Sensata Technologies

Sensata Designs Reliable Tire Pressure Monitoring System With NI AWR Software

Company Profile

Sensata Technologies, formerly known as Texas Instruments Sensors and Controls, is a leading supplier of controls and sensor systems. Sensata’s industry-leading product lines include: automotive sensors, motor protectors, circuit breakers, HVAC pressure sensors, thermostats, and switches.

The Design Challenge

As safety demands escalate for the global automobile market, automobile manufacturers are looking to keep pace using more intelligent monitoring systems to enhance driver safety. Emphasis is being put on increasing the reliability and sophistication of these systems, including tire pressure monitoring systems (TPMS), which have been proven to be effective in reducing single car accidents caused by insufficient tire pressure. Working to fulfill this need, Sensata has developed a TPMS system that leverages RF technology. 

The system comprises four sensors, one fitted on each wheel of a car, which provide the ability to measure pressure and relay it to a receiver. This receiver communicates with the car dashboard electronics to warn the driver of any decrease in tire pressure or sound an alarm in case of a puncture or rapid decrease in tire pressure. Because the TPMS sensors are fully contained units that include power, optimizing the efficiency, cost, weight, and size of the antenna and RF section are crucial design requirements. Additionally, because these systems must be able to survive and operate reliably in extreme temperature, weather, and physical shock/vibration environments, they must be very robust.

Design optimization is a common issue in RF circuit analysis and relies heavily on network/impedance matching. In many applications there are restrictions not only on RF parameters, but also on size, shielding, and more. With the large number of device and system constraints, as well as the parasitics involved, it can be very time consuming to find the most optimum RF design. 

The Solution

Sensata chose NI AWR Design Environment, specifically Microwave Office circuit design software, inclusive of AXIEM 3D planar electromagnetic (EM) simulator and Analyst™ 3D finite element method (FEM) simulator, to simplify and enhance the demanding design process for its RF TPMS system.

Subcircuit design: 
In this particular application, Sensata engineers used standard surface mount device (SMD) components on their printed circuit board (PCB). AXIEM simulator was used to generate a subcircuit that took into account all parasitic elements that could be on the PCB. This provided easily accessible voltage, current, and current density measurements on particular ports and tracks of the design.

Statistical analysis: 
Since Sensata used standard SMD components, the designers were able to more freely perform heavy statistical analysis (Monte-Carlo) within NI AWR Design Environment, specifically a detailed analysis of the load-pull results at the RF output. The sophisticated capabilities of the tools enabled the designers to tune the circuit for a 1 dB output power spread and 1.4 A step current consumption. This was much better than the previous 2 dB output power spread and 2 steps current consumption. 

Additional optimization was done with Microwave Office software using the optimizer, where component values could be swept continuously for a more ideal configuration. Since there are many optimization methods in Microwave Office, a script was written in Visual Basic to run all methods in a loop and search for improvements. Loop after loop, the most appropriate method for circuit optimization surfaced from the algorithms and finally led to the best design based on the required constraints. 

Environment disturbance: 
The use of the Analyst simulator made it simpler to understand the effect the rim shape of the wheel had on the performance of the valve stem, which is used to inflate the tire. This enabled the designers to use the valve stem as an antenna. A plot using Analyst showed null points where the energy was not transmitting. 

Why NI AWR Design Environment

NI AWR software accessibility and ease of use significantly decreased the time and cost of the TPMS circuit analysis and optimization techniques. NI AWR Design Environment with Microwave Office, AXIEM, and Analyst was pivotal in the analysis of circuit parasitics, tuning for optimization, and analysis of the effects of environmental disturbance, enabling Sensata to produce an overall more robust TPMS.