Transient recorder
Transient recorders are systems for data acquisition that can work with very high sampling rates and high memory depths. They are therefore very suitable for measurement recordings in the high-speed range, especially for signals that do not occur periodically ( transients ), e.g. B. in crash tests . The period to be recorded can be limited to a specific event by targeted triggering . Modern transient recorders are closely related in their functional principle to digital oscilloscopes and PC oscilloscopes , but historically they are to be regarded as a further development of the oscilloscope principle .
Working principle
A simple one-channel transient recorder consists of an input amplifier, a trigger unit, a time base , an analog-digital converter and a memory, designed as a shift register or RAM , whereby only sequential memory accesses are necessary. In contrast to oscilloscopes, transient recorders normally do not contain a graphic output of the measurement data. The signals present at the input are digitized in the cycle of the time base and stored one after the other in the memory. The memory is used cyclically, i.e. after reaching the last memory location, the system continues with the first memory location and the values there are overwritten. At the same time, the occurrence of the trigger event is checked. If this occurs, the acquisition is continued for a specified number of clock steps and then ended. The memory then contains values that occurred both before and after the trigger event, with the total number of measured values corresponding to the full memory length. The values are then read out by a microprocessor or PC and processed further.
To increase the acquisition speed, several transient recorder channels can be combined with an offset time base.
Differentiation from digital oscilloscope and data logger
In many applications today, transient recorders can essentially be replaced by digital oscilloscopes on an equal footing if they offer a reliable single event trigger. In contrast to oscilloscopes, however, modern transient recorders offer a larger number (several hundred) channels that can be recorded simultaneously and a superior memory depth in the range of gigasamples (billions of measuring points) instead of the megasamples (millions of points) that are common in oscilloscope applications. In this way it is possible, for example, to record a triggering primary event and a strongly delayed subsequent event at the same time with high temporal resolution. The high number of channels is particularly advantageous when a large number of sensors are to be monitored.
The distinction between transient recorders and data loggers is based on the time scales: while transient recorders record very fast signals (e.g. the course of force and acceleration during a crash test with a total recording time in the range of seconds), data loggers are used for long-term monitoring of slowly changing variables (e.g. B. temperature or air pressure , with typically a few readings per hour). These different parameter ranges are reflected in the technical implementation of the two device classes, so that despite their conceptual similarity they are generally not interchangeable.
Applications
Transient recorders are mostly used in the field of basic and applied research as well as on test and test benches in an industrial environment. Examples are materials research (load tests), ballistics , high-current and high-voltage test facilities , or crash and airbag tests in the automotive industry.
Historical development
Transient recorders were developed back in the 1970s, when fast flash ADCs were not yet available. Therefore, the procedure for fast signals described under functional principle could not be implemented. Instead, analog CCD buffers were used. In this way, digitization could then take place slowly. Due to the progress made in fast analog-to-digital converters, this technology is now meaningless. Early transient recorders also had an analog output based on a slowly clocked digital-to-analog converter . This allowed the stored signal to be displayed on an ordinary oscilloscope.