When a memory recorder replaces an oscilloscope

Memory recorders are often confused with oscilloscopes. The truth is that memory recorders are specfically designed for industrial situations and oscilloscopes are not.

A memory recorder comes into its own when, for example, there is a service problem with a PLC-based control system driving, among other things, a phase-fired SCR industrial process heater.

Without describing a set of circuits in unnecessary detail, it isn’t hard to imagine that the diagnostic methods will involve thermocouples and perhaps RTDs, limit switches (some of which are in power circuits) and measurement of floating voltages.

Measurement of millivolt signals sitting on top of large AC loop voltages, floating at high DC voltage or large voltage value signals (for example, the direct phase angle firing observation on SCRs), becomes a real problem for oscilloscopes but not so for a memory recorder.

Of course, there are input signal modules that can be bought to use with oscilloscopes but the measurement procedures get messy.

With a vibration problem there’s a possible very difficult problem with motor bearings showing early failure. There’s a need to measure harmonics directly on the motor windings - for a delta wired field, its phase-to-phase measurement (no ground reference) because there can be a presence of negative sequence harmonics, like the fifth.

Not only that, a vibration analyser is needed plus something else - like an FFT function to show a frequency spectrum.

A service problem has occured with a recloser on a remote SWER system. The CT/PT relay and three-phase recloser require monitoring - of course, the recloser operating at 19 kV needs to be taken offline but checking on the timing of the three pole sections is a job for a memory recorder, such as a Hioki MR8880-20. Connected to the PT, the fast response of the recorder is well suited to monitoring input waveforms, as well as the operation of the contacts.

A service problem has occured with a CNC - again there’s the complication of control circuitry and large numbers of switches, proximity switches operating at various voltage and current levels.

The situation can be handled by oscilloscopes but often in a very complicated way - and using more than just the oscilloscope - as indicated by the vibration analysis example.

In building maintenance, adjustment of elevator floor levels, operation of brake thrusters and the overall operation of control logic and operation of DALI lighting control as well as HVAC monitoring and building energy management systems are all well within the capacity of a fast memory recorder that facilitates the servicing.

In the case of the MR8880-20 HiCorder, the large number of input modules makes the instrument suitable for the tasks mentioned earlier. Not only does it have a fast sampling time (1 µs), large onboard storage (1 MW per channel) but there is a useful range of input modules including temperature, strain, charge-sensitive, FFT, frequency, current and CAN.

The latter is useful in vehicle and general CAN-bus systems. Charge-sensitive input modules can be used with piezoelectric transducers to measure acceleration and vibration forces.

The FFT modules equipped with antialiasing filters provide a large number of analytical applications including harmonics and interharmonics.

Apart from direct measurement of frequency, the frequency module can also function as an integrator with voltage to frequency converters.

The eight logic channels that are part of a memory recorder are invaluable in allowing, in addition to logic control monitoring, a flexible triggering range using AND/NAND an OR/NOR digital signal combinations.

Analog triggers include in-window, out-of-window, glitch triggering (crossing of a threshold with a pulse width shorter than the reference pulse width) and event triggers based on the occurrence of a number of preset events.

So it is clear a memory recorder is tailor made for industrial service situations whereas an oscilloscope is suited to bench-style servicing.

In a field situation with a large mix of analog and digital signals, floating signals, signals with large common voltage levels and mechanical parameters to be monitored, an oscilloscope, although theoretically up to the task, would in practice be exceedingly clumsy to use.