Replace the Potentiometer reference Voltage with Analog Output

[Steve Cheung 0]

Hi Friends, I have application to replace the old Reliance servo controller with voltage reference output to drive what they call an Inverter. This inverter is variable voltage reference input. In manual mode, we can use the existing Potentiometer to move and stop the Belt conveyor from this inverter. When I move the potentiometer in half, it will move the belt, then if I want to stop it I turen the potentiometer to the zero point with very smooth. Now I have replace it with FX2N PLC, FX2N-1HC and FX2N-2DA analog module. It was able to move and stop the conveyor as the target setting (but not exactly accurate). I have problem also in stop it with more linear (smootly) like using potentiometer. I am using comparation function, for instance.... Let say the setting point 1000mm. I trigger the first time voltage 6V from starting point. If the FX2N-1HC detect the present value is reach 800mm, then I trigger the analog output to the lower voltage, let say 3V, again if FX2N-1HC detect PV reach 850mm then I lower again the output voltage to 2V, until reach 1000mm I trigger it 0V. But, seem the FX2N-2DA cannot act smooth like using potentiometer. Also the belt cannot reach exaclty as the target setting point. Anyone have idea? Best regards, Steve

[panic mode 246]

no, analog output doesn't have same resolution as potentiometer but this is not the problem. the issue is that when you use potentiometer by hand, you are not producing same output as analog out, you need ramp. when you turn potentiometer by hand, there is no sudden change like what your program is currently putting out (jump from one setting to another). wire it to a chart recorder or storage scope and you will see what i mean. program plc to produce same curve as the hand operated potentiometer and result will be the same. also i'm not sure what are you trying to do. it looks like you want to do position control using open loop or use one or two photoeyes as feedback. when you control potentiometer by hand, can you achive what you want and with same accuracy without looking at process (belt conveyor in this case)? for example if you are using photoeyes, try looking at indicator lights controlled by those photoeyes instead of looking at the process. if that still works, plc should also be able to do it. ---- edit ---- my bad, i just re-read the post i noticed high speed counter. this means that you do have position feedback so positioning control is possible. if you ware using servo drive you could just tell it where to go and you would be more or less done. when using vfd to control position (loop closed in plc program) there are always limitations (like positioning speed). you could try pid loop or custom logic to build ramps, slow down before finish etc.

[Steve Cheung 0]

Thanks Panic, Yes, I have slow down the speed before reach the setting point, but still have a different between repeated measure, such as 1mm or 2mm. I will try to using ramps function. Thanks & best regards Steve Edited 13 Jul 2006 by Steve Cheung

[royjohnston 0]

i use fx2n plc with inverters every day. i obtain 100 % positioning on conveyor belts. WHat I do is i attach an omron encoder onto the chain drive. wire this to input X000 and use a high speed counter to reset the output of the plc. in the inverter i use 2 jog speeds and pre-program a ramp down time to rougly 0.5 s. step 1 engage output and an additional output to tell the inverter to ramp up to maximum speed. step 2 count the impulses received back from the encoder . step 3 when a certain amount of impulses are received change the output from high speed to low speed. the inverter will now ramp down to the slow speed over 0.5 seconds. step 4 after the final count you stop the inverter. this is the only acurate way of using position control with an inverter. and is elevently cheap. the only other alternative is a servo motor and controller. this is easy to set up, however is very expensive

[Steve Cheung 0]

Hi royjohnston, Thanks, but I can not adjust any setting inside the inverter, because it very old model and I can't find the manual. I try to build a ramp function to slow down when it reach the near setting point. Best regards steve

[Steve Cheung 0]

Hi Panic, Royjohnston, I have try to use ramp function to decrease the speed, but still the belt conveyor can not stop to exactly the setting value I set. Let say Setting Point is 1000mm (changeable). The conveyor should be stop every 1000mm for printing process, then run again, stop, then run again. But, in real, it would be stop at 1065mm and with diverse length. Even the FX2N-1HC already get the data, but I have problem to stop the conveyor at the position through the FX2N-2DA. The original system is a old positioning unit that drive the Inverter through 0 - ref terminal (analog input). But, unfortunately the positioning unit is lost. So I try to replace it with FX2N-2DA & FX2N-1HC. Any idea? Thanks & best regards Steve