High Precision Linear Stage
Micron Level Accuracy on a Budget
Putting together a few basic requirements to work towards on this project.
A comparable industrial solution is $2500+
A stepper motor with a built-in leadscrew was chosen as an economical solution for motion
The included lead nut had terrible backlash. Backlash is the slop that exists between the male and female threads. It is the "wiggle" that a nut has when thread onto a common screw.
Backlash is bad, because during directional changes, there is lot motion, and that results in inaccuracies of positioning.
Easy solution is to pre-load two nuts against each other. This practically eliminates the backlash. As a bonus, as the lead nut wears due to friction, the spring will automatically take up the excess slop.
A design was formed that embraced these concepts
Ever try to push something across a rough table very slowly?
The object will resist motion, and then all of a sudden lurch forward. This is the difference in material coefficients of static and dynamic friction. This is aptly named stick-slip
Well if you want to move a lead nut a few micron along a lead screw, the same thing will happen.
So the lead nut was made from a special material with coefficients of static and dynamic friction that are almost the same. The greatly reduces the stick-slip phenomenon.
Finite element analysis ensured that the lead nut design would not flex excessively under the load specification.
Linear rails were chosen as the motion support system.
Calculations showed that with the linear rails would support the required load for 10,000 km before failure.
Plenty of life for this application.
The main body of the stage was designed so reference surfaces could be machined in the same clamping on the CNC.
This method allows for incredibly high tolerances to be held without inflating the price.
The blue surfaces show the important surfaces, the linear rails mounts and the stepper motor face mount
Here are the linear rails installed into the body.
The linear rails are pressed up against a machined flat edge. This keeps everything in line, and eliminates any binding that could occur.
Next in is the stepper motor assembly.
The the linear stage carriage is installed with the first part of the sealing system.
If you think that that part looks like it has a few weak flanges...
FEA confirms that this part is more than strong enough too.
Maximum deflection is only 4.7 microns.
Here is the sealing system.
A flexible rubber belt is fed through a pulley system internally in the carriage assembly.
Works great!
Soft limits are achieved with hall effect sensors on each end of the linear stage. The trigger point is adjusted by changing the position of the magnet which is mounted to a fastener.
Just in case the soft limits fail, hard limits are also present to protect the hall sensors from being crushed.
Machining the body
Machining the lead nut
Initial prototype
To measure the position accuracy, a Heidenhain MT-2501 was used. This tool has a travel range of 25mm with an accuracy of +/-0.2 micron. This is 100x more accurate than the linear stage.
Results come in 3 stages.
1. Untuned Driver
A bit complicated, but the bipolar stepper motor driver was running in full step mode. By reprogramming to micro-step (send partial current to both motor poles simultaneously) the cyclical error was reduced.
2. Micro-step Enabled
Much better! But still a linear error trend. This is due a slight inaccuracy in the pitch of the lead screw.
3. Compensated Micro-step
By mapping the linear eror back to the driver firmware, the lead screw pitch error was eliminated. This positioning error down to less than 14 micron per 25mm
Quick shout-out the stellar results to repeatability and backlash
Repeatability, sending the stage to the same position multiple times, and measuring how close it comes back to the same spot.
Backlash, measuring the lost motion when the stage is reversed in direction.
After it was all said and done, the component costs were added up for all the parts in the assembly.
Total cost was $309
Requirements were crushed.
Project a success!
Linear stage in action!