Tilt Switches Information
Tilt switches transfer a change-of-state to another device. These devices receive a signal from the tilt sensor for changes in motion or orientation and turn on or off. They do this by generating an artificial horizon and measuring angular tilt with respect to this horizon. Not all products open or close a switch. some, such as tilt switch alarms, trigger audible or visual responses to notify an operator that a system is out of alignment.
Tilt switches are made of nonconductive tubes that have two electrical contacts and a material which acts as a conductor between the two electrical contacts. Mercury was a once commonly used conductive material but it is now less popular due to its toxicity. Other materials that can activate a tilt switch include metal balls and electric current.
Mercury switches use a drop of mercury in the tube. The sensor is positioned with respect to gravity forces so that the mercury moves away from the contacts when the switch is open. A change in the orientation of the sensor causes the bead to roll in the tube, touching both the contacts and closing the switch.
- Advantages of mercury tilt switches are their low cost and ease of use.
- Disadvantages include the toxic nature of the material and that they can perform only on/off functions.
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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. Multiple contacts can be added to this device so that tilt direction can be determined.
- An advantage of ball-in-cage switches is that this type of switch can have multiple contacts so that the direction of the tilt can be determined, In addition, ball-in-cage products are less toxic than mercury switches.
Force balance sensors are gravity-referenced sensors often used in tiltmeters. The sensor consists of a 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.
Solid state sensors are micro-electromechanical systems (MEMs). They are miniature devices with movable proof mass plates that are attached through a mechanical suspension system to a reference frame.
Electrolytic tilt sensors are capable of producing extremely accurate pitch and roll measurements in a variety of applications. They function by using a glass or ceramic cavity filled with an electrically conductive fluid. As the sensor is rotated an imbalance is created between the electrodes. The imbalance of one of the electrodes is proportional to the angle of rotation.
Capacitive tilt sensors are designed to take noncontact measurements of tilt and inclination. Capacitors 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.
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.
Buyers should consider these product specifications when selecting tilt switches.
Tilt angle range is the range of desired linear output measured in degrees.
Number of axes determines the 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.
Tilt switches are available 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 and 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).
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.
Video displays record and display tilt sensor readings
Tilt switches 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.]
Guidelines for Instrumentation and Measurements for Monitoring Dam Performance. Reston, VA: American Society of Civil Engineers, 2000. Print.