The piezoelectric sensors should be tested prior to and after installation.
Because of the high costs of installation compared to the cost of the sensor, it is imperative that the sensors be checked out prior to putting them into the road to determine if any damage has been done to the sensor during shipping and handling.
The general steps for installing piezoelectric cables are as follows:
- Mark the layout of the sensor installation with aluminium slotted templates and fluorescent orange paint. Ensure sensors are emplaced exactly perpendicular to the flow of traffic and that all lines are straight.
- Verify that the passive cable length is enough to reach the cabinet.
- Wet cut the slot for the sensor. The slot must be 20 mm wide and 25 mm minimum deep. Cut the slot 200 mm longer than sensor length (including lead attachment).
- Cut home run slots for the cables that run from the sensors to the LCR meter.
- Power wash and sweep all slots. All slots must be very clean.
- Dry all slots with compressed air. All slots and the pavement 300 mm on either side must be completely dry.
- Place duct tape along the length of both sides of the sensor slot 3 mm away from the slot.
- Connect sensor up to LCR meter. Test capacitance and dissipation factor. Test the resistance on the standard setting. Capacitance and dissipation should be within ±20% of the enclosed data sheet. Resistance should be infinite.
- Place sensor on tape next to the slot and clean sensor with steel wool or emery pad. Wipe down with alcohol and clean lint-free cloth.
- Place installation brackets on sensor every 150 mm for the length of the sensor.
- Bend the end of the sensor downward at a 30 °angle. Bend the lead attachment end down at a 15 °angle and then 15 ° back up until level.
- Emplace sensor in the slot in the road. Starting at the lead attachment end, use the installation depth gauge to position the sensor so that it is 9 mm below the surface of the road by pressing the depth gauge against the top of the sensor.
- Visually inspect the length of the sensor to ensure it is at uniform depth along its length and it is level (not twisted, canted or bent).
- Run the passive wire the length of the home run slot, 75 mm from the lead attachment.
- Place backer rod under and over passive cable (inside the slot).
- Pour grout into slot using a small bead, allowing the installer to watch the grout flow under the sensor, eliminating air pockets. Start at the end and pour towards the lead attachment.
- Using a putty knife or trowel, lightly spread the grout smooth along the length of the slot. Resin should be slightly higher than tape as it will shrink while curing.
- Remove tape as soon as grout begins to set (2-5 min, depending on grout type and ambient temperature).
- Remove backer rod from the slot. Fill in home run cable and inductive loops with loop sealant.
- Once grout is cured, use an angle grinder or a belt sander to grind/sand the top of the grout flush with the surface.
- Wait allotted period to allow loop sealant and grout to fully cure (45-60 min) and then open the lane to traffic.
- Hook up oscilloscope to sensors and view wave forms as vehicles pass. Ensure signal is clear without noise.
Figure: Positioning and painting of sensor templates
Source: TDC Systems-2005
Figure: Diagram of BL Roadtrax sensor
Source: Central Weighing-2005
Testing after Installation
Once the sensor is installed and the grout has cured, retest the sensor in the same manner as before installation.
In addition, it is recommended that an oscilloscope be connected to the sensor and typical waveforms be collected for a truck and a car. These should then be printed and saved for permanent records.
The output of the sensor will depend on the type of the installation, sensor length, cable length and epoxies used for the installation.
Typical settings for the scope would be 200 mV/div for a voltage setting and 50 ms/div for a time setting.
The trigger should be set at about 50 mV for a positive going signal.
TDC Systems 2005, ‘BL piezo electric sensor installation’, TDC Systems, North Somerset, United Kingdom.