Force Measurement

Force measurement in manufacturing, specially in machining, is very important. This is because:

Measurement methods

There are many force measurement systems, such as strain gages, inductive and capacitive systems. Here is the piezoelectric method given as an example.

The piezoelectric force measurement is based on the piezoelectric effect. The piezoelectric effect was discovered by Pierre and Jacques Curie in 1880. That is, certain crystals have the property to exhibit electrical charges under mechanical loading, as shown in the following figure. A very high input impedance amplifiers can amplify their signals, which are proportional mechanical loading.

Piezoelectric measuring devices are widely used today in the laboratory, on the production floor and as original equipment. They are used in accurate measurement and recording of dynamic changes in mechanical variables such as pressure, force and acceleration.

Piezoelectric measuring systems are active electrical systems. That is, the crystals produce an electrical output only when they experience a change in load. For this reason, they cannot perform true static measurements. However, it can offer excellent quasistatic measuring capability.

Piezoelectric transducers consist essentially of thin slabs or plates cut in a precise orientation to the crystal axes, which is sensitive to either compressive or shear loads.  The finely lapped quartz elements are assembled either singly or in stacks and usually preloaded with a spring sleeve. The quartz package generates a charge signal (measured in picoCoulombs) which is directly proportional to the sustained force. Each transducer type uses a quartz configuration which is optimized and ultimately calibrated for its particular application (force, pressure, acceleration or strain).

In piezoelectric force transducers the sensing element is the same as the transduction element which produces the electrical output signal from an acting force. The resultant high rigidity of piezoelectric force transducers greatly reduces the disturbance (or geometric changes) caused by the measurement and provides an inherently high natural frequency and associated rise time. This permits the measurement of extremely fast events (such as shock waves in solids, impact printer and punch press forces) that otherwise might not be possible.

Direct Force Measurement

As shown in the following figure, the transducer is mounted in line with the force path and the entire force is measured. High accuracy is obtainable and virtually independent of the point of the force application. Transducer size and measuring range depend upon the magnitude of the force to be measured.

Direct Force Measurement

A multi-component force transducer consists of a stack of quartz discs or plates and electrodes installed into a steel housing. Each quartz disc or plate has been cut in a definite crystal axis, and the orientation of the sensitive axes coincides with the axes of the force components to be measured (see Fig. below). Shear (or transverse) forces must be transferred to the transducer surface via friction. Preloading the load washer establishes this frictional contact and allows the load washer to measure specified shear loads. Figure a shows the operating principle of a three component force transducer. Force F acts upon the transducer and is transmitted to each of the discs with the same magnitude and direction. Each disc produces a charge proportional to the force component acting on that sensitive axis. Contrary to other measuring systems, it is not necessary to construct mechanical devices for separating the components.

In some applications, depending on measuring range and force footprint, it may be possible to use only one multi-component element to solve a particular problem. When larger sizes and/or ranges are necessary, a dynamometer or force platform can be constructed with more than one multi-component element.

a b
Three-Component Force Transducer

Another option is to build the multi-component elements into a customized design. In the simplest case, a multi-component dynamometer or platform consists of 4 three-component transducers placed in a rectangular arrangement between a base plate and a cover and preloaded to a specified range (see Fig. below). Since the outputs are in the form of an electrical charge, they can be tied together inside the platform or dynamometer housing before being routed to the connector(s) and ultimately on to the charge amplifier(s). For x, y, and z only measurements, the individual transducer outputs are tied together for a three-channel system.

Multi-component Dynamometer

Indirect Force Measurement

When a force transducer cannot be installed directly into the force path, an indirect (or shunt) force transducer can be used. Only a portion of the total applied force will be measured by the transducer (see Fig. below). Although transducer size and range are dependent upon the magnitude of the force to be measured, they are usually less restricted than direct force transducers. Since the measured output is only a fraction of the total force, on site, comparison calibration is often required.

Indirect Force Measurement

The piezoelectric method can be used to measure torque as well.

Several shear-sensitive quartz plates are arranged in a circle, with their sensitive axis aligned tangentially. When they are mounted under preload, these sensors can be used to measure torque.

Sensors with quartz sensing elements are used in the laboratory for measuring force and torque. These sensors are calibrated in the factory and are therefore immediately ready to be used. They are connected by means of a special, highly insulated cable to a charge amplifier, which converts the electric charge yielded by the sensor into a proportional charge. The sensitivity of the sensor and the required measuring range can be set on the charge amplifier.  The output signal from this calibrated measuring chain can be displayed and further processed as required.