How do I control a chain conveyor with 3 dc drives without having surging problems?


Thread Starter

Brent Edsoren Jr.

Hi, I have a system with 3 dc motors 1 connected at each end of a conveyor and one in the middle. The conveyor has a chain inside a track to drive the conveyor . The conveyor has allot of variable load upsets that have been plotted with a special device made to measure tension forces in various parts of the chain. The drives control the motors in torque mode and the drives are setup to gear 1 to 1 with another device. I have tried to tune the drives, have also put the (A/B 1395)drives in velocity mode, changed encoders all with no success. There is a tension take-up on the system but there are still variable tension upsets through the system. Is there anything you could suggest to help me with this control scheme that I might be able to use to stabilize my system? The reason for torque mode is there is a Sam-ex single axis control card that looks at the device encoder and the conveyor encoder to gear 1 to 1. There is another proprietary board which controls positioning. Torque needs to be used for fast positioning reaction, but even in velocity mode the system still surges. The driv commands are pretty staedy but the drives are still fighting each other.

Alfonso Padilla

Hi Brent, You might need to arrange your system such that the motor at the heading end of the conveyor works as 'master', while the others as slaves. Then working on a scheme to keep the currents balanced within certain bandwidth with PID loops, to avoid sudden surges. It takes a lot of trial and error and becomes more complicated if the load ís variable. You might even need to work on an 'adjustable bandwidth' which adjusts in direct relationship to the torque (current) demand. Good luck ! Alfonso Padilla QPS Control & Automation [email protected]
Brent, Are all the drives mastered from the same frequency source? If not, I would see if you could switch to this method of control. Cascaded references in this type of application rarely work very well. This applications is difficult because the output from each motor affects the remaining motors. You may also need to look at the dynamic responsiveness of the drives. We have found that the AB drives typically have a variable scan time on the analog command input and cannot be tuned to a very high level of performance - sometimes as low as 3 - 4 Hz. One method we have employed to solve this kind of problem is to create an electronic differential using the feedback from all drives. Using a high-end motion controller (Delta Tau) we sum all the encoder feedbacks into a single register and then use this register as feedback for each position loop. (drives should be in torque mode operation). The setpoint is now 3x what you normally would use and each PID loop sees the same setpoint and same feedback and (when configured with identical gains) the same torque command. This solution gives you an inherently stable load sharing system. Good luck! Ken Brown Applied Motion Systems, Inc. http:/
You need to select one master drive and set the other two up for "load sharing". It has been a while since I have done this, but here is the best that I can remember at the moment: Master drive is in velocity mode (or use your position controller for the velocity loop). The slave drives are in torque mode. Transmit the torque demand out of the master drive to the slave drives. The master torque demand becomes the slaves torque setpoint. As the slaves pick up the load, the masters torque drops and they hopefully achieve equilibrium. The total load becomes equally distributed between the three drives. This way, you only have on velocity and/or position loop. That is the key to stability. Bill Sturm

David Lawton Mars

I did a replacement project last year to eliminate this kind of problem and it works extremely well. The original system comprised of a chain approx. 1 mile long, with a main drive motor (using a VSD) and 3 torque motors, all connected to the same chain. The problems with the system were very similar to those you describe. I replaced all 4 motors and connected encoders on the back of the motor shafts. I then fitted 4 off Allen Bradley 1336 Impact drives all configured in velocity mode and used the encoder feedback to close the loops. The "clever" bit was to use a PLC (Allen Bradley SLC5/04) to give each drive an individual speed command i.e. the drives were not in a master/slave configuration. I then fitted potentiometers to the tension points along the chain and fed these back to the PLC as 4-20mA inputs. The PLC code then monitored these signals and adjusted the speed demand to each drive to keep it in "balance". The system has been installed now for nearly a year without any problems. Please e-mail [email protected] if you want more info. Regards David [email protected]