Machine Design

Where the smart rubber meets the road

Get ready. Tire-pressure-monitoring systems will soon be standard equipment in new vehicles.

Building a tiny strap-on sensor package able to withstand road abuse proved a challenge for Smartire engineers. Linear and nonlinear static and dynamic analyses run with MSC.visualNastran software from MSC.Software Corp., Santa Ana, Calif., predicted housing loads and stresses at wheel speeds to 250 mph over temperature extremes. Special attention was paid to high-stress snap-fit joints.
The analyses showed a ribbed housing made of a glass-reinforced nylon-based plastic gave the desired safety factor of two, cut wall thickness from 0.16 to 0.05 in., and lowered material specific gravity from 1.5 to 1.16, compared to a previous design.
Instrumented valve caps from Advantage PressurePro measure tire pressures to 150psi and transmit the information to a dash-mount monitor via an RFID link.

Underinflated tires caused accidents that resulted in 400,000 injuries in 2002, according to statistics compiled by the insurance industry. Underinflated tires were also to blame for the much-publicized Ford Explorer/Firestone rollover accidents, concluded the NHTSA, which prompted the organization to enact the Transportation Recall Enhancement Accountability and Documentation (Tread) Act of 2000. It mandates every new car and light truck by 2005 be equipped with either indirect or direct tire-pressure-monitoring systems (TPMSs).

Indirect systems have no provision for detecting actual tire pressure and instead measure tire rpm using sensors in existing antilock brake equipment. Direct systems, in contrast, put pressure sensors on or inside each wheel and wirelessly transmit inflation data to a dashboard-mounted monitor. NHTSA says direct TPMSs must trigger when tire pressure falls 25% below recommended cold psi levels. For indirect systems that figure is 30%.

One maker of direct systems, Smartire Systems Inc., Richmond, B.C., has two types of sensors that go inside tires. The first type combines (in a special valve stem) a pressure sensor, a temperature sensor, and an RFID transmitter. This setup is primarily for OEMs because it may require that the stem hole in the rim be specially angled and located to receive the unit.

Smartire's other sensor type straps to the smallest diameter of a drop-center rim. A centrifugal switch activates the unit when wheel forward speed reaches 5 mph. The sensors check tire pressure every 7 sec and report a low-pressure condition every 3 to 7 min when they detect a 3-psi pressure loss. Drivers are alerted to an immediate loss of pressure within the normal 7-sec scan time.

Another direct-type system from Advantage PressurePro LLC, Harrisonville, Mo., puts pressure sensors outside tires in special triple-sealed valve caps. Each sensored cap contains an RFID transmitter and battery. Once installed, an in-vehicle receiver queries each cap for an ID number. And installed caps work only with their original receiver, which discourages theft. Receivers are able to read up to 34 tire positions (caps) from 100 ft. Caps are factory sealed and batteries last about 3 years, says the company.

Make contact:

Advantage PressurePro, (816) 887-3505,
MSC.Software Corp., (714) 540-8900,
Smartire Systems Inc., (604) 276-9884,
TAGS: Materials
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