Hi Rich,
No more hijacking, I promise. I had a momentary lapse and decided to try to be a commedian! Consider me properly chastised for the unacceptable beheavior.
I won't pretend to recognise all the components that you cite, but rather I can recount the issues that I encountered in like efforts.
To begin, the large cranes in the steel mill traditionally had a mechanical line shaft from one side of the bridge to the other to connect the wheels together to even out the bridge travel for both acceleration and braking. In later versions, when AC took over, the line shafts began to depart and sometimes so did the sync of travel from one side of the bridge to the other so the crane could become ****ed in the building.
I have seen cranes in long buildings that had infrared lasar distance measuring systems on each end of the bridge to measure the distance to the end of the building so that the independent motor control systems could square up the bridge in the building. This began around the mid 80s and not that it is involved in position control, it was an component in regard to safety and accuracy in the unmanned warehouse concept, becasue the lasers provide at least the gross component of the long axis position measurement. The fine component was sometimes from encoders that were periodically zeroed within a zone, but his scheme was often in error due to the likely hood of wheel slide caused by operators that were in a hurry. Automatic control could reduce this with controlled accel & decel. The better way was to have a target that could be hit with a laser sensor when the bridge was in line with a row in the long axis. The down side of this was that it reduced the flexibility on the floor layout and the fixed flags had to be relocated to take advantage of odd locations on the floor. This can be done with long lasers now.
As for bridge traverse, I have seen infrared used in this axis as well as cable driven rotary encoders and early on, the best was a rack and pinion system that was probably unpopular from the cost standpoint. I would use lasers in this application now. Wheel slip requires consideration in this axis since a hook hitting the top of a finished product will be very espensive.
The third axis is by far the easiest in that there are two stage absolute encoders available that can be attached to the drum which takes any counting errors out of the elevation.
In regard to the ML1500, I've seen them often, but not used one yet. My current activity has been centered on the CompactLogix platform, which also uses the 1769 I/O, and hense my joke about the HSC. I have one in an application that seems to have a mind of it's own in that it appears that I am dropping counts even at a very low input freq. I need to buckle down and tame it in the near future because I need for it to be working. I have used the VHSC in the PLC5 family for many years wtih flawless performance so my expectation is that this one will work as well.
So to review, in regard to your Traverse Wheel, consider wheel slip on the track. For the left hand / right hand prox, consider that their relationship may change if the bridge is long enough to become ****ed.
And lastly, if the Telemec is a hoist encoder, I suggest that you investigate the two stage absolute encoders. They have seperate course & fine sections.
Feel free to try me at
[email protected]
Best Regards,
Bob A.
Note (Commercial Reference), I have written a number of applications in the ML1000 that may coincide with your crane application. Check out this link:
http://www.americansensors.com/Sensors.aspx
If you scroll down, you will see a crane application, that he has added.
