Measurement consultants – fiber SC to SC are often called upon to make measurements at remote locations under unpredictable conditions – value the capability of the solder pin adaptor to make up long cables on the spot. Broken cables can be quickly and simply repaired with the soldering iron. University customers appreciate the economic advantages involved with making and repairing their own low-cost ribbon wire accelerometer cables.
Incorporating built-in microelectric signal conditioning, sealing up the high-impedance circuit inside the accelerometer, and providing for standard cables and connectors have been important factors in moving piezo accelerometers from the clean R&D laboratory into continuous vibration monitoring applications on machinery in dirty factory environments. Machinery monitoring applications can involve unattended operation of hundreds of permanently mounted accelerometers, making high-reliability connections, and running long, low-cost cable through adverse factory environments to a centralized data acquisition location important. And all of this must be done economically.
In order to avoid the cable frustration scenario discussed at the start of this article, good cable management practices can also be applied to multi-channel measurement systems using low-impedance voltage mode accelerometers. One such cable management system involves PCB's "Structcel" modal array system, which was engineered for several-hundred channel structural motion measurements on autos, airplanes, space vehicles, satellites, and a variety of other structures.
The Structcel cable management system involved transistor-like plug-in accelerometer adhesive mounting pads with integral low-mass ribbon cable that is cut to length. Standard insulation displacement connectors (IDE's) are installed on the cable, which conveniently couples into a "patch panel." The long cable connection, running from the patch panel back to the signal conditioner, is made by means of a multi-conductor ribbon wire. Cables can be neatly taped to the structure and since they are cut to length during installation, they are neither too long nor too short. 300-channel modal tests have been successfully accomplished in as little as three days, all the way from sensor installation and end-to-end calibration through data acquisition.
LC fiber cable management system involves standard cables and connectors, from the test structure to the data acquisition. In this system long, low-cost, standard RG-58U or RG-62U coaxial cables with BNC connectors are hardwired from the data acquisition to a BNC patch panel located in the test cell. A short, ruggedized, coaxial adaptor cable with 10-32 to BNC jack connectors couples the accelerometers to the patch panel. A ruggedized cable is made from ordinary coaxial cable by the addition of steel braid covered with heat shrink tubing. Should the cable develop a problem, only the short length would need to be replaced.
Monitoring the continuity of cable connections in low-impedance voltage mode systems is very simple and inexpensive. There is a DC bias voltage associated with the transistor circuit used inside the accelerometer. This DC bias voltage, usually in the 3-5 or 10-12 volt range, exists on the input cable to the power/signal conditioner where it is decoupled from the output. A simple color-coded meter, standard in most ICP constant-current power units, checks cable integrity by continuously monitoring the DC bias voltage. Fiber Optic Patch Cables reading indicates normal circuit connection; red indicates a short circuit, and yellow an open circuit. Each channel is checked for good or bad connections and a specific fault (short or open) is indicated. Most manufacturers of ICP-type constant-current power units offer this cable check-out circuit, which may be accomplished with a meter, LCD display, or lights.