adccommunitymod (AutomationDirect) asked a question.
speed follower
Created Date: January 13,2011
Created By: keklon
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hi, Can someone tell me a good way to control speed on a VFD based on the rate parts past a photoelectric switch. I am thinking PID loop using dl05. any suggestions?
Created Date: January 13,2011
Created by: Bob S BN
What kind of drive?
How will you get the speed reference from the PLC to the drive (analog or comm link)? If comm link, what comm protocal is available to the drive?
These need answering in order to decide what PLC/modules to buy.
Many drives have PID capability built in, although what I 've seen used an analog input signal for the PV, and it sounds like you 're talking about a discrete sensor and measuring time between pulses.
Created Date: January 13,2011
Created by: KPrice
keklon, if you have (2) photo switches and measuring the time it takes the object to travel between the (2) switches, look at using the Dual Edge Timer function in the CTRIO.
If you have (1) photo switch and measuring the time between an object and the next object, look at using the Edge Timer function in the CTRIO.
You would need to do some conversion math to convert object rate (objects/min) to speed (ft/min).
If you have (1) photo switch and measuring the time as the object passes in front of the photo switch, perhaps you are just measuring conveyor speed. In this case, you can measure the conveyor speed directly by measuring RPM of a shaft with the photo switch or using an encoder.
Yes, you could use a PID loop, your measurement (converted to your speed units) would be your Process Variable (PV), your PID output would control the VFD, and you would set a speed SP (I'm assuming you want to control linear speed, ft/min for example. You will probably need to do conversion on your output also so your motor speed converts to (ft/min).
HOWEVER, the DL05 only has (1) expansion slot. So you could not use a CTRIO and an analog output module. Perhaps you could use hi-speed inputs, or perhaps communications to the VFD.
Created Date: January 13,2011
Created by: keklon
speed follower
Thanks for the replies. The drive is a Automation direct GS2 ac drive. I am using a red lion IFMA frequency to analog converter to give the GS2 a 0-10 vdc speed reference based on pulses from the parts on the conveyor. I cannot reference the belt directly due to the random nature of the parts. The belt speed is constant but the receiving equipment needs a backlog. The way it is now works but I have to stop the down stream equipment to maintain backlog which decreases productivity.
Created Date: January 14,2011
Created by: Bob S BN
The drive is a Automation direct GS2 ac drive. I am using a red lion IFMA frequency to analog converter to give the GS2 a 0-10 vdc speed reference based on pulses from the parts on the conveyor.
Have you looked at the PID abilities of the GS2? See page 4-51 of the GS2 user manual. Looks like it should be able to run PID control of motor speed based on the PV analog input signal. I haven't done this though, so hopefully others can help with actual implementation if needed.
Created Date: January 14,2011
Created by: keklon
speed follower
I may need a whole different approach to what I am trying to achieve. I'll try to explain more specifically. I have a conveyor feeding a machine that requires a backlog of approx. 25 parts. That is my target backlog. I want to speed up the machine (GS2)60HZ max when the conveyor gets increasingly full (I can count the # of parts on the conveyor with a photo switch) and slow it down to a minimum speed of 26Hz when the conveyor gets to 25 or less pieces. I will use a dl05 to track the # of parts entering and exiting the conveyor. I hope I am clearer in my description .Suggestions? Thanks keklon
Created Date: January 14,2011
Created by: Bob S BN
Ah Ha, that helps a bunch. The PLC will give you the ability to monitor the amount of product, and output an appropriate speed reference to the drive.
You should be able to connect your part sensor directly to a discrete input on the 05 (no need for the red lion) and do any counting and timing with ladder logic, and calculate the output speed reference value.
The speed reference can be sent to the GS2 via an analog output card added to the 05, or via MODBUS over rs232 (page 5-12 of the GS2 manual)
Created Date: January 14,2011
Created by: keklon
speed follower
I may need a whole different approach to what I am trying to achieve. I'll try to explain more specifically. I have a conveyor feeding a machine that requires a backlog of approx. 25 parts. That is my target backlog. I want to speed up the machine (GS2)60HZ max when the conveyor gets increasingly full (I can count the # of parts on the conveyor with a photo switch) and slow it down to a minimum speed of 26Hz when the conveyor gets to 25 or less pieces. I will use a dl05 to track the # of parts entering and exiting the conveyor. I hope I am clearer in my description .Suggestions? Thanks keklon
Created Date: January 14,2011
Created by: KPrice
Yes, the picture is a little clearer. So you have (2) photo switches? One counts the parts entering? And one counts the parts exiting? And the speed of the conveyor (controlled by the VFD) controls how fast the parts exit, increase speed to increase parts exiting?
I agree with Bob S BN, the DL05 should be able to count without the need for a CTRIO, unless the conveyor is really fast. Reviewing the specs will show if the std or hi-speed inputs will handle this.
The inputs could trigger an up\down counter, entering counts increase counter, exiting counts decrease counter. So the counter value will indicate number of parts in backlog. A simple calculation could give speed control, no need for a PID.
Created Date: January 15,2011
Created by: Rich19551
Can I change the way you are thinking about this more on a one to one ratio.
The more boxes you have the machine runs faster and the less boxes it slows up.
I 've used this to secquence machines together and it works.
Conveyor speed constent so you look at boxes with eye (on time of VFD)
Space between boxes (off time of VFD) more boxes less off time.
Add decel to off time and machine never stops but slows down as to the number of boxes on conveyor. More boxes it speeds up (less off time).
Rich
Created Date: January 15,2011
Created by: milldrone
I have a conveyor feeding a machine that requires a backlog of approx. 25 parts. That is my target backlog. I want to speed up the machine (GS2)60HZ max when the conveyor gets increasingly full (I can count the # of parts on the conveyor with a photo switch) and slow it down to a minimum speed of 26Hz when the conveyor gets to 25 or less pieces.
I do not know your application. I had an application that required a certain backlog for the feeder to work properly. When I tried the count up count down approach I found that I accumulated counting errors. The errors were from no gap between incoming parts and multiple pulses from the exiting photo eye on a single exiting part. Just something to consider.