Air Spring for pick & place 1

[globia]

I would like to use a pnuematic cylinder as an air-spring for pick&place? I need to be able to adjust the pressure from as low as 100 grams, up to 10-20Kg. I am looking at using a double actuated Airpel cylinder which has a graphite piston inside of a quartz tube. These are extremely low friction cylinders. My problem is I'm not sure what I will need on the control end. I need to be able to pick up a part with a lower load (100-500 grams), and place it at a significantly higher load (up to 20Kg). I also need to be able to control the speed for smooth pick/place. Does anyone have an idea of what type of control valves, regulators etc. I would need? I would much appreciate your input!

 

[Barry1961]

When you are picking up does the extra force damage the part? Can you limit the force another way such as moving the pick point to where the cylinder is bottomed out?

Self relieving regulators could be used in the circuit to limit pressure on one side or the differtial pressure between ports. I have used cylinders in the fashion as counter balances for lifting loads. We also include a air tank near the cylinder to keep the pressure constant.

One other trick I have used for a different reason is to plumb both the ports together on a single rod cylinder. This will make the cylinder extend quickly but be very weak. The amount of force is the area of the rod times pressure. This is mostly done to get a larger cylinder extended quickly in a press application then apply pressure to the rear port only after the ram hits the part. This can save a lot of money on the HPU.

Barry1961

 

[globia]

Barry, thanks for your reply. I can't move the pick point because the camera is at a fixed location to view the part (these are surface mount components and flip chip parts with the solder pads on the bottom of the part. the camera looks up and down simultaneously to align the chip to the board. The way the viewer is mounted in the machine, it is not practical to move the camera up/down. the chip needs to be picked up with a relatively low force. Counter balancing was exactly what I have in mind, but instead of lifting, the cylinder will be applying a load. The cylinder will be normally extended (at the focus position). It is mounted to a motorized z-motion which will bring the tool (mounted to the counter-balanced cylinder) down to the stage, and apply the load via over-travel. The cylinder will basically always be normally extended. I chose double actuated instead of spring extend for more control. Does this paint the picture?

 

[Barry1961]

I assume your z-axis is not accurate enough to pick without a little compliance and you looking to counterbalance a standard pneumatic chip gripper which will probably weigh at least 2kg before you add you sliding hardware.

If this is the case remember that while you can counterbalance the weight that the mass will stay the same and at the speed of most board stuffers decell can be a problem. You could end up with the same effect as a sledge hammer in zero gravity. Making any part you need to acell or decell very low mass.

I would also guess your cylinder stroke will not be much, so venting the air should not be a problem. I would go with plumbing the ports together then switching to rear port to P and front to T(vent). Just calculate your difference between push and pull to see if the force will be right for you with your rod size. I would be afraid to try a self relieving regulator at high speed but I may be very wrong about them.

Big ports, big and short hoses. Maybe a small air tank nearby with 10 times the volume of the cylinder.

Barry1961

 

[globia]

Our Z-axis is a cross roller slide with a 50K count encoder. In fact in our standard equipment, the head is attached to a load cell to give feed back of the load...the problem is that we have to travel to a slow crawl at the end of the stroke in order to protect from a crash. The air cylinder is being used instead of a load cell for applications where we want to pick and place at a faster rate (with less accurate loads). The head holds a flat vacuum tool which picks up the parts (not a gripper). We will need to do some calculations as to how much stroke we need to decelerate the z-slide. Thanks for the input.

 

[Barry1961]

This is going to sound a little crazy..........

How about a hard stop on your pick stroke? Maybe with a small hyraulic shock to cushion the blow. ACE and Enydine makes little shocks.

Any compliance at the pick point(under tape?) would be a lot less mass to deal with.

It just seems to me that you should be able to stop accurately enough to pick up a part quickly when using a servo system. Maybe your reflected inertia is to high or tuning is off. If you have any integral gain in your tuning I would dump it and try 10:1 P ratio. Turn up your proportional until it whines then back it off a bit.

Good luck

Barry1961