Guest
Show Posts
2371
General Mach Discussion / Re: Half Speed Drive
« on: June 26, 2019, 08:08:10 AM »
Hi,
I take it your ballscrews are direct coupled? If so the steps/per unit are fixed by the pitch of the ballscrew and the microstepping
regime you have in place on your stepper drives.
A two phase stepper advances 1.8 deg/step or 200 steps per rev. If you use a five microstep per full step regime that would
mean 1000 pulses would produce one revolution of your stepper. One revolution of your stepper will cause 10mm of axis
travel, assuming direct coupled ballscrews. Thus the 'steps per unit' value in your motor tuning page should be
1000 (steps)/10 (mm movement) =100 'steps per'
Note this is DETERMINED by the pitch, any gear or belt reduction and the microstepping regime.......it is not a free choice.
To test whether you have done the calculation correctly zero the axis and MDI:
g0 x200......the axis should move 200mm, not 199 or 201 but 200mm EXACTLY. This sets the basic accuracy of your machine.
You will do NOTHING successful until you have this perfect.
With 10mm pitch ballscrew and the stepper doing 500 rpm the axis would travel at 5000mm/min. Per my previous recommendation
I would suggest that is a reasonable value for max velocity in the motor tuning page.
Are you using a parallel port? If you are the parallel port has a limited maximum output pulse rate, called the kernel speed.
Machs default parallel port kernel speed is 25kHz.
At 500rpm and 1000 pulse per rev each stepper will require a maximum pulse rate of 500 x 1000 /60=8333.3 pulse per second.
So 500 rpm will mean that Mach can loaf along at about 1/3 its default kernel speed and still produce enough pulses to spin your
steppers to 500 rpm. In fact you could, with the same default kernel speed, have Mach signal your steppers (1000 pulse/rev)
to spin at 1500rpm for an axis speed of 15000mm/min. I would expect your stepper to wimp out and stall long before
you could get that fast though.
steppers spin faster without losing torque. What inductance are your steppers? Less then 1mh is very good to good, about 2mH
is average and 4mH or more is poor.
Note that first time stepper buyers tend to buy on the basis of holding torque.......the higher the torque the better right?
Well......no actually. It is quite possible to wind a motor to have high torque but commensurately high inductance. Thus it would
have a spec of 4Nm and 6mH and so would be lucky to have 0.2Nm at 1000 rpm. The same size motor wound differently
might have only 2Nm holding torque but a commensurately low inductance of 0.9mH and so still have 0.6Nm torque at
1000rpm. Thus the lower torque motor is better overall because it maintains its torque at speed.
The classic way to overcome the drop of torque with speed is to use a high voltage driver. About 80V is the current state
of the art maximum voltage for two phase steppers. What voltage power supply are you using?
Craig
Quote
In the motor tuning my steps per are set to 400, this allows for a maximum speed of 3750mm/min (Cant go any higher ??)
My stepper drivers are set to 1000 pulse/rev (can I set different than that)
I take it your ballscrews are direct coupled? If so the steps/per unit are fixed by the pitch of the ballscrew and the microstepping
regime you have in place on your stepper drives.
A two phase stepper advances 1.8 deg/step or 200 steps per rev. If you use a five microstep per full step regime that would
mean 1000 pulses would produce one revolution of your stepper. One revolution of your stepper will cause 10mm of axis
travel, assuming direct coupled ballscrews. Thus the 'steps per unit' value in your motor tuning page should be
1000 (steps)/10 (mm movement) =100 'steps per'
Note this is DETERMINED by the pitch, any gear or belt reduction and the microstepping regime.......it is not a free choice.
To test whether you have done the calculation correctly zero the axis and MDI:
g0 x200......the axis should move 200mm, not 199 or 201 but 200mm EXACTLY. This sets the basic accuracy of your machine.
You will do NOTHING successful until you have this perfect.
With 10mm pitch ballscrew and the stepper doing 500 rpm the axis would travel at 5000mm/min. Per my previous recommendation
I would suggest that is a reasonable value for max velocity in the motor tuning page.
Are you using a parallel port? If you are the parallel port has a limited maximum output pulse rate, called the kernel speed.
Machs default parallel port kernel speed is 25kHz.
At 500rpm and 1000 pulse per rev each stepper will require a maximum pulse rate of 500 x 1000 /60=8333.3 pulse per second.
So 500 rpm will mean that Mach can loaf along at about 1/3 its default kernel speed and still produce enough pulses to spin your
steppers to 500 rpm. In fact you could, with the same default kernel speed, have Mach signal your steppers (1000 pulse/rev)
to spin at 1500rpm for an axis speed of 15000mm/min. I would expect your stepper to wimp out and stall long before
you could get that fast though.
Quote
I'm using pretty hi power 2Nm steppers on all axis2Nm is not earth shattering, although I'm sure will be adequate. The important specification is the inductance. Low inductance
steppers spin faster without losing torque. What inductance are your steppers? Less then 1mh is very good to good, about 2mH
is average and 4mH or more is poor.
Note that first time stepper buyers tend to buy on the basis of holding torque.......the higher the torque the better right?
Well......no actually. It is quite possible to wind a motor to have high torque but commensurately high inductance. Thus it would
have a spec of 4Nm and 6mH and so would be lucky to have 0.2Nm at 1000 rpm. The same size motor wound differently
might have only 2Nm holding torque but a commensurately low inductance of 0.9mH and so still have 0.6Nm torque at
1000rpm. Thus the lower torque motor is better overall because it maintains its torque at speed.
The classic way to overcome the drop of torque with speed is to use a high voltage driver. About 80V is the current state
of the art maximum voltage for two phase steppers. What voltage power supply are you using?
Craig