How to Select Tilt Sensors
Image Credit: Rieker, Inc. | Comus International | Fredericks Company.
Tilt sensors are devices that produce an electrical output that varies with angular movement. They are sometimes referred to as inclinometers, but inclinometers produce both a signal and readout; tilt sensors just produce a signal. As their name suggests, tilt sensors are used to measure tilt or slope within a limited range of motion. They generally function within one to two axis of a place, whereas full motion would use three or more axis and additional sensors.
Segway Transporters use tilt sensors to adapt to user movement. Image Credit: Tradenote.net
Tilt Sensors Construction
The way a tilt sensor works depends upon its type. They are generally force balance sensors, solid-state MEMS device, or fluid-filled sensor. Each type is described below.
The GlobalSpec SpecSearch database allows industrial buyers to search for tilt sensors by type of senor, type of switch, specifications, electrical outputs, display types and applications.
Types of Tilt Sensors
There are five tilt sensor types. This product classification includes many technologies and device types to measure slope, tilt, inclination, elevation and slope.
- Force balance sensors are gravity-referenced sensors that are often used in tiltmeters and inclinometers. The sensor consists of a flexure-supported torque balance system in which a pendulous mass is suspended between two position detectors. When the mass tries to move in the direction of tilt (due to the force of gravity), the position of the mass is detected by the position sensors and induces a current change.
o Advantages of force balance sensors include their superior performance, high accuracy, insensitivity to temperature change, and a broad measurement range.
o The disadvantage of this type of device is its high cost.
Force balance tilt sensor. Image Credit: Geoffrey Bainbridge
- Solid state micro-electromechanical systems (MEMS) are miniature sensors with movable proof mass plates that are attached through mechanical suspension system to a reference frame. Springs suspend the MEMS structure above the substrate such that the body of the sensor (also known as the proof mass) can move in the X and Y axes. Acceleration causes deflection of the proof mass from its center position. Around the four sides of the square proof mass are sets of radial fingers. These fingers are positioned between plates that are fixed to the substrate. Each finger and pair of fixed plates make up a differential capacitor, and the deflection of the proof mass is determined by measuring the differential capacitance.
- Advantages of MEMS sensors are that they feature integral signal conditioning and are easy to install.
- The disadvantage of this type of device is that the thermal coefficients are extremely high and compensation is normally required to obtain acceptable accuracy in most applications
MEMs Motion Sensor. Image Credit: Sensorland.com
- Fluid-filled sensors can either be electrolytic or capacitive
- Electrolytic tilt sensors are capable of producing extremely accurate pitch and roll measurements in a variety of applications. These devices lend themselves to both narrow and wide angular range measurements and easily maintain their small size and high accuracy. They function by using a glass or ceramic cavity filled with an electrically conductive fluid. The fluid conducts between a common, positive and negative electrode. When the sensor is level, both the electrodes are evenly submerged within the fluid producing a balanced signal output. As the sensor is rotated an imbalance is created between the electrodes. This imbalance of one of the electrodes is proportional to the angle of rotation.
o Capacitive tilt sensors are designed to take noncontact measurements of tilt and inclination and can operate both as sensors and switches. Capacitive sensing is independent of the base material and relies on variation of capacitance when the geometry of the capacitor is changing. The device consists of a central proof mass, comb drive capacitors and suspension beams. The suspension is flexible in one plane but stiff in all others. When tilt occurs, the central mass moves towards one of the combs and capacitance increases on one side and decreases on the other. The difference between the values provides a parameter related to tilt.
- The advantage of capacitive tilt sensors is their low cost to performance ratio.
- The disadvantage is their limited response time.
Design Tip: All fluid filled type tilt sensors are limited to a total sensing range of less than +/-90 degrees, as the electrodes will become either totally retracted or submerged when approaching that angle. Once this occurs, the sensor is saturated, and no variation in the output will be observed. This limitation can be overcome by incorporating a second tilt sensor offset 90 degrees (from the main axis) to the first. This design successfully provides a full 360 degree sensing range, but requires a sophisticated multiplexing routine within the electronics to discern the correct angular position.
- Accelerometers are instruments for measuring, displaying, and analyzing acceleration and vibration. They can be used as tilt sensors but the resolution is limited in the tilt direction and they have high power consumption.
Types of Tilt Switches
In general there are two tilt switch types. For more information about tilt switches, please visit the Tilt Switches Selection Guide on GlobalSpec.
- Mercury switches use a drop of mercury in the tube. A change in the orientation of the sensor causes the bead to roll in the tube, touching both the contacts and closing the switch.
- Ball-in-cage switches use a small metal ball held in a housing unit with two or more contacts. When the housing is tilted the ball shorts the two electrical contacts, turning off the switch.
Buyers should consider these product specifications when selecting tilt sensors.
- Tilt angle range is the range of desired linear output measured in degrees.
- Number of axis determine direction of tilt
- Bandwidth is the frequency used to measure changes in acceleration.
- Accuracy and frequency range should be considered together, as they indicate the range of frequencies over which tilt switches and sensors meet their accuracy specifications. Accuracy is degraded at lower and lower frequencies unless the device is capable of DC response; and at higher frequencies near resonance and beyond, where its output response rolls off. Frequencies are usually the 3-dB roll off frequencies.
GlobalSpec provides information about tilt sensors with different electrical outputs.
- Analog current represents tilt sensed as current (e.g. 4-20 mA).
- Digital transistor-transistor logic (TTL) uses transistors to preform both the logic gating function and the amplifying function.
- Analog frequency or pulse is a signal representing the sensed tilt.
- Analog voltage represents tilt sensed as voltage (e.g. 0-10 VDC).
- Serial interfaces transmit one bit of data at a time. Serial communication tends to be faster than parallel communication.
- Parallel interfaces transmit multiple bits of data at a time using different channels.
- Switched or alarm signals are designed to alert the user when a tilt sensor has been turned on or off indicating that the orientation of the sensor has changed.
Display types influence how the user interacts with the device.
- Analog meters are basic devices with markings to determine orientation
- Digital numerical displays use LCDs to display orientation angle, voltage or current.
Digital display for tilt sensor. Image Credit: Rieker
- Video displays record and display tilt sensor readings.
Tilt sensor are used in many different applications. These include:
- Aircraft flight controls
- Automobile security systems
- Video cameras
- Construction equipment
- Automobile air bags
- Studying human movement
- Video game controllers
Dunnicliff, John, and Gordon E. Green. Geotechnical Instrumentation for Monitoring Field Performance. New York [etc.: Wiley, 1993. Print.
Fraden, Jacob. Handbook of Modern Sensors: Physics, Designs, and Applications. New York, NY [etc.: Springer, 2010. Print.