Cable runs and Transsmitters
Hi Dr_Ac
The cable runs are not that long i guess, at 12m, but it is a 650 Tonne machine, that's 650 tonne of steel, which i figured might have an electrical influence. the mA transducers are meant to be preferred in a bad electrical environment, i thought, as is the twisted pair cable Peter M mentions. I'm getting spiking in the 0-5V transducers i am using now. Also, for some of the non specialist trades people i have to work with two wires = less mistakes that three wires.
To purchase, Transducers 0-5V and 0-10V are the same price as 4-20 mA transmtters. As Retrolefty mentioned earlier in the thread the transmitters are more industrially common, as these are typically used with PLCs - expensive alternative to the Picaxe.
Also, someone mentioned "live zero". If i use the resistor combo, as per attached sketch, plagiarised from the ocean controls module,and use say a precise 240 ohm resistor between input and ground, then the output range for the 0-4 Bar would be from 0.96 V to 4.8 V. I could use code to highlight a "Transmitter error" on the LCD screen if the voltage was below 0.96 - Open circuit, or above 4.8 = short circuit. This would prompt a repair, rather than aimlessly collect useless data. The 10 - 50K resistor would limit the current input to the picaxe ADC to milliamps if there was a short, so it would not be damaged (well the Op amp in the Ocean Control circuit isn't)
I have not prototyped, but these things are pre calibrated and fixed, 4 mA = zero Bar, and 20 mA is the maximum range of the transducer. In my case 4 Bar or 60 psi would be about perfect. I will do a calibration check with actual pressure.
There should not be a need to convert to digital and RS232. If i was going 500 metres I would be using x Bee modules, much cheaper than paying guys to run cables and having it damaged.
So
1) Does my circuit look OK, and
2) do we really need the zener if the current to the ADC is limited to milliamps?
Michael V