Quite an Unusual one

[Jonathan]

Answers to questions:

1) Ballscrews give a very large mechanical advantage. You do not need to worry about the weight of the Z-axis so much. See here. You'll be fine with a generic 3Nm motor and 70V stepper driver. The closed loop ones are a good concept, but cost a huge amount more for little gain.

2) Don't use RM1610 - it doesn't gain anything.

3) I wouldn't go with the long belt idea, especially when using servos as servos work best when they are rigidly coupled to the load - i.e. without a long stretchy belt.

You may well be fine with two stepper motors - have you tried putting the values in this spreadsheet?

If using one servo, the formula you need is this:

Je=M*(L/(2*pi))^2

Je is the equivalent inertia, L is the pitch of the screw (so 0.005m with 1:2 ratio) and M is the mass of the gantry.

Next find the inertia of the servo motor rotor (Jr), e.g. it says in the datasheet for 110ST-M04030 is 7.61kg-cm^2. You then need to find what 'inertia ratio' the motors will tolerate - typically it might be Je/Jr<5. So just use the above formula to calculate Je, divide it by Jr (7.61kg-cm^2) and if the result is less than 5 the motor is probably suitable.

e.g, M=120kg, L=0.5cm (1:2 ratio):
Je=120*(0.5/(2*pi))^2=0.76kg-cm^2

Je/Jr=0.76/7.61=0.1.
0.1 is much less than 5... so that servo is plenty big enough. Another way to look at it is rearrange the formula to find the mass a particular servo will drive, e.g:

M=Je/(L/(2*pi))^2
Je=Jr*5=7.61*5=38.05
M=38.05/(0.5/(2*pi))^2=6000kg(!)
Or if the ratio is 1:1, that's
M=38.05/(1/(2*pi))^2=1500kg
(So note the quadratic relationship - if you double the ratio the drive-able mass goes down by a factor of 4)

The rated speed of that motor is 3000rpm, so the gantry would go at 3000*0.01=30m/min with 1:1. You need to check the Je/Jr ratio - Je=Jr gets the most efficient energy transfer, but the drives do tolerate a mismatch so you need to find how much, as going with them equal generally results in an oversize motor. You should also include the inertia of the pulleys in the system - just work out their inertia (assuming cylinders is near enough) and add them to the Je value, remembering to take into account the drive ratio.

Either way, you can see that that servo is far bigger than you need... so I suggest you put the formulas in a spreadsheet and find which motor is best. Also try the stepper motor spreadsheet I linked to earlier, as I expect you'll find that there are suitable stepper motors available.

4) If you use stepper motors, and select the correct size motors, then they wont stall unless you crash the machine. The only time the closed loop servos help therefore, is if the machine crashes. For the same amount of money as the closed loop stepper you could get a much higher torque standard stepper motor and be certain that it wont stall.

[Jonathan]

I forgot to mention...I'm not keen on the placement of your Z-axis ballscrew. If a large force is applied parallel to Z, the axis will begin to skew (i.e. rotate in the Y-Z plane), due to the ballscrew placement being off center. The effect can be reduced by increasing the spacing of the Z-axis linear bearings, or go back to two ballscrews, or get the ballscrew closer to the center. That's why on the new machine I placed the ballscrew intersecting the Z-axis ram - it got it closer to central, but admittedly still not ideal.