What types of force sensors are there?

    Newton's first law of inertia states that "every body will remain at rest or in uniform motion in a straight line unless forced to do so." In other words, "if no force is exerted on a body, its acceleration (a) will be equal to zero."

    To understand force sensors, let us first understand force. As mentioned earlier, when a force is applied to a free object, it will cause the object to accelerate in the direction of the force. According to Newton's research, the acceleration is directly proportional to the applied force "F" and inversely proportional to a property of the object called mass "m".

    Then a is acceleration, F=am; F is force, which is a vector, and m is mass, which is a scalar value.

    The unit of force is the Newton, which refers to "the force that would accelerate a 1 kg mass to 1 m/s2".

    Newton's third law of motion describes the relationship between two objects. It states: "For every action, there is an equal and opposite reaction; or, the forces which two bodies act upon each other are equal and opposite."

    In engineering, density of a medium refers to the amount of substance per unit volume. Measurement of force is very important in the field of civil and mechanical engineering for various measurements. Whenever pressure is measured; it requires force measurement. Force is usually measured when dealing with solids. Whereas pressure is measured when dealing with fluids such as liquids and gases. In other words, force is considered when the action is applied to a point, while pressure is measured when the force is distributed over a large area.

    Force sensors fall into two main categories: quantitative and qualitative.

    Quantitative sensors actually measure force and represent its value as an electrical signal. Examples include gravimeters and load cells.

    Qualitative sensors are threshold devices. Their function is that when the magnitude of the force exceeds a predetermined threshold level, an output signal will be available. This is subject to strong forces. Computer keyboards are examples of such force sensors. This type of qualitative force sensor is used in motion and position detectors. Qualitative pressure sensors are composed of a pressure sensitive pad and a piezoelectric cable.

    The following method is used in a force sensor to sense force.

    1.) By balancing the unknown force with the gravity of a standard mass.

    2.) By measuring the acceleration exerted on the hand mass by a known force.

    3.) Balance the relationship between each other through electromagnetic force.

    4.) By converting force into fluid pressure and measuring that pressure.

    5.) By measuring the strain produced in the elastic member by the unknown force.

    Methods 3, 4, and 5 are most commonly used with modern sensors.

    In most sensor designs, force is not converted directly into an electrical signal. Some intermediate steps are taken. For example, a force sensor can be developed by combining a force transducer and a position sensor.

    The former can be designed using a simple helical spring whose compression displacement x is defined by the spring constant k and the force F as x = k * F

    Another type of sensor constructed using a spring and pressure sensor is shown in the figure. The pressure sensor is combined with a fluid-filled bellows that is subjected to a force. The fluid-filled bellows acts as a force transducer for the pressure by distributing the local force at its input across the sensing diaphragm of the pressure sensor.