What is a Tilt Sensor and How it is Used in Geo-technical Fields?
What is a Tilt Sensor
A tilt sensor is an instrument that is used for measuring the change in tilt and monitoring of inclination and vertical rotation in vertical structures. Tilt sensor produces an electrical signal which is proportional to the degree of tilt in multiple axes (Uniaxial & Biaxial).
Tilt sensors measure the tilting position with its original reference. This sensor provides valuable information about both the vertical and horizontal inclination tilt.
Main purpose to use a tilt sensor is to measure tilt changes in structures due to construction activities such as tunneling, excavation and de-watering that affect the ground that support the structure.
The tilts sensor works on the Micro-Electro-Mechanical Systems (MEMS) technology to provide a digital output.
Tilt sensor are of different type, shapes and size
How is a Tilt Sensor used in Geo-technical Fields?
The tilt sensors are intended to measure inclination from vertical for long term measurement. As movement occurs, the tilt sensor will move with its mounting, thus changing the inclination of the internal sensors. The tilt sensor measures tilt over a range of ± 15º. In the bi-axial model, a second MEMS sensor is mounted at 90º to the other sensor and measures tilt in the orthogonal direction, on the horizontal plane. Once mounted on a structure they are normally adjusted to read close to zero, their midpoint. These are designed to attach to the structure so that they can sense and measure the tilt of structure in uniaxial or bi-axial directions.
Encardio-rite tilt sensor is works on two types of technology (MEMS and Electrolytic)
1) MEMS technology that is composed of a suspended mass between a pair of capacitive plates. When tilt is applied to the sensor, the suspended mass creates a difference in electric potential which is measured as a change in capacitance. That signal is then amplified to produce a stable output signal in digital, 4–20mA.
2) Electrolytic tilt beam Sensor consists of an electrolytic tilt sensor attached to a rigid metal beam. The tilt sensor is a precision bubble-level that is sensed electrically as a resistance bridge. The bridge circuit outputs a voltage proportional to the tilt of the sensor.
Models of tilt sensor provided by Encardio-rite are as follows:
· Electrolytic Level type Beam sensor/tilt meter:
The Encardio-rite model EAN-31EL/EAN-31EL-B Electrolytic Level type tilt meter/beam sensor is designed for monitoring differential settlement by measuring change in tilt. It is designed to monitor rotation and deflection of structures such as buildings, retaining walls etc.
These are rugged and high-resolution tilt meters and housed in a compact weatherproof enclosure. These are generally used as a stand-alone unit for measuring tilt. It is either attached directly to the structure whose tilt is to be monitored using suitable mounting brackets or fixed to horizontal or vertical beams.
It is suitable for monitoring small change in inclination and vertical rotation of structure at a point. A beam which is available from 0.5m to 3.0m long, to be mounted on structure to detect inclination or rotation between two fixed points. Movement of the structure changes the tilt of the beam and the output of the sensor.
· MEMS type tilt and beam sensor:
1) EAN-41M/EAN-42M (SDI-12) Uniaxial Beam sensor:
The Encardio-rite model EAN-41M/EAN-42M (SDI-12) Uniaxial MEMS Beam sensor equipped with latest SDI-12 technology. It can be mounted both vertically and horizontally for monitoring of differential movement and tilting of structure. For monitoring deflection and deformation of retaining walls, sheet piling etc, the beams sensors are mounted in vertical strings.
The beam sensor can also be installed in long horizontal strings to measure differential settlement & angular distortion along railway tracks, tunnels, pipelines, embankments etc. the beam sensor is available from 0.5m to 3m beams. Beams can be linked in a daisy chain fashion to provide cumulative settlement data over long distances in tunnel and tracks.
2) EAN-90M/EAN-92M Biaxial Tilt Sensor:
The Encardio-rite model EAN-90M/92M tilt sensor is suitable for monitoring of inclination and vertical rotation in structures. Especially these tilt sensors are used to monitor an individual location. It is a high resolution tilt meter, is rugged in construction and has excellent temperature stability.
Model EAN-90M tilt sensor consists of basic sensor, mounted inside stainless steel housing. The tilt sensor (Uniaxial and Biaxial) is fixed on to a vertical or horizontal surface by means of an adjustable bracket and expandable anchor. Model EAN-90M has a voltage output, which can be read by model EDI-53UTM read out logger or any indicator that measures differential voltage output, and can also be directly connected to our automatic data acquisition system.
Model EAN-92M tilt sensor is equipped with SDI-12 digital interface such that all sensors can be connected through simple bus cable to EDAS-10 system and ESDL-30 data logger.
3) EAN-91M/EAN-93M Uniaxial Tilt Sensor:
The Encardio-rite model EAN-91M tilt sensor can be directly fixed on a wall/structure. Option is also available to fix the EAN-91M tilt sensor to a beam of length from 0.5m to 3m and used as a beam sensor for monitoring of differential movement and rotation in structures. It is also used for monitoring deflection and deformation of retaining walls.
The Model EAN-93M sensor (box type) is with a SDI-12 digital interface such that all sensors can be connected through a single bus cable to EDAS-10 system and ESDL-30 data logger.
Applications of Tilt Sensors
It has applications in the measurement of tilt in structure, including:
· To monitor rotation of retaining walls, structures, piers, and piles during adjacent excavation, tunneling
· Stabilization measures
· Monitoring structure for effects of excavation and tunneling
· Monitoring the behavior of structure under internal and external load
· Monitoring deformation of retaining wall
· Monitoring convergence and other movements inside tunnels
· Stability of structures in some landslides area
· To evaluate performance of bridges and struts under load
· To measure angular distortion along railway tracks and tunnels
These tilt sensors are used in various applications and are presented in a nutshell:
Tilt/beam sensors are commonly used for instrumenting the following types of structures:
Bridges and Piers:
Installation of tilt sensor in a bridge and pier will provides data to evaluate the amount of tilt occurs during construction. It can also be used for convergence measurement and on crown for crown settlement by changing the fitment orientation.
Pile and retaining wall
Installing a tilt sensor in pile & retaining walls is to measure the lateral deflection during the excavation.
Metro tracks and tunnels
Model EAN-42M 0.5m Beam sensor use to measure the settlement between the tracks. Model EAN-42M 3m Beam sensor use to measure the settlement along the tracks. Beam sensor with 3m long is connected in a series of over 15m to monitor the cumulative settlement of the track.
Model EAN-93M 0.5m Beam and EAN-42M 0.5m beam sensor were deployed every 20 meters to monitor the displacement of tunnel in both lateral and vertical directions.
Tunneling activities especially in congested areas can lead to notable settlement of the ground surface which can affect the adjacent buildings and other structures. To avoid this, tilt sensors are used to monitoring for settlement/upward movement during tunneling.
Tilt sensors provide a single point rotation data it leads us to monitor to rotation of adjacent buildings. Tilt sensor with desired beam length which can be connected together in a chain provide a settlement profile which can be used to control the construction in safer manner.
Landslide and slope stability
A monitoring of buildings and structure using tilt sensors provide us early warning comprise one of the most promising ways to reduce disasters induced by landslides, surface failure of slopes and slope instabilities.
What is the major factor that can affect tilt sensor readings?
Temperature and sunlight:
It is always recommended to install tilt sensor out of direct sunlight or else it should protected from sunlight by using protection cover/ wrap up with thermocoal to avoid seasonal temperature effects. After installation, initial tilt reading and temperature to be recorded for future data reduction like temperature correction if required.
It is good practice to see the actual measurement during early morning. This is because structure is at thermal equilibrium with ambient conditions and thermal stresses are less during that period. Ideal time to monitor daily would have been early morning between 12:00 AM to 4:00 AM.
(A): Tilt meter graphs showing variation in readings at different time of the day
(B): Tilt meter graphs showing stable readings at 2 a.m.