Sensors arrive ready for military duty

  • 18-Feb-2011 04:00 EST
Zed T233.jpg

A Sherborne Sensors T233 gravity-referenced servo inclinometer, an extremely sensitive transducer that converts angle of tilt into an electronic signal, is able to determine horizontal and vertical inclination with high precision.

The core sensor technology powering inertial inclinometers to measure angle relative to gravity has remained relatively unchanged for several years and has been field proven in a wide range of defense applications. Yet the need to consistently deliver measurement precision and repeatability, under the harshest operating conditions and often in space restricted applications, continues to drive the need for further customization.

According to Frost & Sullivan, the sensors market in Europe earned revenues of $12.5 billion in 2009 and is estimated to reach $19 billion by 2016. Meanwhile, in the U.S., demand for sensors, transducers, and associated equipment is forecast to increase 5.2% annually to hit $12.3 billion in 2014, according to research by Publications. This rise will be spurred by economic recovery from a weak base in 2009, says the company, with the aerospace/military market seeing moderate growth and benefiting from technological innovations and increased shipments of sensor-containing equipment.

At the same time, there has been renewed focus on revisiting tried and trusted sensor solutions to solve new technological challenges. One recent example is the Helicopter Alert and Threat Termination–Acoustic (HAALT-A) project in the U.S., where commercial sensors were reconfigured onto a helicopter airframe to acoustically detect incoming fire and localize the shooter(s). Since an established sensor design was used, a fully developed and detailed deployment package was delivered within seven months of the contract award.

Survivability and mission assurance

A servo inclinometer is an extremely sensitive transducer that converts angle of tilt into an electronic signal and is able to determine horizontal and vertical inclination with high precision. A torque balance flexure servo inclinometer essentially consists of a pendulum and unbalanced mass that aligns itself with the Earth’s gravitational field, the deflection of the pendulum being proportional to the sine of the angle of tilt.

Typically, the pendulus mass is mounted on a flexure suspended coil that rotates within a magnetic field provided by a permanent magnet not dissimilar to that employed in a dc motor. The pendulus mass’s position is detected by an electronic proximity sensor, and the resulting error signal is used to feed current back into the moving coil, restoring the pendulus mass to its original position. The restoring current is itself the analogue signal of the sine of the angle of tilt.

For applications demanding precision and repeatability several orders of magnitude greater than conventional open-loop inclinometers, a closed-loop torque balance servo system is preferred and delivers resolutions to 0.1 arc second (0.00003°). Closed-loop, torque balanced servo inclinometers are extremely robust and can offer high precision after withstanding severe levels of mechanical shock and vibration. Survivability via robust design is essential for sensors deployed within military platforms. And all the components on a platform must perform to specification within the defined environment if "mission assurance" is to be delivered.

Techniques employed by sensor-based devices to enhance survivability while maintaining accuracy include fluid damping and mechanical stops. The latter is invoked within an inclinometer (or accelerometer) to prevent damage when a device is exposed to acceleration beyond its calibrated range of operation.

Meanwhile, fluid damping employs a specialized viscous fluid that encapsulates the mechanical servo mechanism to provide both shock protection and to act as an additional filter attenuating unwanted background vibration that ensures highly accurate output signals. Fluid damped servo systems are available for applications demanding the most robust sensors and offer up to 1500 g of mechanical shock protection.

Electronic damping is added in the form of active low pass output filters to maximize the usable frequency range of the inclinometer if critically damped. Alternatively, the response can be over-damped to further attenuate the effects of environmental vibration and shock on the signal output.

In military environments, the sensor often has to operate reliably within a background of electro-magnetic interference due to the proximity of other electrical or electronic equipment. Additional filtering is therefore provided to ensure this electromagnetic compatibility.

Meeting military demands

Sensor devices are employed in defense applications where situational awareness is critical, i.e., to provide users with as much information as possible concerning their present circumstances to take the most appropriate action. Today, sensors play a crucial role in enabling SA, and there has been a huge increase in the volume of information being made available to military personnel as a result. Significant military applications for inclinometers are:

•    Fire control aiming systems, which demand robust sensors able to deliver precision measurements after exposure to severe levels of mechanical shock from the firing process. The reliability of the sensors' performance over many years is paramount in these harsh operating environments.

•    Naval ship communications-antenna alignment to satellites, which requires that precise total error bands in two axes are achieved over wide operational temperatures by thermally compensating the sensors' outputs at specific temperatures. Precision, robust, and environmentally protected sensors are required to withstand the rigors of these applications.

•    Military vehicle systems, including mobile communication and weapon guidance system leveling, in which it is imperative that such vehicles are accurately leveled when stationary to ensure their onboard systems are correctly referenced. The robustness of the inclinometers during transit over rough terrain is also essential to ensure the sensors’ precision is maintained.

The scope of applications for inclinometers within military platforms is extensive and all demand the highest levels of precision, repeatability, ruggedness, and reliability. Precision measurement capability describes the need for the sensor to be able to accurately translate the exact angular position of the platform to which it is affixed during or after being exposed to severe environmental conditions.

The demands placed upon the sensor in many defense applications will often call for a customized design based on proven technology that fully addresses the specific requirements of the application.

Marching forward

Sensors will continue to pervade all aspects of daily life but especially in the defense markets. This trend is driven mainly by the need for end users to have ever more precise and diverse information about the environments in which they are operating.

Minimization of size, form factor, and weight has become necessary given that the cost associated with carrying extra weight and/or increasing the size of the overall form factor can be significant in reducing the ability of the military platform to carry out its primary mission.

As a result, industry investment will continue to increase to further develop existing technologies so they can adapt to ever more stringent requirements. As in most cases, this pathway is more cost-effective than developing, testing, and fielding a new technological solution.

Fortunately, many existing sensor technologies have proven their capabilities in the field, are familiar to end users, and with incremental improvement offer core capabilities that can meet operational demands for many years, if not decades, to come.

Robin Butler, Sherborne Sensors, wrote this article for SAE Magazines.

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