Force Feedback on a Stepper Motor?

[arpus4KM]

The SMC Actuators I mentioned have a specific force pushing feature built into them that allow you to set their moves based on how much force is being applied instead of distance traveled.

 

[Peter Nachtwey]

The spring idea may work. It has the advantage of adding compliance like the rubber pad I mentioned in #13. The rubber pad would do the same thing. Now the question is which would be more consistent.

Controlling the position of pneumatic cylinder is tricky. The cylinders tend to get sticky because of the dirty air.

 

[parky]

I tend to dissagree with the pneumatics problem, we had over 20 packaging machines with very large cylinders like 220mm bore, 270mm stroke, these were driving the tray nests up, stopping at 2mm before compression to allow gasing then again up to compression for sealing & cutting the film, the gas position pretty critical although not to thousand's of an inch perhaps +- 1-2mm these worked at a rate of 40 per minute rarely had problems, usual things perhaps after about 2-3 years operation the seals in the cylinder needed changing odd magnetic switch replacement etc. over the years newer version of these machines had servos instead, just as good but for our application not much better slightly higher output rate at 45 per minute but the servo took up nearly double the room, the linkage would wear faster etc. so in all it was not an improvement the cost was a lot higher, maintenance definitely higher, more difficult for our maintenance engineers to diagnose the problems. Even replacing the servo took nearly 3 times longer than removing the cylinder re-furbing & replacing it. My thought was this was a backward step in some respects. although the reliability of the servo components were good, the mechanical linkage & such caused far more PPM's & breakdowns.
To sum it was one step forward & two steps back for me.

 

[Peter Nachtwey]

Parky, those are relatively big cylinders. The OP wants only 10-20lbf. Also, big cylinders still have seal friction but it is not as severe when the cylinders are big. The seal area goes of the the radius of the cylinder, the area goes up with the radius squares. This means the seal friction is a much smaller factor compared to the overall force. The problem is even with 220mm diameter cylinders, will the seal friction still be less that a few pounds?

 

[parky]

Yes I'm aware of that, my point was that the position i.e. gasing position was always pretty accurate even after 2-3 years service & millions of operations. for the application I proposed to the OP there is no worry about any of those problems even the position of the sensor is not a real big problem as it is the spring that gives the pressure required to do the weld, just like a shock absorber even the flag for the sensor can be quite large it does not need an accurate position the pressure the spring will exert on the parts with the correct spring the travel could vary by quite a lot without any real extra pressure on the parts. From what I found in pack leak testing the amount of travel could vary without causing to much or to little pressure on the pack. Obviously, without seeing what this welder system actually does it is impossible to know if this will work, how the required parts would mount etc.
Sometimes I find people overthink these things Again on a packaging machine an upgrade was to fit servo drives on pack positioning, it was a bit of a disaster the cost very high a simple solution one of our engineers did was solenoid operated stops that once the packs were in position retracted allowing the packs to be placed into the sealing nest, they designed quick change pre-set stop bars ( the solenoids were on slides, the stop bars had catchments on them positioned for the pack size) so all the operators had to do was remove the existing stop bars fit the new ones & hey presto the pack spacing was correct, cheap to do, no electronics easy for our mechanical maintenance staff to sort out.