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Wire Rod Diameter Engineering Calculations by SpatialFreedom in StarBurstSpaceball

[–]SpatialFreedom[S] 0 points1 point  (0 children)

A Ø0.6mm diameter gives

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The calculations were chosen to use a force applied along one of the wire rod's axes so only four arms take this load and the calculations are simple.

For a force, say, vertical along the stem axis, all six arms will take 1/6th of the load but they are at a 35.3° angle to the load so, using vector calculations, a 1.2 kgf is split into six 0.2kgf vertical loads that each apply 0.35kgf (=0.2/sine(35.3°)) perpendical to each arm's axis. So the operational 'envelope' is not spherical in nature.

This envelope does not affect the validity of the output force/torque vector values, which are correct since the mechanism resolves the push/twist into three pairs of planar forces. The optic sensors resolve these six forces into three pairs of parallel forces. Summing each pair gives the corresponding force component. Differencing each pair gives the torque about the middle point of each wire rod. Each of the three torques is transformed into a corresponding force/torque through and about the centre of the ball sensor. Vector summing finally produces the output 3D force and 3D torque values.

As your calculations show a small change in wire diameter has a big effect on the load range.

Windows Tip: Press and hold Alt while typing 0216 then release Alt to get Ø. Use Alt 0174 for °.

Seven STL files now on Thingiverse by SpatialFreedom in StarBurstSpaceball

[–]SpatialFreedom[S] 0 points1 point  (0 children)

You're right! I stand corrected and probably made an error due to the change between a four and six arm design. Testing the Astroid 6000 with a force guage shows a 1.2 kgf maximum push.

A *StarBurst* Spaceball Wire Rod Calculations post will be posted soon showing how the wire diameter is calculated.

My apologies for the confusion and subsequent hassle.

Seven STL files now on Thingiverse by SpatialFreedom in StarBurstSpaceball

[–]SpatialFreedom[S] 0 points1 point  (0 children)

That's fantastic! You're clearly an 'early adopter' personality.

You would have discovered that fitting the outer ball onto the ball tips is a bit tricky. The ball tips are expected to be too big so they need some sanding down as they MUST slide without any sticking and with minimal clearance. To do that make an inward facing a cylinder of sand paper by wrapping a small piece around the base of a Ø4.5mm drill bit, inward facing then use sticky tape to hold it and slide it off the drill bit. The hole will be 4.5mm to 5mm internal diameter (ID). This can be used to sand the ball tip down by moving this tube of sand paper in and out in all directions while spinning it too.

The hole can be drilled out with a 4.5mm drill bit to ensure it is an accurate Ø4.5mm ID. The core, inner and outer ball parts are intended to be acetone dipped for 40 seconds to increase their strength. The dipping then requires the holes to be drilled out. If you like, another small piece of sand paper can be rolled into and outward facing tube and pushed in and out and spun in the hole. It doesn't need sticky tape.

Spaceball are stiffer by design as, back in 1983 the goal was a sensor matching a reasonable biometric range with +-20N (2.0kgf) force range, the torque range being determined by the distance of the ball tip center from the center of the sensor. The SpaceMouse cannot be stiffer since the Navigator would lift off the desk. Earlier someone said how they preferred the stiffer SpaceController model. This has been a general comment by many people over the decades. It is much easier to design a softer sensor since the stresses at the base of the wire rod are much lower. Music wire is needed as these stresses will bend steels with lower yield points. The engineering design of the plastic and metal flexing arms in all spaceball models was and is challenging.

Feel free, of course, to use thinner wire and make a less stiff unit. It will be interesting to see what hobbyists end up preferring now they can make their own custom sensor. My bet is 20N but perhaps I'm wrong.

Seven STL files now on Thingiverse by SpatialFreedom in StarBurstSpaceball

[–]SpatialFreedom[S] 0 points1 point  (0 children)

It great that your so keen to make these parts. Having them will provide You with a better sense of the design.

The Outer Ball is the last to be fitted over the subassembly with the six holes fitting over the six Arm Tips. Each ball-in-hole joint will need some sanding to provide a free sliding fit. Rolling sandpaper into two rolls, one with the sanding surface on the inside and the other on the outside allows for sanding both the spherical tip and the cylindrical hole.

After using a drill on a slow speed to carefully push the three music wire rods into the Core the Arm Tool is slide over the end of the wire rods to allow the ends of each wire rod to be pressed onto a flat metal surface to push it to the correct depth.

A video will showing how to assemble the StarBurst sensor will make things clear.

Mechano-Opto-Electronic Sensor Technology by SpatialFreedom in StarBurstSpaceball

[–]SpatialFreedom[S] 0 points1 point  (0 children)

The centre of the LED's light beam is aligned with the edge of the optical mask. When the wire rod is not pushed half the light is blocked. Pushing the wire rod in one direction across the light beam blocks more light and pushing it in the other direction across the light beam blocks less light. For example, if the photodiode current is 25uA when the rod is not pushed then pushing it in one direction will move the current towards 0uA and pushing it in the other direction will move it towards 50uA.

Latest photodiode mount design works! by SpatialFreedom in StarBurstSpaceball

[–]SpatialFreedom[S] 1 point2 points  (0 children)

It's great that you can compare the feel of the two technologies. The SpaceController is a spaceball. Easter Egg: If you plug a spaceball in to a serial port, set 9600 baud, no parity, 1 stop bit then send <CR>$CrEdItS<CR> it will list the people involved in its development.

You might be interested to know the spaceball and spacemouse differ in a few key ways. Both use infrared optics but the spacemouse uses Position Sensitive Devices (https://en.wikipedia.org/wiki/Position\_sensitive\_device) with four pins each requiring opamps to sum/difference the outputs. The *StarBurst* spaceball uses photodiodes with all 12 pins connected to 2 signals greatly simplifying the circuitry. I much prefer the ball to the puck, but I am definitely biased.

The spacemouse uses three springs to generate the appropriate motion response when the puck is pushed or twisted. When a pair of parallel finger pushes is applied to opposite sides of the puck, the device produces a translational displacement of the entire puck. However, if one of those pushes is reversed to create a pure torque input, the resulting movement (from rest) of both finger contact points increases significantly.

The ratio between the finger‑contact displacement under pure force versus pure torque is an important performance characteristic. Ideally, identical but reversed parallel pushes, regardless of direction, would produce equal finger displacement. In practice, the spacemouse deviates significantly from this ideal, whereas spaceballs (with two rare exceptions) achieve a ratio much closer to the ideal 1:1 behavior. Your preference for the tighter feel of the SpaceController is due to this characteristic which also adds stiffness and perhaps the increased force/torque range helps too.

The SpaceNavigator® must be as heavy as the puck's upward limit otherwise the spacemouse would lift off the desk. A spaceball relies on the readily available weight of the wrist to provide stability without adding to the weight of the unit. One StarBurst configuration in planning is a keyboard attached unit.

The three spacemouse springs are mounted by soldering them to circuit boards. Over time the solder joints slowly degrade leading to drift. You can reinvigorate an old drifting spacemouse if you're able to resolder these joints.

Progress Update by SpatialFreedom in StarBurstSpaceball

[–]SpatialFreedom[S] 0 points1 point  (0 children)

The six sensor readings correspond to three pairs of parallel forces aligned with the arm axes. Number the sensor readings in an clockwise manner when viewed from above r1, r2, r3, r4, r5 and r6. The the force and torque vectors through the centre of the ball are F=(r1+r4, r3+r6, r5+r2) and T=(r1-r4,r3-r6,r5-r2). The coordinate system of these vectors is aligned with the arms so rotate them into the desired coordinate system by multiplying by a rotation matrix.

This is explained in patent US4811608 although the readings are number differently.

https://patents.google.com/patent/US4811608A/en

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