Counterweights for heavy lenses on vertical rails?

[genera]

I have some very heavy lenses, some more than 10 kg. This is far above the rated vertical capacities of most motorized rails. One idea I've had is to use a counterweight to balance the weight of the lens, like elevators use. Has anyone tried this? Any advice?

I have a Nikon MM40 stand that uses a large coil spring as a counter-balance (think of a spring in an auto retracting tape measure, only much thicker and much wider). The top of the moving stage is attached to the spring, on the back side of the column, with two small diameter wire ropes. The wire ropes are guided by idler pulleys. One pair above the sliding stage and a second pair above the spring.

I also have an old Nikon Video Measurement Microscope stand, similar to the MM40 stand but much more robust. Instead of a coiled spring it uses a heavy iron counter weight suspended from the same sort of wire rope set up. The weight is guided by a rod running through a vertical hole in the weight's center. I think a guide of some sort, especially in a high resolution set up, is essential to reducing unwanted movements of the sliding stage. I'd take it a little farther though and use, instead of a loose fitting rod, a guided rail to help control the motion of the weight, similar to the first item Ray linked to in his post just a little above this one. Some high end milling machines use a similar setup.

Both of these systems use a stepper motor on the vertical axis that's the same size as the motors used on the relatively low load xy stages. This leads me to believe that the counterbalance setups help in regard to moving the stage efficiently. I think the torque component may still be a problem though unless you find a way to suspend the weight of the camera and lens instead of just suspending the slide where it meets the rail. I think the Nikon columns get away with it because they have long bearing contact areas, essentially performing the same function as what Ray describes as an idler rail.

You might also consider an air spring of the type used to hold open the hood on a car. That could be installed on the front side of the column, one end on the base and the other above the camera.

[Lou Jost]

Nice to hear that the counterweight idea is being used in microscopy. That plus idler rails is probably the DIY route I will take for now. I appreciate everyone's help!

[enricosavazzi]

Counterweights work fine for manually operated focusing racks, but stepper motors typically rotate in a stepwise fashion, rather than smoothly. With a counterweight that uses mass (literally, one weight used to oppose another), the moving parts end up weighing twice the mass of the lens, camera and moving stage, and the resulting increase in inertia can be a problem for the motor and other mechanical parts.

The problem can be lessened if you use springs instead of a massive counterweight. Another thing to do is use microstepping and gradually accelerate and decelerate the motor in software, rather than starting/stopping it abruptly. Using a stronger motor and driver may also be necessary.

[ray_parkhurst]

Please excuse the quick and badly-scaled drawing but this should convey the idea.

Your drawing conveys the approach very well! Thanks for this.

So the idler rails constrain motion to a single axis of translation, leaving the focus-stacking rail to perform only one job: provide motive force within that axis. Elegant!

--Chris S.

Exactly.

Rotate the system 90-deg CCW and it works as a moving optical table, eliminating some major sources of vibration and motion artifacts.

[ray_parkhurst]

Counterweights work fine for manually operated focusing racks, but stepper motors typically rotate in a stepwise fashion, rather than smoothly. With a counterweight that uses mass (literally, one weight used to oppose another), the moving parts end up weighing twice the mass of the lens, camera and moving stage, and the resulting increase in inertia can be a problem for the motor and other mechanical parts.

The problem can be lessened if you use springs instead of a massive counterweight. Another thing to do is use microstepping and gradually accelerate and decelerate the motor in software, rather than starting/stopping it abruptly. Using a stronger motor and driver may also be necessary.

The additional dynamic load is indeed a potential problem.

Note that you don't need to maintain counterbalance over a very wide range, only across the depth of the stack. There may be a clever way to implement a simple spring mechanism that can be removed for major movements of the system, then put in place before stacking.

[enricosavazzi]

Since the movements of the stage will be slow, you could look into the industrial spring tool counterbalancers used to carry most of the weight of hand tools on assembly lines. The best of these springs have internal mechanisms that give an almost constant spring force throughout the extension of the tool suspending rope (usually 1 m or more).

Like in most things today, there are good expensive springs of this type made in Europe and the US, and cheap ones of unknown quality made in China and sold on eBay.

The force is also adjustable within a given range, so you need to know the approximate weight of the camera, lens and moving platform to choose the right model of spring.

[Lou Jost]

This is excellent advice. Thanks very much, the spring route has clear advantages over counter-weights.

[genera]

. . . With a counterweight that uses mass (literally, one weight used to oppose another), the moving parts end up weighing twice the mass of the lens, camera and moving stage, and the resulting increase in inertia can be a problem for the motor and other mechanical parts.

It helps a little that you don't need to balance the moving components with the counterweight, you're just trying to lessen the load on the motor and leadscrew.

If you're using a StackShot I don't know what the options are for controlling acceleration/deceleration. They do say the ramp time is variable but don't say anything about the range. With a more conventional motion controller you can tune acceleration so that inertia won't be an issue.

[ray_parkhurst]

. . . With a counterweight that uses mass (literally, one weight used to oppose another), the moving parts end up weighing twice the mass of the lens, camera and moving stage, and the resulting increase in inertia can be a problem for the motor and other mechanical parts.

It helps a little that you don't need to balance the moving components with the counterweight, you're just trying to lessen the load on the motor and leadscrew.

If you're using a StackShot I don't know what the options are for controlling acceleration/deceleration. They do say the ramp time is variable but don't say anything about the range. With a more conventional motion controller you can tune acceleration so that inertia won't be an issue.

Not using the counterweight will for sure help with dynamic load. From the specs on THK rails, dynamic load rating is ~50% of the static load rating, not taking torque into account. So adding a counter weight is exactly the wrong thing to do. Using a spring to balance the static load will not change the dynamic load much, but if done properly can greatly reduce the static torque. The problem of dynamic torque still exists, and although it is not specified I'd expect the rating to be similar to the static torque numbers, which are not very high for THK and I'd expect them to be even lower for the StackShot.

edited to add: I think the best solution is an idler to reduce both static and dynamic torque and spring-loading to reduce static load, plus (if possible) motion optimization to minimize effects of dynamic load.