We're in the process of updating part of our steam turbine control system. While commissioning I've found a minor problem with noise on the 4-20mA signal from the Balluff BTL5 linear position transducer which we have just installed on the hydraulic servo valves. I've used these transducers elsewhere on the same site, and the feedback signal is deathly quiet.
The interconnect cables are multicores with individual screened pairs, with overall steel wire armour. Each transducer has one pair carrying the signal, and a second pair carrying the DC power. The screen is terminated at the 'quiet' ground in the equipment room housing the control system. The field end is floating. Power to the transducers is 24V DC and is a shared supply to all eight transducers. The noise problem does not improve when isolated supplies are used. The noise signal is also present on the power supply pair, but at a much reduced amplitude.
The interfering signal is what is really puzzling me: a brief burst of HF, or possibly RF, noise of a couple of microseconds duration, repeating every 4ms. On the 'scope the envelope of the waveform has the vague appearance of HF commutation noise from a large rectifier, but I can't account for the 250Hz. I haven't been able to trigger reliably on the HF interfering waveform itself, only on the envelope of the waveform, so I'm not sure of the nature of the HF signal.
I've proven that I can suppress the signal on the 4-20mA loop to an acceptable level with a 1uF polypropylene film capacitor, and the results are further improved with the signal pair passed a couple of times through a ferrite toroid.
We have a digital trunked radio system at site which I have some suspicion of as being the source of the interference, but that is only a guess. If any of you are familiar with the unpleasant noise that the signal from a mobile phone causes when it is next to the speakers on a PC, the radios have a similar effect. They're also implicated in a few other mishaps with electronic apparatus on site, but it is politically very sensitive to criticise the system.
Does anyone have any other ideas where my interfering signal might originate? Although the immediate problem is 'solved', I don't like mystery signals getting in to the control system and I am concerned where else it may be propagating to.
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I don't suffer from insanity. I enjoy it...
The interconnect cables are multicores with individual screened pairs, with overall steel wire armour. Each transducer has one pair carrying the signal, and a second pair carrying the DC power. The screen is terminated at the 'quiet' ground in the equipment room housing the control system. The field end is floating. Power to the transducers is 24V DC and is a shared supply to all eight transducers. The noise problem does not improve when isolated supplies are used. The noise signal is also present on the power supply pair, but at a much reduced amplitude.
The interfering signal is what is really puzzling me: a brief burst of HF, or possibly RF, noise of a couple of microseconds duration, repeating every 4ms. On the 'scope the envelope of the waveform has the vague appearance of HF commutation noise from a large rectifier, but I can't account for the 250Hz. I haven't been able to trigger reliably on the HF interfering waveform itself, only on the envelope of the waveform, so I'm not sure of the nature of the HF signal.
I've proven that I can suppress the signal on the 4-20mA loop to an acceptable level with a 1uF polypropylene film capacitor, and the results are further improved with the signal pair passed a couple of times through a ferrite toroid.
We have a digital trunked radio system at site which I have some suspicion of as being the source of the interference, but that is only a guess. If any of you are familiar with the unpleasant noise that the signal from a mobile phone causes when it is next to the speakers on a PC, the radios have a similar effect. They're also implicated in a few other mishaps with electronic apparatus on site, but it is politically very sensitive to criticise the system.
Does anyone have any other ideas where my interfering signal might originate? Although the immediate problem is 'solved', I don't like mystery signals getting in to the control system and I am concerned where else it may be propagating to.
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