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4 Decade Logrithm to Linear Conversion Step 7 Basic

Created by: SeanC. at: 8/16/2011 4:50 PM (6 Replies)

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8/16/2011 4:50 PM	Rate (0)
SeanC. Posts: 8 Rating: (0)	Dear all, I've been through everything on here for the S7-1200 and Step 7 Basic and haven't seen this talked about so I will ask. What I'm trying to do is take an analog input from a radiation sensor that is a 0-50mV run through a converter to 4-20mA and compare that to an adjustable set point to trip an alarm. All well and good up to the point where the signal coming in is a 4 decade logarithmic scale and I need to be able to display this in mR/hr. Thanks.
SeanC. Posts: 8 Rating: (0)	Last edited by: SeanC. at: 8/16/2011 4:50 PM
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8/16/2011 5:03 PM	Rate (0)
Denilson Pegaia Platinum Expert Joined: 9/23/2005 Last visit: 12/8/2023 Posts: 4284 Rating: (1380)	Hi, Can you express this conversion in a formula?
	Denilson Pegaia Siemens Industry Sector Technical Support
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8/16/2011 5:31 PM	Rate (0)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 12/7/2023 Posts: 13989 Rating: (2282)	Dear all, I've been through everything on here for the S7-1200 and Step 7 Basic and haven't seen this talked about so I will ask. What I'm trying to do is take an analog input from a radiation sensor that is a 0-50mV run through a converter to 4-20mA and compare that to an adjustable set point to trip an alarm. All well and good up to the point where the signal coming in is a 4 decade logarithmic scale and I need to be able to display this in mR/hr. Thanks. 0 - 50 mV =0 - ? mR/hr 0-50mV = 0 - n counts per second, correct? You need the conversion factors associated with your counter. You can calibrate the values using a calibrated mR/hr meter. I think the formula is : a = b log(count/sec) where a = mR/hr and b is your conversion factor The signal being provided is the 'count/sec' so you need the 50mV = ? mR/hr value to complete the calculation for b so you can scale the value. Once to the PLC, you'll have to use the 4-20mA range's counts to properly set your mR/hr values. I do not know if I'd use a {non-nuclear-rated}PLC to trigger a high radiation alarm or alert if that is the intended application.
	science guy
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8/16/2011 6:09 PM	Rate (0)
SeanC. Posts: 8 Rating: (0)	Thanks for the quick response. Honestly I'm not sure if that formula is correct or not but it gives me a place to start which is more than I had before. 0-50mV is 0.01 to 100 mR/hr btw. Luckily the nuclear rating isn't a problem since this is for a remediation project and will be taken out of service probably by the end of the year.
SeanC. Posts: 8 Rating: (0)	Last edited by: SeanC. at: 8/16/2011 6:10 PM
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8/16/2011 6:56 PM	Rate (0)
Denilson Pegaia Platinum Expert Joined: 9/23/2005 Last visit: 12/8/2023 Posts: 4284 Rating: (1380)	Hi, in this case I suggest you use a ready made block for 4..20mA Sample program: SCALE and UNSCALE /tf/WW/en/Posts/31698#top Adjust 4mA (0mV) = 0.01 20mA (50mV) = 100.0
	Denilson Pegaia Siemens Industry Sector Technical Support
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8/16/2011 8:31 PM	Rate (0)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 12/7/2023 Posts: 13989 Rating: (2282)	Pegaia Hi, in this case I suggest you use a ready made block for 4..20mA Sample program: SCALE and UNSCALE /tf/WW/en/Posts/31698#top Adjust 4mA (0mV) = 0.01 20mA (50mV) = 100.0 If the PLC input is linear, you should do as Pegaia suggests. If it is not, find the formula for the relationship between the input and the mR/hr. It will be similar to the one I posed above. If you have something measuring counts per unit time (cps), the big question is whether your log is already handled by the instrument or whether the plc has to do the math to obtain the value. Alternatively, you can generate a lot of sample values in a table, and simple do a table lookup in the PLC code to get the value. Easier than RTD bridgelook up tables!
	science guy
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8/17/2011 12:51 PM	Rate (0)
Denilson Pegaia Platinum Expert Joined: 9/23/2005 Last visit: 12/8/2023 Posts: 4284 Rating: (1380)	Hi, as complementar suggestion, after do the linear conversion with SCALE blocks, you can use the block CALCULATE to perform the non linear correction.
	Denilson Pegaia Siemens Industry Sector Technical Support
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