Manual machining still possible?

[tblough]

True manual movement refers to a handwheel physically connected to the ballscrews where the operator moves the machine. In that case, Acorn has no provisions to read the encoder output of the scales to know position has changed.

With MPGs, the operator is telling the control to move the machine, so the control knows it how far it was commanded to move and adjusts the position accordingly. Unfortunately, the Acorn control has no way to verify the position.

[suntravel]

With Acorn Six you can have full closed loop with scales or rotary encoders, you can also have the scale position on the DRO.
But turning an axis by hand is no option, even with disabled servos, only moving the axis with buttons/mpg ect.
I also considered using an analog joystick, but for me, it is on the save side to have one MPG for each axis with 1x 10x 100x buttons.

The s curve acceleration is also active with MPG or jogging with the VCP.

Uwe

[Muzzer]

I'm not a fan of joysticks or "jog" controls, as they turn movement of the stick into velocity, whereas an MPG handwheel operates rather like a handwheel on a manual lathe, with the added bonus of adjustable "gain" (how much movement results per turn of the wheel) and a wireless MPG controller allows you to operate the machine from any convenient (or safe!) vantage point.

As long as you have your machine set up correctly, you won't lose steps and position with either manual or automatic control. As Uwe says, you can go for fully closed loop control on Acorn 6 but it doesn't really change the way it works. I have closed loop control on one of my lathes (under Linuxcnc) and although it may be slightly more accurate than open loop, it's not radically better and took more fiddling to set it up optimally.

All my machines have some form of MPG and I couldn't imagine operating them differently when I use manual mode. They also allow program start/stop/pause, feed override, macros etc. Marvellous things!

[MartinC]

True manual movement refers to a handwheel physically connected to the ballscrews where the operator moves the machine. In that case, Acorn has no provisions to read the encoder output of the scales to know position has changed.

I should have explained that better - I did not mean to disconnect the servo from the ballscrews for manual machining (as I understand this wouldn't work anyway as the ballscrews are not self-locking as the trapezoidal screws are).
What I meant was whether you could ergonomically handle the machine (like using an edge finder or touching off with a tool) using the controls provided by Acorn. So far noone seems to complain...

[MartinC]

I'm not a fan of joysticks or "jog" controls, as they turn movement of the stick into velocity, whereas an MPG handwheel operates rather like a handwheel on a manual lathe, with the added bonus of adjustable "gain" (how much movement results per turn of the wheel) and a wireless MPG controller allows you to operate the machine from any convenient (or safe!) vantage point.

I see the point... yes, with a joystick you have to be careful, particularly with progressive operation (gong from 0.01 mm/s to full speed within a few mm of joystick travel.

[MartinC]

With Acorn Six you can have full closed loop with scales or rotary encoders, you can also have the scale position on the DRO.

Can you explain how this works? Currently I have glass scales on all axes wired to the DRO.

The s curve acceleration is also active with MPG or jogging with the VCP.

VCP are the buttons on the (touch-)screen? Do they have an acceleration built in (increasing speed)?

What about s curve acceleration with the MPG - as I understand it, if it is set to 1 mm, one click of the wheel gives 1 mm, regardless of speed?

Martin

[suntravel]

You can display the commanded position or the scale position on the DRO part of the Centroid screen.

Every move is with s curve acceleration, one click of the MPG or a lot of fast ones, but even if you turn the MPG very fast, it will not exceed the max rate of the axis. You can also dial in the acceleration time. This is making the axis movement very smooth.

Uwe

[GBCues]

I have a slightly different question on manual machining.
If you have the Acorn powered off (and everything else), you can turn the stepper motors to move the axes. So in that sense you are manually machining. However, is there a danger of generating reverse voltage from the steppers back into the drives or the Acorn and damaging them?
Thanks
Gary

[MartinC]

I guess this is independent on Acorn.
Turning any PM motor (regardless if PM servo or stepper) will generate a back EMF voltage (proportional to speed) that goes back into the driver. Whether the driver could be damaged or not depends on the driver. I'd guess mostly not, but you are generating uncontrolled voltages that go into the bus voltage. In the extreme case (probably impossible by hand cranking, but very real by decelerating a rotating axis) the generated voltage could exceed the max. voltage of the driver (and the acorn if connected to the same e.g. 48 V). For that reason higher power drives have brake resistor control. In the stepper world, system builders mostly seem to ignore this (so it can't be that much of a problem). But I have killed a lab PSU once by braking a small motor from full speed to zero. I'll use AC servos as described lately by Uwe and I hope they handle braking properly.

Besides the (probably not so likely) potential damage to the driver, the main reason why I would avoid turning motors with power switched off is that the current you induce by turning the motors create a (roughly velocity proportional) counter torque, which feels like someone has filled your motors with honey. So if you want to turn the motors by hand, apply power to the system, but set the driver enable to false. This way the driver power stage has bus power (like 48 V) and unless the generated back EMF of the motors is larger (which would happen if you rotate the fast enough, unlikely by hand), no current will flow (through the power stage free-wheeling diodes): No honey-like feeling and no danger to drivers or other connected electronics.

Martin

[centroid467]

I'll use AC servos as described lately by Uwe and I hope they handle braking properly.

Check the manual for the drives you will be using. It should say what which models come with a braking resistor installed or if an external resistor should be provided. Most AC servos systems I have seen between 400 W and several kW include the braking resistor.