RadarRay
Electrical
- Oct 18, 2002
- 80
In response to a previous thread I replied to, JOmega asked that I further describe some of the advice I gave as to the control of punch presses. Now, I do not consider myself the all knowing advisor with punch presses, but since I did once do a punch press, here is some information learned while doing it, along with the specific areas JOmega wanted me to answer.
It always takes some co-ordination to get the feeder to feed and the press to punch at the correct time. The system I responded to had a fixed cycle Press and the feeder has to accomodate this cycle. I called this system uncoordinated, which perhaps is a poor choice of words. In fact there is some coordination between the press and feeder in any system. Since the stroke rate was fixed on this press, it some how has to trigger the feeder when to feed. Most likely there is a sensor which tells the feeder that the press is open and it is now possible for it to advance material into the press. Note that this does not have to be at the top of the press cycle. It can be just after the press has opened. The feeder then must advance material under the press and stop before the press is closed again. The feeder is most likely position controlled. This is the only way I have seen feeders, although before position control technology I am sure there were other ways (limits switches and prox's) of doing this. Note that if you set up a system like this, you have to account for some mechanical tolerances and other delays. So you have to have some dwell period before the press tells the feeder it is ok to advance.
In what I referred to as a Coordinated system, I should have called a two axis controller. One position controller controls the feeder, while a second axis controls the punch. You could take some liberties without having the press position controlled but IMO it is better to know were the press is at all times and with the cost of axis controllers these days, it is not that costly to implement.
So with the feeder and the punch at home, the process follows like this, the feeder starts to feed, typically on a trapezoidal move. It can signal the press when to start. Note that it does not have to wait to come to a stop after feeding, as the press takes time to come down. So you can coodinate the two. Usually based on a max speed, minimum feed lengh. As the press can then be controlled both down and up, following its position all the way. Again, after the first cycle, the feeder does not have to wait for the press to go all the way up. Only for the press to open and the part to clear. So it can get a jump on the press relative to the first pressed part. ALL OF THIS IS BASED ON THE POSITION OF THE TWO, RELATIVE TO EACH OTHER.
Now the main advantage of the Uncontrolled press app, is it is cheaper to implement.
The main disadvantage is a limited range of product can be run and the sysem is not easily opitimized for maximum output, when compared to the two axis controlled system.
Advantages to the two axis system are 1) the Press force based on speed could be controlled for different applications by changing the speed of the down movement. 2) the feeder can be controlled more accurately. Lets say the feeder can only accelerate so fast, without the material slipping in the feeder, which is usually a nipped roll. By having full control of the feeder, with the press following, it accel rate of the feeder can be adjusted so no slip occurs. Note most feeder applicatins must be very accurate. 3) the Dwell times can be minimized, and therefore the net output optimized.
One disadvantage of the controlled press can be the size of the press motor/drive. If it was decided to not have a flywheel, then the drive must supply all the energy and it must sized to accel and decel the press each cycle. You could perhaps leave the flywheel and use a delay to get the press up to speed, trying to only trim the position during the cycle with the drive, this would take less power but would also create a less versatile machine (i.e. you cannot move the press to a position very rapidly).
A fixed cycle press can be modified. It only takes time and money, and good engineering practice. Both mechanical and electrical( controls). Again as in all things that end up taking engineering, it comes down to cost vs payback. Although sometimes payback can be more than just output, it can be in quality.
To accomplish a rebuild, you first have to find out the max/min cycle times required, and from that the powers of all the motors and power units required. This is the most important step. Blow it here and the machine will never work correctly. Once the power/inertia, or I should really say torque/inertia relationships are known. The cycles can be verified and the system designed.
There are several multiaxis position control cards/systems around. I would probably use the card that sits in an AB plc rack, the product line name escapes me right now. It is available with analog out, so it could be used with any good vector controlled drive. Here my choice would be Eurotherm. You could also do it totally with Allen-Bradely using their integrated servo line. I am not sure which would cost out the cheapest.
I am not the biggest fan of AB but I find the commands of their servo controller to be quite powerfull. One command can do quite a lot for you, the negative side in applying it is in understanding how they designed it to operate verses how you interpret the command to operate. But then, that amounts to product experience.
I hope this post supplied the answers you were looking for and that it provides others with information for thought. This two axis system in similar to many other systems in design, so the discussion may prove useful in other applications.
It always takes some co-ordination to get the feeder to feed and the press to punch at the correct time. The system I responded to had a fixed cycle Press and the feeder has to accomodate this cycle. I called this system uncoordinated, which perhaps is a poor choice of words. In fact there is some coordination between the press and feeder in any system. Since the stroke rate was fixed on this press, it some how has to trigger the feeder when to feed. Most likely there is a sensor which tells the feeder that the press is open and it is now possible for it to advance material into the press. Note that this does not have to be at the top of the press cycle. It can be just after the press has opened. The feeder then must advance material under the press and stop before the press is closed again. The feeder is most likely position controlled. This is the only way I have seen feeders, although before position control technology I am sure there were other ways (limits switches and prox's) of doing this. Note that if you set up a system like this, you have to account for some mechanical tolerances and other delays. So you have to have some dwell period before the press tells the feeder it is ok to advance.
In what I referred to as a Coordinated system, I should have called a two axis controller. One position controller controls the feeder, while a second axis controls the punch. You could take some liberties without having the press position controlled but IMO it is better to know were the press is at all times and with the cost of axis controllers these days, it is not that costly to implement.
So with the feeder and the punch at home, the process follows like this, the feeder starts to feed, typically on a trapezoidal move. It can signal the press when to start. Note that it does not have to wait to come to a stop after feeding, as the press takes time to come down. So you can coodinate the two. Usually based on a max speed, minimum feed lengh. As the press can then be controlled both down and up, following its position all the way. Again, after the first cycle, the feeder does not have to wait for the press to go all the way up. Only for the press to open and the part to clear. So it can get a jump on the press relative to the first pressed part. ALL OF THIS IS BASED ON THE POSITION OF THE TWO, RELATIVE TO EACH OTHER.
Now the main advantage of the Uncontrolled press app, is it is cheaper to implement.
The main disadvantage is a limited range of product can be run and the sysem is not easily opitimized for maximum output, when compared to the two axis controlled system.
Advantages to the two axis system are 1) the Press force based on speed could be controlled for different applications by changing the speed of the down movement. 2) the feeder can be controlled more accurately. Lets say the feeder can only accelerate so fast, without the material slipping in the feeder, which is usually a nipped roll. By having full control of the feeder, with the press following, it accel rate of the feeder can be adjusted so no slip occurs. Note most feeder applicatins must be very accurate. 3) the Dwell times can be minimized, and therefore the net output optimized.
One disadvantage of the controlled press can be the size of the press motor/drive. If it was decided to not have a flywheel, then the drive must supply all the energy and it must sized to accel and decel the press each cycle. You could perhaps leave the flywheel and use a delay to get the press up to speed, trying to only trim the position during the cycle with the drive, this would take less power but would also create a less versatile machine (i.e. you cannot move the press to a position very rapidly).
A fixed cycle press can be modified. It only takes time and money, and good engineering practice. Both mechanical and electrical( controls). Again as in all things that end up taking engineering, it comes down to cost vs payback. Although sometimes payback can be more than just output, it can be in quality.
To accomplish a rebuild, you first have to find out the max/min cycle times required, and from that the powers of all the motors and power units required. This is the most important step. Blow it here and the machine will never work correctly. Once the power/inertia, or I should really say torque/inertia relationships are known. The cycles can be verified and the system designed.
There are several multiaxis position control cards/systems around. I would probably use the card that sits in an AB plc rack, the product line name escapes me right now. It is available with analog out, so it could be used with any good vector controlled drive. Here my choice would be Eurotherm. You could also do it totally with Allen-Bradely using their integrated servo line. I am not sure which would cost out the cheapest.
I am not the biggest fan of AB but I find the commands of their servo controller to be quite powerfull. One command can do quite a lot for you, the negative side in applying it is in understanding how they designed it to operate verses how you interpret the command to operate. But then, that amounts to product experience.
I hope this post supplied the answers you were looking for and that it provides others with information for thought. This two axis system in similar to many other systems in design, so the discussion may prove useful in other applications.