zcut
Member
I'm currently working on a project which requires shaft synchronization. Anybody have any experience in this type of project? any help would be appreciated. z
zcut said:????
CharlesM said:I assume this is a new system. If you want good motion control go to a motion controller.
It is best if all the axes follow the computer generated master. That way all the axes have the same master. The master is perfect because it is computer generated. Gearing to a physical master causes a lot of problems. The master position is quantized by the resolution of the encoder. This adversely affects the master velocity and acceleration calculations. The master will also suffer from wow and flutter. This will make synching to the master even harder. As stated above, a computer generated master position is perfect and so are the calculations for the velocity, acceleration and jerk etc. These higher order derivatives are important for calculating feed forwards. The four axes can use the same master or target psition by just issuing exactly the same command to all four axes at exactly the same time. This is easy. If all four axes are tuned well then they will follow their target or master position, velocity and accelerations with little error.zcut said:ok sorry, new to forum. what I have is a four axis indexed machined.I am using mitsubishi fx3cu, inc encoders. I set the least loaded axis as the master.
I would consider using SSI encoders because they are absolute and don't need a homing routine. On start up all the axes should be commanded to a common point. That may be an average position as you stated. When all axes are in position then all the master are target positions will be at that same position. From then on you should give the same command at the same time to all the axes at exactly the same time. This way the masters or target positions will following exactly the same motion profile on a millisecond to millisecond basis as the targets move to the new position.I test for position relationship in a defined part of the rotation, currently between 0 and 180 degrees. I sample the error during this time, average it and pass to a PI loop.
the old system I am trying to replace had a synchro and tach for feedback. It also had a very small range of operation +- 5% of rated speed. The customer required the machine to be slowed down to 1/6 of that. While it seems to "sync" up relatively quickly on initial startup, it does not upon stopping one axis "which they have to do for maintenance".
This is why I prefer SSI absolute encoder for these applications. Sick/Stegmann and Danaher make excellent absolute SSI rotary encoders.
I thought you said you told all the axes to go the same average position on restart? In anycase good controller can handle that easily.Upon restart sometimes it locks in within 5 indexes, other times not at all.
Again, this will not be a problem for a good motion controller because the master or target position for all the axes will match exactly at the end of the day.The original also had an independant master synchro signal. When looking at the encoder feedback it was in fact varying in frequency, altho it was slight enough that I felt the loop would overcome it. Also the allowable error between axis' is +- 2%. So it is somewhat forgiving.
A good motion controller can make your life much easier because the issues you bring up are not a problem.I have never had the opportunity to set one of these up before, so I know there is a better, more efficient, and tried and true method. Should I provide a separate, non-machine based master reference? again any suggestions are appreciated... z
You also mentioned incremental encoders. They will work but will require homing on start up and power loss. If there production on the line that must be cleared off during homing then I would stick to the SSI encoders. If power goes off during production then the four axes should drift to a stop. They should be close enough on start up where a go to the average postion will get them back in sync with little motion so the production that is on the system is saved and no time is wasted homing.
I have an idea........
I wonder what I can do with windows movie maker and a four axes motion controller. Hmmm.
This is where a 'phasing' command is handy to get your axes in 'phase' in a controlled manner while moving on-the-fly. In this case you do pick one axis to be the master and then the phasing command is issued to each of the 'slave' axes. The first parameter is the offset from the slave position which is usually 0. The second parameter is how much distance the master will move before the slaves reach the desired offset. This parameter is usually calculated on-the-fly for optimal performance. If the axes are far out of synch then it is best to let the master move a farther distance before expecting the slaves to be in sync. If the slaves are just a little out of phase then the distance the master moves before getting into phase can be very short. This calculation is a trade off between minimizing the phasing time and distance without exceeding the speed of the slave axes.zcut said:position sync. On startup I run all axis at a fixed speed after the servos have ramped up for a period of time. Then I release control to the pi loops, which are only sampling if they are in the same quadrant. If after one rotation they are not in the same quadrant I pulse the function to sample them where ever they are. The loop responds and brings them closer.