My macro stand

December 10, 2013

Back in the Spring of 2012, I spotted an item that looked like some kind of granite table being auctioned as surplus at the University here in Tucson. I was right in the midst of trying to decide what to do about welding up a really stiff and solid (and vibration free) camera stand. I bid on the item after inspecting it in person, and got it for something like $65 and hauled it home. I shudder to think what this whole setup probably sold for when it was a new item. It was on a pallet and was gently placed in my pickup truck with a forklift. Once home, two of us were able to move it safely with some care.

Look at this thread for some discussion of what I am doing with it.
Here is what the original instrument looked like:

It turns out this is (was) a Taylor Hobson surface roughness measurement device. I did not get all of the electronics in the auction (nor did I want it). In fact not having all of the parts made my decision to "gut" what I had and convert it to my purposes easy and remorse free.

What I did do after getting it home was to disassemble it and use a bronze wire brush in a dremel tool to clean rust off the precision ballscrew. I then worked out the electronics used to control the ballscrew motor and wired up an AC line plug so I can move the big head up and down via the paddle switch. It might be possible to replace the AC motor with a stepping motor and do focus stacking directly, but I do not intend to go that route. One caution I received was that the original AC motor had enough gearing that the stage would not back-drive under gravity. If this motor was replaced with an ungeared stepper motor, it likely would backdrive and crash optics into the table when power was removed !! People who have made such a motor swap have also found it prudent to add a brake that clamps the shaft when power is removed. This is something I don't wish to get involved with.

The following images shows the stand in use in a temporarily thrown together setup. I have been gratified that it has entirely solved the vibration problems that previously plagued my photomacrography efforts. Ultimately I intend to machine a brace that will clamp onto the Arca-Swiss style bracket that I have mounted on my camera. I also intend to mount a geared stepper motor screw with controller that I will use to do focus stacking.

Translation Stage

At my favorite electronics store they had a box of stepper motors, and next to the box were two translation stages (precision screw with motor attached). The price was $25.00 each, so I bought them both. I chose the one with a motor and a belt to work with.

This stage has a screw with an 0.2 inch pitch. The motor is coupled to the screw with a 2:1 pulley ratio. With this arrangement, 2 full rotations of the motor shaft yield one rotation of the screw, so 1 full motor rotation yields a linear motion of 0.1 inch. With 200 steps per rotation, we get a resolution of 0.0005 inches per step (half of a thousandth of an inch). This is .0127 millimeters or 12.7 microns.

Weights

The whole point of this rig is to achieve sharp photographic images. This system is using several strategies to this end. First of all, the stand is stiff. This means that if the stand does move, it all moves as a unit, so the motion does not appear in the photograph. Secondly, the stand is heavy, making the amplitude of any motions very small. Lastly, the stand is isolated by rubber blocks from whatever table is supporting it. In truth the mass of the stand plus the elastic blocks are a resonant system which could be analyzed. The weak point of the system is probably the table I use to support it. All told, the whole apparatus weighs 211 pounds. Moving the stand requires two strong men.

Here are some pictures with the newly machined motorized focus stage (in 11/2013) prior to working up the control electronics.

Motor

The motor on the translation stage is a Superior Electric "Slo-Syn synchronous stepping motor". These little blue motors are high quality and familiar to everyone who works on precision instruments. This one is a type M061-LE08 with a 60 oz-in holding torque, rated at 1.25 volts and 3.8 amps. The voltage specification has never made sense to me as everyone runs these with at least 12 volts and sometimes 24 volts or more.

The motor presents 6 wires. I join black to white and orange to black/white, leaving me wih 4 wires to connect to my controller. This is appropriate for a bipolar motor driver, which is what I intend to use. The other wires allow flexibility in wiring. I could access the junctions and drive the motor with a unipolar driver. Alternately I could wire windings in parallel rather than in series which would provide more high speed torque with a bipolar driver like I am using. I am opting for more low speed torque. With the windings connected in series, I measure 0.8 ohms of resistance. I have one winding on the green and green/white pair, another on the red and red/white pair. I can connect these to my stepper driver, find out which way things move, and if I want to reverse direction, switch one of the windings.

The R208 stepper driver

I thought about using transistors (perhaps FET's) to build a stepper driver circuit. I would not have been hard, but I opted to spend the money and buy a decently priced commercial driver. The unit I chose is the R208 microstepping driver from RMS technologies. It can run with a 12-24 volt motor supply and deliver up to 2 amps per phase. Also it allows 1x, 2x, 4x, or 8x microstepping which gives 200, 400, 800 or 1600 steps per rotation with the usual 1.8 degree motor.

The stage I am using has a screw with an 0.2 inch pitch. The motor is coupled to the screw with a 2:1 pulley ratio. With this arrangement, 2 full rotations of the motor shaft yield one rotation of the screw, so 1 full motor rotation yields a linear motion of 0.1 inch. With 200 steps per rotation, we get a resolution of 0.0005 inches per step (half of a thousandth of an inch).

As discussed above, my translation stage gives a resolution of 0.0005 inches per step, which is 0.0127 millimeters or 12.7 microns. With a microstepping controller we can multiply this resolution by 2, 4, or 8 giving a choice of 6.35, 3.175, or 1.5875 microns.

Feedback? Questions? Drop me a line!

Tom's Mineralogy Info / [email protected]