By Dan Hebert, PE, Senior Technical Editor
NEW REQUIREMENTS and regulations are constantly being foisted on you and your plant, and many of these requirements necessitate acquisition and recording of data. These data may be needed to satisfy regulatory edicts, comply with internal audit procedures, or meet customer demands. Unfortunately, existing control systems often can’t easily accept new inputs, and store data from same. Problems with adding inputs can range from the simple, such as running new wires from a remote monitoring point, to the more complex, such as revalidating part of a complex control system just to add one data-monitoring point.
Past solutions tended to be simple but expensive. A sensor had to be installed to measure the desired parameter, let’s say temperature, and the sensor had to be connected to a data acquisition point. This data acquisition point typically was a PLC input module or a PC-based data acquisition card. Then the temperature value needed to be recorded on some non-volatile media like a PLC flash drive or a PC hard drive. This configuration is quite expensive for simply measuring and recording a single data point or a small group of data points.
Compact data loggers offer a solution to this problem. Although these products have been around for years, recent technical advances combined with open standards have cut costs and eased implementation.
Compact data loggers are available to fit most any data acquisition application. Some units combine the multiple functions of sensing, converting sensed signals to digital form, and storing digital data. Other units just store digital data, typically prodigious amounts given their relatively small size. Still other units can be installed on a PDA, and directly connected to sensors.
Many of these compact data loggers are battery-powered, so no new wiring is required from a power source to the measurement point. NEMA and IP ratings vary, and can accommodate environments ranging from benign to hazardous.
A typical multi-function compact data logger is made by Onset Computer Corp.. Its battery-powered Hobo data loggers range in size from 2.8x1.3x2.3 in. to 6x3.7x2 in., and some models weigh less than an ounce. These data loggers contain a sensor, an A/D converter, and non-volatile data storage. Prices start at $55 and range upward as features are added. Different models of Onset data loggers are available to measure 18 parameters, including temperature, humidity, voltage, current, watts, pressure, carbon dioxide, light intensity, vibration, water level, and wind velocity.
Onset’s data loggers are connected to a PC for initial configuration, and reconnected later for downloading stored data to the PC. “Our data loggers can be deployed in most any situation or environment, and left unattended for up to a year to collect data,” says Frank Deshaies, Onset’s product marketing manager.
For applications with sensors and A/D converters, a compact data logger that just stores data can be a perfect fit. Si-Gate GmbH makes compact flash data loggers that typically are used to monitor existing sensors. Robert Newberry, Si-Gate’s managing director, says its product has some advantages over traditional, PC-based data loggers. “Our flash data logger is smaller, weighs less, and costs less than a PC-based system,” says Newberry. “Boot-up time is very fast, just a few milliseconds, and our compact flash media stores large amounts of data in a format compatible with nearly all PC-based applications”.
Acumen Instruments also makes compact flash data loggers. Its units are designed to accept an RS-232 serial data input, and its uses removable CompactFlash storage. “Our data logger captures ASCII or binary serial data, and is smaller, more rugged, uses less power, produces less heat, and requires less boot time than a PC. Many of our customers come to us because they understand the high total cost of ownership for custom-programmed PC-based loggers,” says Joel Hagen, Acumen’s vice president.
A third type of compact data logger is designed to work with a handheld portable PDA. One example is Datastick Systems’ vibration spectrum analyzer. This PDA-based vibration analyzer has a module that attaches to certain brand PDAs running the Palm OS. The module is connected to standard ICP-type industrial accelerometers. The module includes software that collects and stores vibration measurements. The module also displays measured vibration data in time waveforms and FFT spectra complete with alarm levels on the PDA display. A PC-based software program based on Microsoft Excel is included for use with vibration data exported from the PDA to a PC.
“The Datastick Spectrum software that runs on the PDA places all the vital waveform or FFT spectral information on one screen,” reports Michael Scandling, Datastick’s marketing vice president. “Data can be shown in terms of acceleration, velocity, or displacement.”
