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Is 8 bit enough?

Created by: kokov at: 3/29/2011 9:33 PM (5 Replies)

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3/29/2011 9:33 PM	Rate (0)
kokov Advanced Member Joined: 3/28/2011 Last visit: 9/10/2024 Posts: 12 Rating: (0)	I hesitate whether to use the SM 334 (AI 4/AO 2 x 8/8 bit) for thermo control in range -10 .. +30 °C. Sensor will be Pt100 with transducer 4..20 mA. Resolution of 0.15 °C/unit is enough for mesurement of our process, but is it enought for PID control. Can anyone advise me? Thanks in advance!

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3/30/2011 5:32 AM	Rate (0)
hdhosseini Diamond Expert Joined: 1/28/2009 Last visit: 9/10/2024 Posts: 6849 Rating: (1365)	kokov I hesitate whether to use the SM 334 (AI 4/AO 2 x 8/8 bit) for thermo control in range -10 .. +30 °C. Sensor will be Pt100 with transducer 4..20 mA. Resolution of 0.15 °C/unit is enough for measurement of our process, but is it enough for PID control. By selecting lower resolution for analog input, process value between 2 successive points may change noticeably for PID algorithm. PID algorithm calculate manipulated value based on error signal which is error=setpoint-process value. setpoint is fix in most of the time. Any abrupt change in process value may cause rapid change for manipulated value. Consider the following cases for a mere P controller: case1: setpoint=20 , pv=19,gain PID=4, error=1 , LMN=4 case2: setpoint=20 , pv=18.8,gain PID=4, error=1.2 , LMN=4.8 According to this, I think by using options provided by PID function wisely , you can minimize the effect of these minor jumps in manipulated value.Tempreture loops are sluggish .This may help you to ignore the difference between 2 points of measurement. regards
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3/30/2011 7:40 AM	Rate (0)
kokov Advanced Member Joined: 3/28/2011 Last visit: 9/10/2024 Posts: 12 Rating: (0)	Yes my process is really slow and fluctuation of manipulated value will be noticeable. Thank you hdhosseini! Тhis clear my view. With12 bit module SM 334resolution will be 0,01 *C/unit,but ithave not 4..20mA input. This module have only Voltage and Thermo input, so I can't use transducer. I doubt is it safty (even with 4 wire) to connect If Pt 100 directly to SM 334 if distance is around 50m? Regards!

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This contribution was helpful to 1 thankful Users hdhosseini

3/30/2011 9:15 AM	Rate (0)
Aret Posts: 2348 Rating: (264)	Generally 4wire scheme guarantees no resistance increase due to cable length. BUT for this module (6ES7 334–0CE01–0AA0) resistance measuring is not allowed according to manual - it's allowed for 6ES7 334–0KE00–0AB0 - and climatic range also allowed for this module - no transducer required (for up to 100m meters of shielded cable). Also a question - what deadband width would you have?
Aret Posts: 2348 Rating: (264)	Last edited by: Aret at: 3/30/2011 9:17 AM
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3/30/2011 11:00 AM	Rate (0)
hdhosseini Diamond Expert Joined: 1/28/2009 Last visit: 9/10/2024 Posts: 6849 Rating: (1365)	Dear kokov Also check this manual: Modular PID Control check the following function as well designed function to make your control loop robust. section : 2.1.5 DEADBAND: Dead Band (make loop insensitive of small changes around set point) If the process variable is affected by noise and the controller is optimally set, the noise will also affect the controller output. Due to the high switching frequency (step controller), this increases wear and tear on the actuator. Suppressing the noise prevents oscillation of the controller output.. When the dead band is used to form the error signal, the offset DEADB_O must be set to 0.0. section 2.1.13 LMNGEN_C: Output Continuous PID Controller The block is used to structure a continuous PID controller. It contains the manipulated value processing of the controller.
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This contribution was helpful to 1 thankful Users Niranjan Prasad

3/31/2011 7:08 AM	Rate (0)
kokov Advanced Member Joined: 3/28/2011 Last visit: 9/10/2024 Posts: 12 Rating: (0)	Thank you for your reply Aret, Finally we decide to use 6ES7 334–0KE00–0AB0. 4 wire circuit. Distance wil be 45 m max. About dead band: I read many publications about controlling of slow processes and most of them advice to set dead band to 0.0 to reduce dead time. Regards!

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