In the two-wire system, there are brief fluctuations in pressure of 10–20 Pa upward and downward; I’m not sure if this is normal
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The analog module of Siemens 7KF02 is used for two-wire pressure measurement, with a range of 0–6000 Pa. The FB41 valve is used for adjustment, but the pressure value always fluctuates by around 20 Pa upward or downward; the adjustment effect provided by FB41 is not good either, as the pressure remains below the set value and cannot be increased. I think it’s because the pressure entering keeps fluctuating up and down, which causes FB41 to be unable to figure out the situation and thus fail to output signals accurately. Look, everyone? The shielding of the stress line is not grounded; it’s from an old project, and the work wasn’t done carefully enough. I changed the HMI configuration software to WinCC a few days ago; it was originally MCGS. I felt that the original program was written in a messy way, and I didn’t really like it; haha, his original program even didn’t call FB41 from OB35. So I wrote a completely new program – it’s not large nor complicated; it involves just dozens of temperature and pressure values, along with PID control for 6 actuators. After finishing, one of the actuators did not perform well in terms of adjustment, which is the situation mentioned above. How can this be resolved? There’s another issue: I originally wrote an OB31, which is that OB block that triggers a 2S interrupt once, but it wouldn’t allow me to download it. I remember a message indicating a problem with priority, and I’m not sure why.
1. Use minimum signal filtering. 2. The full-scale range of a two-wire pressure transmitter is 6 kPa; fluctuations of up to 20 kPa are considered somewhat large; a variation of +_6 kPa is within the normal range. 3. Use a multimeter with high precision to rule out issues at the installation site first. 4. Connect it in the control room for shielding – it will be difficult to do this at the installation site
It seems that the fluctuation of 20PA isn’t very large; perhaps the field pressure transmitter should be taken apart for calibration to check for any zero drift.
There are instantaneous fluctuations of 10-20 Pa, which seem to cause interference; the control room does not have a ground wire for the instruments, so shielding cannot be established； I don’t have a spare pressure transmitter on hand to replace it. I’ll go take a look when I go to work tomorrow. Would it be a good idea to replace the isolator with a four-wire system? Also, a friend upstairs mentioned zero drift. I’m not really worried about zero drift; as long as there are no sudden fluctuations, it’s fine. Since the measurement range is quite large, I don’t really care about any slight shifts in the zero point, hehe. This post was last edited by *nshiji168 on 2009-3-15 13:15]
Could it be that the pressure signal fluctuations are caused by vibrations in the pipeline at the pressure measurement point? Try reducing the flow rate by closing the valve a bit. Another thing is to apply filtering to the measurement points.
The 20PA fluctuation is not very large; it should be normal.
20 is very small compared to 6000. It’s necessary to consider the manufacturing process – whether there are already fluctuations in the process pressure~~:)
I consulted a Siemens expert regarding this PLC issue; below is his response, which I hope will be helpful to you. “Personally, I think writing FB41 within OB35 and setting its sampling time parameter to an integer multiple of the OB35 call interval (such as 100ms/200ms/...) should help improve the situation you described; it is also a good approach ； Or perhaps placing FB41 in OB1 and increasing the sampling time parameter should also help. ” Additional note: “There’s another issue. I originally wrote an OB31, which is that OB block that triggers a 2S interrupt once, but it wouldn’t allow me to download it. I remember a message indicating a problem with priority, and I’m not sure why.” ” Not all CPUs support all organization blocks; for loop interrupts, a CPU with a model number of 300 only supports the loop interrupt with the identifier OB35. This post was last edited by chmyh on 2009-3-17 15:19]
I simply place FB41 inside OB35. The actual value of the pressure used for adjustment is also stored in OB35, obtained through FC105. I don’t think it’s necessary for OB1 to retrieve this actual pressure value, as OB1 has a short cycle time; there’s no need to take samples that frequently. A sampling interval of 100ms in OB35 should be sufficient. For FB41, I use a default sampling time of 1 second, while for OB35 the default interrupt time is 100 milliseconds. I’ve heard that for FB41, the sampling time can be any multiple of OB35’s interrupt time; I see that many people use the default value of 1 second. I’m not sure whether using a smaller value would be better, but I haven’t tried it yet. I went to reduce the P-value of the PI the day before yesterday; it seemed that there was some improvement at that time, but I didn’t observe it for a long period that day. I’ll go and check again another day. This reduces the sensitivity of FB41, so its output will not respond sensitively to the instantaneous fluctuations in the actual pressure value. This seems to be a good way to address the problem I mentioned earlier.