I would like to know if anyone has any experience with the Modicon Momentum High Speed Counter Module (170 AEC 920). I'm have a bit of trouble with one. My incoming signal is from a Faure
Herman turbine meter and preamp. The magnetic pick-up in the meter sends a mV signal to the preamp where it is shaped and put on the wire by pulsing the current between 3mA and 44mA. At the counter module a 1 watt 270 ohm resistor was put across the input as directed by Faure Herman to give us a voltage pulse instead of a current pulse. Using a oscilloscope I discovered
that the square wave was not from 1VDC to 11VDC as would be expected at 24 VDC, but from 9.7VDC to 22VDC. I increased the resistor at the input until my square wave was from 3VDC to 22VDC. According to the spec, the module sees logical state "1" at 11VDC or higher and logical state "0" at 5VDC or lower. So my square wave should fall well within the spec. However, when
the frequency is measured at the magnetic pick-up and after the pre-amp, the frequency is the same, but the frequency I calculate from the counter
register in the PLC is higher. As I initially increased the resistor value at the input module, the calculated frequency got closer and closer to the input frequency. But at 1.8 KOhms, the PLC started giving wild readings. Right now the resistor is at 1.5 KOhms and my square wave is at the above mentioned levels 3VDC for the low and 22VDC for the high, and I'm about 1Hz higher in the PLC than the frequency measured at the input. However, if I use a pulse generator, to input a signal to the PLC, the frequency in the
PLC matches that produced by the generator. But the pulse generator produces a square wave from 0VDC to 16VDC.

If anyone could shed some light on this problem, I would greatly appreciate it.

Thanks,
Brady Matherne
PLC / SCADA Tech
Panel Specialists, Inc.
[email protected]

 

At 14:16 22/05/01 -0400, Brady Matherne wrote:
<clip>
>I would like to know if anyone has any experience with the Modicon Momentum
>High Speed Counter Module (170 AEC 920).
>I'm have a bit of trouble with one. My incoming signal is from a Faure
>Herman turbine meter and preamp. The magnetic pick-up in the meter sends a
>mV signal to the preamp where it is shaped and put on the wire by pulsing
>the current between 3mA and 44mA. At the counter module a 1 watt 270 ohm
>resistor was put across the input as directed by Faure Herman to give us a
>voltage pulse instead of a current pulse. Using a oscilloscope I discovered
>that the square wave was not from 1VDC to 11VDC as would be expected at 24
>VDC, but from 9.7VDC to 22VDC. I increased the resistor at the input until
>my square wave was from 3VDC to 22VDC. According to the spec, the module
>sees logical state "1" at 11VDC or higher and logical state "0" at 5VDC or
>lower. So my square wave should fall well within the spec. However, when
>the frequency is measured at the magnetic pick-up and after the pre-amp, the
>frequency is the same, but the frequency I calculate from the counter
>register in the PLC is higher. As I initially increased the resistor value
>at the input module, the calculated frequency got closer and closer to the
>input frequency. But at 1.8 KOhms, the PLC started giving wild readings.
>Right now the resistor is at 1.5 KOhms and my square wave is at the above
>mentioned levels 3VDC for the low and 22VDC for the high, and I'm about 1Hz
>higher in the PLC than the frequency measured at the input. However, if I
>use a pulse generator, to input a signal to the PLC, the frequency in the
>PLC matches that produced by the generator. But the pulse generator
>produces a square wave from 0VDC to 16VDC.
>
>If anyone could shed some light on this problem, I would greatly
>appreciate it.
<clip>

I don't know anything about your high speed counter module or turbine meter, but the voltage readings you mentioned sound very unusual.

Experiment #1 (270 ohm resistor) V=I*R
So: 3ma * 270 ohms = 0.003 * 270 = 0.81 volts - but you read 9.7 V
44ma * 270 ohms = 0.044 * 270 = 11.88 volts - but you read 22 V

Experiment #2 (1.5k ohm resistor) I = V/R
So: 3V/1500 ohms = 0.002 A = 2ma (not to spec.)
22V/1500 ohms = 0.015 A = 15ma (not to spec.)

I think it is likely that where you are seeing 22 volts, it is because the current transmitter is at its limit of compliance (maximum
voltage output). This means that in both cases the resistor would be too *big*. If the current pulse transmitter is operating at its voltage limit, it might possibly not switch cleanly, and your high speed counter is fast enough to pick up the extra transitions (which is why you read higher counts). There could be other reasons why you wouldn't get a good signal if things are not working correctly, and you may not happen to see the extra counts on your scope. If this is the case, then trying to do something to the counter is not a good solution.

If we look back at your first set of data, then if you are reading 9.7 volts across the resistor, then the current being transmitted must be (9.7/270 = 36ma) for a "low", and (22/270 = 81ma) for a "high". The second number here though is not meaningful, as the transmitter is at its limit. In either case, this does not match the specifications you mentioned.
The data seems to be more what we would expect to see from a 3k ohm resistor (9.7V/0.003A = 3222 ohms, while high value is beyond the range of the transmitter). If we assume a higher resistance in the circuit, then the readings sound more reasonable.

If we assume that the specifications of the turbine meter are correct, then it is safe to assume that the problem is one of the following:
1) Something is hooked up wrong.
2) Something is wrong with the current transmitter.
3) Something in the specifications or configuration is not correct.
4) The resistance in your circuit is higher than you suspect.

I would suggest trying to get the correct readings at your resistor before worrying about the high speed counter. I should think your resistor would need to be slightly less than 500 ohms to work with a 24 volt system (3ma * 500 ohms = 1.5 volts, 44ma * 500 ohms = 22 volts).


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Michael Griffin
London, Ont. Canada
[email protected]
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