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|Federal Highway Administration > Publications > Research > Descriptive Script for Video: Calibrating the Falling Weight Deflectometer|
Publication Number: N/A
Descriptive Script for Video: Calibrating the Falling Weight Deflectometer
FHWA Pooled Fund Study TPF-5 (039) July 2009
Technical Report Documentation Page
[A trailer-mounted FWD pulls up to a testing location on a highway. A flagger with a stop-slow paddle is directing traffic.]
Narrator: Falling Weight Deflectometers are used by many states to help manage their pavements as part of rehabilitation analysis or network management. [Narrator is shown in an office setting] They are also used to help with forensic studies of distressed pavements. [Title on screen: David Orr, Senior Engineer, Cornell Local Roads Program] An FWD can also help evaluate soft sections of pavement, determine the transfer efficiency of concrete slabs, and facilitate the design of pavements from total reconstruction to the determination of overlay thickness. [FWD is shown testing a spot on the pavement]
Narrator: The speed of the FWD allows an agency to go down the pavement and test multiple points fairly quickly with limited amounts of time needed for traffic control; which is very expensive. [FWD operator is shown examining data from the FWD laptop computer] [A close up of the FWD weight being raised and dropped during testing]
Narrator: The FWD can help determine the expected response of the pavement surface, and to some extent, the response underneath the surface, due to the passage of trucks and other large vehicles. It is critical that the data from the FWD be as accurate and precise as possible. Errors in the data will lead to errors in the pavement analysis. [Close up of the load plate of the FWD]
Narrator: Without calibration, the bias error in an FWD could be as large as plus or minus 2 percent for each sensor. [FWD load plate is raised off the pavement] It is important to reduce these systematic errors in the deflection and load readings as much as possible. During FWD calibration we can reduce the bias errors to about 0.3 percent. [FWD begins to drive away from the camera] This greatly improves the quality of any pavement analysis.
[The title of the video "Calibrating the Falling Weight Deflectometer" appears on the screen and then the background showing the FWD driving away fades to black]
[The FWD operator is shown in an office setting using a phone to call the FWD Calibration Center] [The Calibration Center Operator is shown in office and answers the phone when it rings]
Calibration Center Operator: Hello. Pavement evaluation. This is Cal.
FWD Operator: Hi, I'm interested in getting my FWD calibrated.
Narrator: The first step in getting your FWD calibrated is to contact the closest calibration center to set up an appointment.
[Calibration Center Operator's office]
Calibration Center Operator: I can tell you what to do before you get here. [FWD Operator reviewing the FWD trailer using checklist provided by Calibration Center Operator. Operator is checking electrical connections] You will need to make sure your FWD is well maintained and in good operating condition. You might look at your owner's manual. It will set you straight on what you need to do for the maintenance on the unit. [Calibration Center Operator's office] I can also send you a checklist and some other items that you can go over before you get here. [FWD Operator reviewing the FWD trailer using checklist provided by Calibration Center Operator. FWD Operator is checking weight assembly and hydraulics]
Narrator: It is important to fill out and properly follow the checklist sent by the FWD calibration center operator. This checklist contains typical maintenance activities that should be performed before leaving for the calibration center. [Narrator is shown in an office setting] If these steps are done prior to arrival, the calibration process will go much smoother and faster.
[FWD Operator driving down road to Calibration Center, turning left into the driveway of the center, and pulling into the center through a roll up door.]
Narrator: If possible, drive to the center the night before the scheduled calibration. Once on site, get the FWD ready and warm it up. [Title on screen: PennDOT Calibration Center Harrisburg, PA] This is done typically outside, but sometimes it has to be done inside. Then drive into the calibration center facility and maneuver the FWD into position. [Transition FWD operator shown in rear view mirror backing FWD into position for calibration. Center operator is shown directing the FWD backwards. Finally, a close up of the FWD trailer in position near the ball joint anchor is shown.]
Narrator: First will be calibration of the FWD deflection sensors and then the load cell. After this is done, paper work will be completed to make sure the FWD has been set up properly before leaving the calibration center.
[Backup alarm sounding]
Narrator: One of the first steps after getting to the calibration center is removal of the deflection sensors from their holders. They have to be removed from the FWD and placed in stands used in the calibration of the deflection sensors. The sensors are usually removed by the FWD operators because they know their own machine and are usually more efficient at it than the calibration center operator. [FWD Operator removing deflection sensors of the trailer of the FWD]
[Calibration Center Operator calibrating the reference accelerometer. The calibration software is shown on the screen of the center's laptop.]
[Title on screen: Reference Calibration of Accelerometer]
Narrator: While the FWD operator is removing the deflection sensors, the center operator performs a daily calibration of the reference accelerometer used in deflection sensor calibration. [Close up of the flipping of the accelerometer box during reference calibration, then a close up view of the calibration software] This is done to improve the accuracy of the data from the accelerometer.
[The Center Operator setting the Deflection sensor stand into the ball joint behind the FWD]
[Title on screen: Reference Calibration of Deflection Sensors]
Narrator: Calibration of the deflection sensors starts with reference calibration versus the accelerometer. [Close up showing deflection sensors being placed into the stand] The goal here is to calibrate the sensors versus a reference device to ensure accuracy. This is done by placing the deflection sensors from the Falling Weight Deflectometer, along with the reference accelerometer, in a large stand. [Overview showing the FWD during sensor calibration. The FWD Operator is at the passenger's door of the tow vehicle, the Center Operator is sitting next to the center computer, and a helper is holding the calibration stand vertically. The FWD is performing a drop.]
Narrator: A series of drops are performed, where the reference accelerometer and the sensors from the FWD see the same deflections. After one trial is done, the sensors are rotated relative to the accelerometer and a second trial is completed. [Close up of the weights of the FWD being raised and dropped] A third and fourth trial may be needed in some cases. [The camera transitions to a computer monitor showing the calibration software collecting data] The results from this reference calibration are used to create a series of interim gain factors for the deflection sensors. [Back to overview shot]
[Title on screen: Relative Calibration of Accelerometer]
Narrator: After reference calibration, a relative calibration improves the precision of the deflection sensor calibration. [A series of close ups of the FWD weight being dropped at several different angles] To accomplish this, a large number of drops are done with the FWD to tighten up the data. These results are used to produce the final gain factors for the FWD.
[Title on screen: Reference Calibration of Load Cell]
[View showing placement of the reference load cell into position under the FWD]
Narrator: The first step of load cell calibration is to make sure that the reference load cell is in line with the FWD load cell. This has to be done manually. [Overview showing the FWD operator and Center Operator going to their respective locations for calibration after placing the reference load cell into position] Before load cell calibration, the center operator adjusts the settings on the front of the signal conditioner. [Close up showing Center Operator adjusting the settings on the signal conditioner as per instructions shown on laptop screen]
Narrator: As with deflection sensor reference calibration, a series of drops are completed to calibrate the FWD load cell versus the reference device. The reference load cell is accurate enough that only a reference calibration step is needed. [Music playing along with sounds from the FWD]
[A series of drops from the FWD are shown while the music plays. Several different angles are shown including an overview, closes up from behind and the side and a shot of the computer monitor with the calibration software in action]
FWD Operator: Here you go. [The FWD Operator walks toward the camera and the Center Operator. The FWD Operator hands the Center Operator a thumb drive]
Center Operator: Thanks Dan. [Center operator attaches the thumb drive to the laptop and reads in the data from the FWD]
FWD Operator: Load Cell 5
Center Operator: Ok, ready. [Close up of the computer screen showing a regression graph of the reference load cell versus the FWD load cell] These numbers look pretty good. Let's take a look at the graph. Ah. Look at the graph. It's beautiful.
Center Operator: Here is a copy of your calibration Dan. It has the final gain factors on it. We will need to input them into your computer. And sensor 3, you might want to keep an eye on during the course of the year. [Center operator reviewing the calibration results with the FWD operator. The Center operator is standing next to the tow vehicle and the FWD operator is sitting in the passenger seat with the FWD computer rotated toward the camera]
FWD Operator: OK. Do you want to read them off to me?
Center Operator: Sure. [The FWD operator turns to put the new gain factors into the FWD computer while the Center Operator begins to read them off]
Narrator: The final step is to take the results from the calibration, put them back into the FWD, and make sure they were put in properly. The center operator and the FWD operator typically go over this information as documentation is produced.
Narrator: One of the reasons we calibrate Falling Weight Deflectometers is to improve the quality of the actual data that we use in pavement analysis. [A paving operation shown from ahead of the paver] Systematic errors in FWD data will affect the design. [Same paving operation from just behind the paver with some handwork near the curb of the street]
Narrator: Using an overlay design as an example, under predicted deflections could lead to a thinner overlay that may not last as long as planned. Over predicted deflections may result in excess costs due to extra material being placed. [Narrator is shown in an office setting] These moneys could have been applied to other projects.
Narrator: A recent study by the National Cooperative Highway Research Program on Falling Weight Deflectometer usage by the State Departments of Transportation looked at the issues of calibration and the quality of the data collected. One of the studies they reviewed, from Indiana, showed that a calibrated FWD could save 24 percent in the area that had to be undersealed. This is a tremendous cost savings just due to having a calibrated FWD.
Narrator: Almost everybody can get their FWD to a
calibration center within an eight hour drive.
Narrator: Falling Weight Deflectometer calibration
centers are located in Pennsylvania, Minnesota, Colorado, Montana,
California, Texas and Florida.
[Music plays and credits start] [Screen fades to black except for credits]
[Calibrating the Falling Weight Deflectometer
[Thanks to Commonwealth of Pennsylvania Department
[Pooled Fund TPF-5(039) Technical Advisory
[FWD Calibration Center Contacts
California DOT/UC Davis
Texas DOT/Texas A&M Univ./Bryan
*Australian Road Research Board/Melbourne
*Foundation Mechanics/El Segundo, CA
*Grontmig|Carl Bro A/S /Kolding, Denmark
Main Roads Western Australia/Perth
* Calibration services offered at FWD Owners' site]
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Topics: research, infrastructure, pavements, LTPP, falling weight deflectometer, calibration, FWD calibration, analysis, data
Keywords: deflection measurement, falling weight deflectometer, backcalculation, calibration, FWD calibration, analysis, data