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Material Type: Lab; Professor: Duva; Class: INSTRUMENTATION & MEASUREMENT; Subject: Mechanical; University: Wentworth Institute of Technology; Term: Unknown 1989;
Typology: Lab Reports
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Measuring speed is one of the most common applications of using electronic sensors to record mechanical events. Non-contact optical, magnetic, and capacitive sensors are often utilized because of their versatility, high reliability and low cost. These sensors do not measure speed directly, but rather act as simple counters to record the number of revolutions of a cyclic process, or the number of discrete items on a moving conveyor or similar process. Once the count has been obtained, a comparison to a known time interval can be used with a scaling factor to provide standardized units of speed or number of revolutions per unit time. In its most basic form, the number of counts per unit of time provided by the sensor and acquisition device result in a measured frequency for the application. The two major components to be used in this lab in addition to the NI-ELVIS system are a small permanent magnet, direct-current (DC) motor and a packaged photo interrupter. The photo interrupter consists of an infrared light emitting diode (IR LED) and a phototransistor detector. The emitter and detector are separated by a 3mm gap, and objects passing between this gap provide an output that can be used for counting purposes. As seen in a previous lab, if the output signal form the phototransistor creates a periodic signal, the FFT algorithm can be used to determine the fundamental frequency. Based on the equation, T f
the period can be calculated if the frequency is known. Alternatively, if the period is measured, the frequency can be calculated. Power to the DC motor will be provided from the +12V variable power supply terminal of the ELVIS workstation. To reduce programming complexity, the variable power supply will be used in the MANUAL mode which allows the user to adjust a ¾ turn potentiometer on the front of the ELVIS to vary the output from 0 to 12V potential. Be sure the voltage is set at “0” before hooking up the motor leads. Information on the Panasonic CNZ-1021 photo interrupter can be found on the digi-key.com website, and the motor (P/N 206949) can be found on the robotstore.com site.
The goal of this lab is to build a photo interrupter circuit that can be used to measure pulses from a disc that is attached to a motor shaft. The output of the measured pulses will need to be converted to revolutions per minute (RPM) with the appropriate mathematical conversions. Characterization of the motor will be based on a comparison of the voltage supplied to the motor