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Our project's goal - break the existing record of 763 mph (1,228 km/h) by reaching 800 mph (1,287 km/h)!

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NAE™ Project: Data Acquisition System

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Introduction

Our data acquisition tasks serve several needs. The overall guiding principle for applying sensor technology is to enable the North American Eagle™ to achieve our target speed while maintaining a reasonable margin of safety. Other very important aspects include system reliability, system maintainability, and (unfortunately) minimizing expense. (Clicking on the image below will give you a larger view on a new page.)

dataimage7

We get data in many forms and from many sources. Some data is collected from sensors installed throughout the vehicle. Other data is obtained from onboard embedded computers that monitor system status. Additionally, the vehicle has three video cameras and a stereo audio monitoring system. (Clicking on the image below will give you a larger view on a new page.)

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Information derived from all this data is used to inform our driver of vehicle health and system status. By being constantly informed of how the vehicle is performing (or failing), Ed makes informed decisions about mission management and his driving technique. At extreme speeds, even Ed can not detect and react fast enough to "bad events". With safety in mind, the vehicle has a "Mission Abort" processor for shutting things down and stopping the vehicle in response to sudden unfavorable events or a combination of conditions.

dataimage8

Here's a sample of data from our recent El Mirage test session.

Sensors

Sensors play an important role in controlling the two canards installed just forward of the cockpit and the active suspension/damping hydraulics. Screaming along the fine line between flight and plowing a trench at 1,174 feet per second requires constant minute adjustments. Control reaction-times of less than two milliseconds is needed.

Another guiding principle: Keep it Simple!

Our hardware is off-the-shelf if possible. Signals are passed from sensor to amplifier via time-honored shielded-twisted-pair aircraft signal wire obtained from the Boeing Surplus Store.  Where possible, the signal amplifier is integrated into the sensor (ICP-type). Cabling runs are terminated with military-spec Bendix connectors. To preserve bandwidth in our telemetry system, raw data is written to a hard drive for post-run analysis. A Larson Davis "DSS" system handles data acquisition, signal conditioning, data recording, and signal computational tasks. Go/No-Go parameters are passed to Allen Bradley Programmable Logic Controllers (PLC's) for local vehicle systems control, PCM-encoded telemetry provides real-time data for transmission to the Base Control station.

Wireless Ethernet also allows access to each computer for programming tasks when the vehicle is parked.

Strain Gauges

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What is it?

The Strain Gauge is a device that when bonded to a structural member, reacts to bending by changing its ability to conduct current. We use strain gages mounted to the components within the vehicle we are most concerned about from a structural integrity standpoint. In order to measure strain with a bonded resistance strain gauge, it must be connected to an electric circuit that is capable of measuring the minute changes in resistance corresponding to strain. Strain gauge transducers usually employ four strain gage elements electrically connected to form a Wheatstone bridge circuit A Wheatstone bridge is a divided bridge circuit used for the measurement of static or dynamic electrical resistance. The Wheatstone circuit is well suited for temperature compensation.

How is it a good thing?

Strain gauges are highly linear in response, can be affixed to almost anything, and are very sensitive. Because of this linearity, and knowing the elasticity of the structure, we are able to extract accurate force, as well as, amplitude from the readings. This gives us useful data for design alterations, if required. Measurement of static and dynamic loads are possible without being concerned about signal decay -which is not true with a piezoelectric sensor.

What makes it bad?

Strain gauges are somewhat delicate and require a high degree of skill for installation. They do not perform well in wet environments. To minimize noise, signal amplification circuits must be as close as possible to the sensor.

Where are they on the North American Eagle™?

Canards, Suspension, Bulkheads

Accelerometers 

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What is it?

The accelerometer is used to sense vibration / acceleration. We use accelerometers fastened to structures like the vertical stabilizer to monitor vibration. Early detection of severe vibration will prevent permanent damage.

Piezoelectric accelerometers rely on the piezoelectric effect of quartz or ceramic crystals to generate an electrical output that is proportional to applied acceleration. The extremely small signal generated by the accelerometer is conditioned and amplified by transistor microelectronics. The accelerometers we use have the amplification circuit built into the accelerometer package itself. -This allows for longer distance between the sensors and the data acquisition equipment.

How is it a good thing?

Accelerometers are very rugged and are easily affixed to any surface. A wide range of frequencies may be measured.

What makes it bad?

Accelerometers tell nothing about the structure they are mounted to -other than the fact that the structure moved.

Where are they on the North American Eagle?

Suspension beams, Vertical Stabilizer, mid-Fuselage, Wheel frames

Thermocouples 

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What is it?

The thermocouple is one of the simplest of all sensors. It consists of two wires of dissimilar metals joined near the measurement point. The output is a small voltage measured between the two wires.

A thermocouple circuit has at least two junctions: the measurement junction and a reference junction. Typically, the reference junction is created where the two wires connect to the measuring device. This second junction it is really two junctions: one for each of the two wires, but because they are assumed to be at the same temperature (isothermal) they are considered as one (thermal) junction. It is the point where the metals change - from the thermocouple metals to what ever metals are used in the measuring device - typically copper. The output voltage is related to the temperature difference between the measurement and the reference junctions.

How is it a good thing?

They can be very rugged, are immune to shock and vibration, are useful over a wide temperature range, are simple to manufactured, require no excitation power, there is no self heating and they can be made very small.

What makes it bad?

Thermocouples produce a relative low output signal that is non-linear. These characteristics require a sensitive and stable measuring device that is able provide reference junction compensation and linearization. Also the low signal level demands that a higher level of care be taken when installing to minimize potential noise sources.

Where are they on the North American Eagle™?

Engine Inlet, Engine Exhaust, Wheel Bearings, Cockpit, Electronics Bay

Piezoelectric Sensors

 dataimage4

What is it?

The Piezoelectric Sensor is used much like a strain gage

in that it supplies a signal in response to bending. Like an accelerometer,

it is packaged with amplification circuitry and may be mounted to most any surface.

How is it a good thing?

Piezoelectric Sensors exhibit fast response, ruggedness, stiffness comparable to solid steel, wide frequency and amplitude range and the ability to measure quasi-static forces.

What makes it bad?

The quartz crystals of a piezoelectric force sensor generate an electrostatic charge only when force is applied to or removed from them. However, even though the electrical insulation resistance is quite large, the electrostatic charge will eventually leak to zero through the lowest resistance path. In effect, if you apply a static force to a piezoelectric force sensor, the electrostatic charge output initially generated will eventually leak back to zero. This means that although the amplitude of vibration IS mappable to forces, the data must be corrected using a time-constant for the static decay phenomenon.

Where are they on the North American Eagle™?

Difficult-to-reach locations within the vehicle structure such as aerodynamic surfaces and low-strain structures where a particularly sensitive sensor is required, such as various locations on the main keel beam.

Rate Gyroscope 

dataimage3

What is it?

Inertial gyros are sensors used to measure angular rates and x, y & z linear acceleration. Applications include vehicle instrumentation, robotics, and attitude reference systems.

Up to three angular or rotary axes, as well as 3 linear axes, can be measured using inertial gyros. A number of technology options for angular measurement are available. Inertial gyros with a gyro reference have an inertial mass used as a reference for rotational movements. More modern designs use a "ring laser" to optically sense these motions.

How is it a good thing?

The Rate Gyroscope provides its own stable reference and can be mounted to the object in motion. They are very accurate and the technology is quite mature. 

What makes it bad?

They are very expensive, and the electronics required for operation are highly specialized. 

Where are they on the North American Eagle™?

Only one place in the Electronics Bay. The Rate Gyroscope provides data for monitoring and controlling the vehicle's angle-of attack.

Thermostats (Fire Sensors)

What is it?

Thermostats are based on the differential expansion of two different metals (brass and iron, for example), a principle discovered in the 18th century. Differential expansion causes an assemblage of the metals to change shape when heated; this change of shape can then be used to actuate a mechanical linkage such as an electrical switch.

How is it a good thing?

Thermostats are very reliable, rugged, inexpensive, and can be made to actuate at a specific temperature. 

What makes it bad?

Thermostats are somewhat bulky. They only measure whether a specific pre-set temperature is reached.

Where are they on the North American Eagle™?

16 locations inside the Engine bay, and in the mid and forward wheel wells.

Wire break sensor

What is it?

The wire break sensor is a simple device for alerting the team to a structural failure. A very thin, fragile wire or foil trace is bonded to critical structural components so that if a crack develops across the path of the wire, the wire will break and the loss of conductivity will trigger an alarm. This is much like foil window-break sensors commonly used in burglary alarms.

How is it a good thing?

The wire break sensor is very simple and inexpensive.

What makes it bad?

Once a break occurs, it may be very difficult to locate the actual break point and the break can be intermittent in nature.

Where are they on the North American Eagle™?

Main Keel Beam (the bottom center structural frame of the body)

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