Electromechanical measurements with an oscilloscope

-April 16, 2014

Making measurements on electromechanical devices and systems often requires a variety of transducers or sensors to convert mechanical parameters such as force, acceleration, pressure, and rotational speed into electrical signals. Using an oscilloscope or data-acquisition systems, you can measure these parameters and calculate others.

Transducers come in a wide variety of form factors and interconnection types. Traditional transducers offer mature technology and high reliability while newer MEMS (micro-electro-mechanical sensors) offer smaller packaging and lower cost for mass market applications. The selection of a transducer depends on the specific application. Considerations include the environment (wet, dry, explosive…), range of measurement (maximum and minimum values of the measured parameter), loading (how the transducer affects the measurement, interconnection method, and cost among others. Table 1 lists transducers commonly used in Electro-mechanical measurements.

Table 1. Popular transducers for physical measurements

Connectors and power
Most transducers require power supplies, signal conditioning, and cabling. Transducer suppliers will have all the necessary hardware to connect the transducer to the oscilloscope or data-acquisition instrument. Cabling can be a bit bothersome because many transducers use connectors that aren't common in electronics. Take, for instance, a piezoelectric accelerometer. 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 is a diagram of a typical electromechanical measurement setup on a small air compressor. Measurements of the internal pressure and vibration acceleration are being made using a pressure transducer and an accelerometer, respectively.

Transducer mounting is another factor that must be considered. The pressure transducer is screwed into a port that allows access to the top of the internal cylinder. The accelerometer is glued onto the cylinder head. Again, the application will determine the mounting method(s). These can vary from transducers being screwed to the device, glued, or connected using magnetic mounts. Accelerometer mounts must be rigid while pressure transducer mounts should allow internal access without mechanical interference or leaks.

The transducers and mountings should be small and light enough to not affect the measurement. Transducer "loading" in the mechanical world is much like probe loading in electrical measurements. Both affect the accuracy of the measurement.

Figure 1. The setup for measuring internal pressure and vibration on a small air compressor. Each transducer has its own power supply.

Figure 2. The piezoelectric accelerometer and pressure transducer in this setup each have their own power supply (lower left). The pressure transducer is mounted though the cylinder head and reads cylinder pressure. The accelerometer is mounted on an adhesive mounting base that is glued to the outside of the cylinder head. The optical tachometer senses the passing of a reflective tape mounted on the motor cooling fan and outputs one pulse per motor revolution.

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