sensor, microchip, and transmitter. These batteries are usually lithium-ion and are designed to last for several years, but they are not rechargeable. Battery life can vary based on sensor design, transmission frequency, and environmental conditions. When the battery depletes, the entire TPMS sensor often needs to be replaced, as the battery is typically sealed within the sensor unit for durability and safety.

Microchip

The microchip (or microcontroller) in a TPMS sensor acts as the brain of the unit. It processes the electrical signals from the pressure and temperature sensors, converts these signals into readable data, and controls the operation of the transmitter. The microchip is programmed with software that includes algorithms for measuring pressure and temperature, battery management routines, and communication protocols for transmitting data securely and efficiently to the vehicle's onboard computer.

Operation and Integration

When the vehicle is in operation, the TPMS sensors continuously monitor the pressure and temperature inside each tire. The microchip processes this data and, if a significant deviation from the preset thresholds is detected, sends an alert via the transmitter to the vehicle's onboard computer system. This alert then triggers a warning indicator on the dashboard to inform the driver of the issue.

It's important to note that while this explanation covers the general operation of direct TPMS sensors, there can be variations in the specific technologies and materials used, as well as in the design and integration of these components, depending on the manufacturer and the vehicle model.

Maintenance and Limitations

Battery Life: In direct TPMS, sensor batteries have a limited lifespan and need replacement when depleted. They usually last anywhere from 5-10 years or up to 100,000 miles.

Accuracy: Direct TPMS provides more precise measurements of tire pressure compared to indirect TPMS which does not use tire pressure sensors.