Analyze mechanical measurements with digitizers and software

August 31, 2015 //By Arthur Pini , Greg Tate and Oliver Rovini
Analyze mechanical measurements with digitizers and software
Arthur Pini , Greg Tate and Oliver Rovini explain how to analyze mechanical measurements with a combnation of digitizers and software.

Measurements on mechanical devices and systems using a digitizer often require a variety of transducers or sensors. Such sensors convert mechanical parameters such as force, acceleration, pressure, rotational speed, and their kindred into electrical signals. Digitizer selection must match the transducer characteristics. Additionally, In order to read mechanical data out in the correct units, the amplitude values must be rescaled to read those units. We present a primer on making such measurements with a modular digitizer. You can, though, apply the concepts presented here to most measurement applications.

Digitizer selection
The bandwidth required for most mechanical measurements is generally under 100 kHz. Thus, sampling rates of 200 kHz or greater will work. The digitizer's resolution should match the dynamic range of your sensors, with 16 bits being the most common today. The number of channels should reflect the number of individual measurements you need to make. Keep in mind that if your transducers use differential outputs you will need two channels for each transducer.

Transducers

Time-honored transducers are available in a wide variety of form factors and interconnection types. They offer mature technology and high reliability and are generally used for developmental measurements. Newer MEMS (MicroElectroMechanical sensors) are available in smaller packages at lower cost and are intended for mass market applications. Transducer selection depends on the specific application. Considerations include the dynamic range (maximum and minimum values of the measured parameter), bandwidth, environment (wet, dry, explosive, etc.), loading (how the transducer affects the measurement), interconnection method, and cost.

Most transducers require power supplies, signal conditioning, and cabling to connect to instrumentation. Transducer suppliers will have all the necessary hardware to connect the transducer to the digitizer or other measuring instrument. Cabling can be a bit bothersome as many transducers use connectors which are not common in the electronics world. Take, for instance, a piezoelectric accelerometer shown in Figure 1. The standard connector is the microdot coaxial connector that uses a 10-32 thread. Transducer manufacturers offer adaptors and cables to get you to the more familiar BNC connectors.

Figure 1. A typical piezoelectric accelerometer needs its own power supply.
Source: PCB Piezotronics

Let's look at a simple mechanical measurement to see how the transducers are employed with a digitizer.

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