grade sensor placement rev aug 2009 grade sensor placement asphalt paving grade sensor placement

Grade Sensor Placement rev Aug 2009

Grade Sensor Placement

Asphalt PavingGrade Sensor Placement


Grade Sensor PlacementWhere should a Grade Sensor be placed on the screed?

Topcon Grade Sensors:

• Sonic Tracker

• Contacting Sensor

• Smoothtrac SAS

• Laser Receiver

• mmGPS Receiver

• Fore and Aft Placement in reference to the tow arm

• Front, middle, or rear?

• End gate or Tow arm?


Grade Sensor Placement Where should a Grade Sensor be placed on the screed?

• Rear of screed is desired grade position, relative to the grade reference

The grade sensor needs to measure and control the screed position

• Grade control is adjusted by the cylinder at the front of the screed tow arm

The grade sensor needs to measure the changes that have been made

Curb – Grade Reference

Desired Grade

Grade Adjustment



• The tow cylinder moves immediately as grade adjustments are made

• The screed will change grade as the screed moves forward


Desired Grade

Grade Adjustment




Desired Grade

Where should a Grade Sensor be placed on the screed?

• The tow cylinder moves immediately as grade adjustments are made

• The screed will change grade as the screed moves forward

Grade Adjustment




Desired Grade

?Grade Adjustment

Where should a Grade Sensor be placed on the screed?


The grade sensor needs to measure the changes that have been made




Desired Grade

Grade Adjustment

Grade Sensor placed at the back of the screed



Grade Sensor Placement Grade sensor positioned at the back of the screed

• No feedback indicating the tow point position has changed

• Tracker continues to apply grade corrections


Desired Grade

?

Grade Adjustment




• Grade sensor at the back of screed

Will produce long waves

A grade sensor can not be placed at the back of the screed

X




Grade Sensor placed at the tow cylinder

The grade sensor needs to have position feedback from the cylinder it is controlling

The grade sensor can keep the tow point exactly at the desired grade

Grade Adjustment




Grade Sensor placed at the tow cylinder

• Changes in the forces acting on the screed still exist!

• Manually adjusting the thickness adjustment screws changes screed height

Grade sensor continues to control tow cylinder keeping it on desired path

The grade sensor does not see changes in screed position

Increase in material height

Screed rises

Pivot point



• Grade sensor at tow cylinder

No correction for changes in screed height

A grade sensor can not be placed at the tow cylinder

X




•The grade sensor needs to measure and control the screed position

•The grade sensor needs to measure the changes that have been made


Screed position

Grade Adjustment


Grade Sensor Placement Rule of Thumb = Middle 1/3 of the tow arm

• Sensor has feedback of screed height

• Sensor has feedback of grade control changes

1/3 1/3


Grade Sensor Placement Sensor placement affects screed reaction

• Grade sensor maintains distance to the grade reference

• Sensor position becomes control pivot point = effective tow arm length

3’

9’ effective tow arm length

d Grade Reference


Grade Sensor Placement Grade sensor at rear 1/3 of screed - Step disturbance change in grade

• Sensor sees step disturbance, unit of 1

• Sensor drives the tow cylinder until the sensor raises 1 unit

• Tow cylinder raises 3 units = large change in screed angle of attack

3’

effective tow arm length

d


Grade Sensor Placement Step disturbance change in grade – Sensor raises unit of 1

• With increased angle of attack, screed begins to raise

• Grade Sensor lowers cylinder to maintain desired distance from reference

• After 1 tow arm length (3’), screed has risen 63% of the grade change

One tow arm length

d



•Grade Sensor continues to lower cylinder as the screed rises to maintain desired distance from reference

• After 2 tow arm lengths (6’), screed has risen 87% of the grade change

One tow arm length

d

One tow arm length



•Grade Sensor continues to lower cylinder as the screed rises to maintain desired distance from reference

• After 3 tow arm lengths, screed has risen 95% of the grade change

d

9’


Grade Sensor Placement Sensor placed farther back – Screed is more reactive

• Screed quickly and accurately follows grade reference

Jobs with quick changes in grade; city streets, parking lots, etc.

3’

9’ effective tow arm length


Grade Sensor Placement Grade sensor forward, 2/3 of screed - Step disturbance change in grade

• Sensor sees step disturbance, unit of 1

• Sensor drives the tow cylinder until the sensor raises 1 unit

• Tow cylinder raises 1.5 units = small change in screed angle of attack


d

6’


Grade Sensor Placement Grade sensor forward, 2/3 of screed - Step disturbance change in grade

• After 3 tow arm lengths, screed has risen 95% of the grade change

6’ 6’ 6’

18’




6’

Sensor placed farther forward – Screed is less reactive

• Screed is slower to respond to grade deviations

Jobs where smoothness is primary concern

Good job practice regarding Principles of Paving is more critical

9’


What is the best place to mount a grade sensor – Tow arm or end gate?• Two primary factors;

• Stability of sensor• Grade reference location



tow arm

end gate

tow arm

end gate

Tow arm or end gate?• In theory, the tow arm and the screed extensions are one rigid unit• As tow arm raises and lowers, extensions and end gate move together


tow arm

end gate


tow armtow armtow arm

end gate

Flex between tow arm and end gate can occur• Wear in crank and adjustment bushings• As screed extension is extended out, physical structure is not as strong



Tow Arm or End Gate?Two primary factors

• Stability of sensor – Tow arm is always best choice• Grade reference location – When the grade reference changes position relative to the paver, such as curbs with lane changes, the end gate is a more practical solution


mmGPS or Laser Sensor MountingTow arm is always best choice• Laser receiver or mmGPS – Grade reference location is not a problem• Stability of the sensor is primary concern



Asphalt PavingGrade Sensor Placement

grade sensor placement rev aug 2009 grade sensor placement asphalt paving grade sensor placement

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