The reliability of the measurement system is no better than that of the input cable, whose primary function is to transmit electrical signals from the accelerometers to the data acquisition system.
Ideally, the electrical characteristics and length of the cable should have no effect on signal quality. The cable and connectors should be physically durable to ensure reliable operation in the same vibration or shock environment the accelerometer is operating in. Cables should also be able to withstand about any imaginable combination of environmental conditions including temperature extremes, humidity, dust, oil, radiation, EMI and RFI, salt spray, and vacuum. But, just as one type of accelerometer cannot be expected to meet all these environmental conditions, neither can one type of cable or connector.
Cable and connector unreliability is a major source of concern for the measurement engineer who attempts to adapt vibration measurement instruments for use in tough environments such as laboratory, field, underwater, shipboard, and ESS combined vibration/thermal cycle product test applications. Depending on the type of instrumentation used, the measurement engineer’s choice of cable may be limited to special “low-noise” cable required in “charge mode” systems. In the low impedance “voltage mode” system s/he can select from a wide variety of standard cables best suited for his or her specific application.
Cables in charge mode systems
The typical charge mode system includes a piezoelectric (PE) accelerometer that generates a high-impedance electrostatic charge output, which is expressed in terms of pC/g (picocoulomb per unit of gravity). The PE accelerometer is coupled to the charge amplifier by means of special low-noise cable (see explanation below). The main function of the charge amplifier is to convert the accelerometer’s high impedance charge output signal (usually 109 to 1014 ohms) to a usable low-impedance voltage signal (<100 ohms) compatible with standard recording instruments.
Charge mode systems require the use of a special low-noise input cable between the accelerometer and charge amplifier. This is because ordinary cables, when flexed or bent, generate an electrostatic charge output that may be of greater amplitude than the signal being generated by the accelerometer. Since the charge amplifier cannot tell whether the charge-generated signal is from the accelerometer or the cable, special low-noise treated cable must be used. “Triboelectric Noise,” as cable noise is commonly referred to, is generated by frictional effects within the cable due to motion between the dielectric and shield.
What is low-noise cable? It is specially made cable with a conductive lubricant (usually graphite) applied between the dielectric and braid (shield). The lubricant minimizes friction and reduces the charge-generated noise when the cable is flexed or bent. Low-noise cable usually has a solid single conductor, PTFE dielectric for high insulation resistance, and a tightly wrapped outer jacket. Low-noise cables depend on the conductive graphite lubricant and tight construction to minimize movement between the layers of materials and reduce the charge-generated noise.
In order to maintain low-noise cables, they must be treated with more care than ordinary cables. Care should be exercised to avoid kinking, coiling tightly, stretching, crimping, stepping on, running over, or any other action that might loosen the tightly wound cable construction.
Because low-noise cables operate in the high-impedance side of the charge mode system, they must be kept “surgically clean.” Cable and connector contamination lowers resistance, which allows the charge signal from the accelerometer to leak off to ground, causing loss of low-frequency response and excessive drift. As a result of charge systems’ sensitivity to contamination, they are generally not well suited for operation in tough factory, field, underwater, or other dirty environments unless a costly maintenance commitment is made to try to seal connections and keep everything clean.
Other concerns involve low-noise cable in long cable driving applications. One of the main advantages of the charge amplifier over earlier signal conditioners is that system sensitivity is independent of input cable length.
While sensitivity may be independent of cable length, system noise and resolution are not. This is due to the increased capacitive loading on the charge amplifier caused by longer cable lengths. In order to reduce the noise caused by the long input cable, charge amplifiers are commonly installed closer to the accelerometer and long output cable used. The added complexity of operation of the charge amplifier controls remotely (e.g., remote ground switch) led to the development of more specialized miniature line driving type designs.
Economics is another concern involving low-noise cable, which is quite costly. It can run anywhere from $1.25 to $5.00 per foot for special lengths. In many cases a 100-foot low-noise cable may cost more than a low-impedance voltage-mode accelerometer, which uses ordinary low-cost coaxial or ribbon wire cable. The high cost of low-noise cables has driven many users into making their own cables and to other types of sensors that do not require special-purpose cable.
What can be done to improve cable problems associated with charge mode systems?
Follow manufacturer guidelines relative to installation and maintenance of cables. Tie cables down to stress-relieve connectors. Use ruggedized low-noise cable. When operating in dirty or humid environments, seal cable connections with RTV and heat shrink tubing. If necessary, clean cables with approved solutions. Do not use cleaning solutions that may leave a film, creating a path of low resistance on insulating surfaces. (The Miller Stephenson Company of Danbury, Connecticut, makes a variety of aerosol cleaning solutions suitable for cleaning high-impedance circuits.) When long input cables are required, adapt to a hybrid voltage mode system by installing a miniature charge amplifier close to the sensor, and drive long, low-cost ordinary coaxial cable.
Most accelerometer manufacturers offer such a hybrid voltage mode system. Better signal/noise ratio is the added technical advantage of this type of circuit, compared to a charge amplifier located at the end of a long input cable.
Cables in voltage mode systems
In voltage mode system, the charge-to-voltage impedance conversion is accomplished within the accelerometer by means of a microelectronic, integrated circuit amplifier. All high-impedance circuitry is safely sealed inside the accelerometer. Power to operate the integrated circuit is supplied from a simple constant-current source that may be located in the readout instrument, or by a separate battery or line power source. Output from the integrated circuit piezoelectric (ICPo) accelerometer is a low-impedance voltage signal that operates as a two-wire circuit through any ordinary coaxial, twinax, ribbon wire, or twisted pair.
Since the output from most ICP accelerometers is a low-impedance voltage mode signal, the whole world of standard cables and connectors is opened up. The measurement engineer can now choose the type of cable and connector best suited for his or her application including high temperature, cryogenic, rugged industrial, MIL spec, and underwater types. Cables can be selected from any physical, electrical, or cost consideration to operate in almost any environmental condition.
Standard coaxial, ribbon wire, and twisted-pair type cables with compatible connectors can be readily made by the user or purchased from the manufacturer. Aside from exercising good assembly techniques, there are no special precautions required, no special conductive low-noise lubricants to remove, no costly noise checks, and no troublesome high-impedance connections to maintain. In other words, the ICP low impedance voltage mode system puts the measurement engineer back in charge of the cable/connector selection.
To facilitate the use of a wide variety of cables, low-impedance type accelerometers are often provided with solder pin connections. The connector adaptor converts the standard 10-32 micro connector to a two-pin-solder connection.
The two-pin adaptor has proven very successful in ruggedizing accelerometer connections. Potted solder terminal connections have ruggedized accelerometer cable connections involved with impact velocity measurements on building support “piles” in somewhat “demanding” environments. The one-piece design of the solder connector adaptor has also performed well in high-shock applications to 100,000g.
Measurement consultants – who 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.
A good 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. A green meter 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.
Cables and connectors are major items effecting the performance and cost of a vibration measurement system. Concerns involving cable type, length, connections, operating environment, maintenance, availability, and cost should be addressed before purchasing a system. Too often problems associated with ignoring cabling concerns surface after the instrumentation has been placed in service.
Some measurement systems require highly specialized low-noise cables; however, these systems can often be converted to low-impedance operation with the addition of miniature charge or voltage converters located near the accelerometer. Other systems using accelerometers with built-in electronics output a low-impedance voltage signal, which is compatible with standard lower-cost cables and connectors from which the measurement engineer can select the best technical cable solution for his or her specific application.