Hello Guest it is April 25, 2019, 12:37:02 PM

Author Topic: Servo Basics  (Read 6090 times)

0 Members and 1 Guest are viewing this topic.

Offline Tef9

*
  •  89 89
    • View Profile
Servo Basics
« on: January 19, 2013, 05:33:19 PM »
Hi There,

I have bought a servo system (DMM tech) I am now in a process of learning how to set this up.  Here is my problem:

I have bought oversized motors, so I can direct drive the mill.  I am trying to work out the steps per value, I have been told to use the value of 2000 as my ballscrew is 5mm pitch and 1:1 ratio this seems right to me. However this seems to limit the mill to 750mm/m do I need to modify the kernal speed or is there another way to get higher speeds?  The encoder is a 14bit enconder, any help would be appriciated as I would like to understand how to set these values and the DMM documentation is of little help in this area.

Thanks,

Andy

Offline Hood

*
  •  25,785 25,785
  • Carnoustie, Scotland
    • View Profile
Re: Servo Basics
« Reply #1 on: January 19, 2013, 06:13:02 PM »
You will have to use a higher kernel if you need faster rapids. That is why a lot of people that use servos, especially when the servos have high res encoders, use external devices such as SmoothStepper or CSMIO tc as they can pulse very cleanly at up to 40 times more than even the highest kernel in Mach when using the parallel port.
Hood

Offline Tef9

*
  •  89 89
    • View Profile
Re: Servo Basics
« Reply #2 on: January 19, 2013, 06:25:45 PM »
Hi Hood,

Thanks for the response, do you alter the steps per value in acordance with the kernal speed.  Do you loose anything by having a higher kernal speed?  I have no interest in getting supper fast rapids (though it would be nice to see) would just like the system to work faster than 750mm/m.  Is there a calculation I can follow? Is smooth stepper recomended if you are using a servo system?  Sorry for all the questions.

Thanks,

Andy


Offline Hood

*
  •  25,785 25,785
  • Carnoustie, Scotland
    • View Profile
Re: Servo Basics
« Reply #3 on: January 19, 2013, 06:38:55 PM »
No steps per unit stays the same as that determines how far the axis moves when you command a move.
Some computers can handle higher kernels, some can not, you will just have to see.
You can work out the max rapids you can possibly achieve by doing the following.
For 25KHz it would be (25,000/2000) x 60 gives you units per minute max in that kernel
45KHz would give you 1350units per min max etc etc

External devices are nice if you use high resolution encoders and want reasonable rapids.

Hood

Offline Tef9

*
  •  89 89
    • View Profile
Re: Servo Basics
« Reply #4 on: January 19, 2013, 07:07:20 PM »
Ahhh ok, how does the encoder affect the rapid? I have a 14 bit encoder, so I belive this to be quite high...perhaps a smooth stepper would be advisable..
I also found the rapid overide in the settings, is this adviseable or not?

Thanks,

Andy

Offline Hood

*
  •  25,785 25,785
  • Carnoustie, Scotland
    • View Profile
Re: Servo Basics
« Reply #5 on: January 19, 2013, 07:49:43 PM »
The encoder count determines the steps per unit you require. Yours is 2000 you said so what kernel you will need will be dependant on the rapid speed you require.
Some computers can manage 100KHz kernel, some struggle at 45KHz, you will just have to see.
If you did manage 100KHz kernel then the max rapid you can achieve with that is 3000units per min. Thats not really that fast but suppose it depends on type of machine. My Beaver NC5 Mill is 8000mm/min, big lathe 10,000mm/min and Chiron VMC is 20,000mm/min.
How fast a rapid will also depend on your motors max RPM.

Hood

Offline Tef9

*
  •  89 89
    • View Profile
Re: Servo Basics
« Reply #6 on: January 20, 2013, 08:46:16 AM »
how do you derive the step per from the encoder, whats the calculation?  I was told to use 2000 however I still have no actual calulation to preform.  The encoder is 14bit where would I go from there?
I see what you mean about rapids, however it sounds like you have larger machinery and enclosed, I would be happy with around 5000mm/m but 3000mm/m is a good start point.  The servo is limited to 1000 rpm.  I have 16mm dia screws with a 5mm pitch.

Thanks,

Andy

Offline Hood

*
  •  25,785 25,785
  • Carnoustie, Scotland
    • View Profile
Re: Servo Basics
« Reply #7 on: January 20, 2013, 09:09:36 AM »
Ok 5000mm/min is the max you will get with 1000rpm motors and 5mm pitch screws direct coupled.

Regarding the encoder, I am not sure how they are worked out in that setup. That is not normal encoders so you will have to look in the DMM manual to see how they derive that number.
Hood
Re: Servo Basics
« Reply #8 on: January 20, 2013, 03:41:55 PM »
A 14 bit encoder has 16384 steps per revolution which provide you 5mm/16384 minimum step size or .0000012 inch per step. This choice of encoder with a 5mm pitch leadscrew will be slow with any computer and unless you have it on a machine that has significant stiffness and flatness, is an overkill. On most home builds or lower level mills like republic, the most you can dream for is .0001" and really better than .0002 is unlikely. .0002 would represent 16 steps on your encoder leads crew combination. you would be better served by a 10 bit encoder which is 16 times faster at the same kernal speed. or a compromise such as a 12bit with a 4 times improvement in max speed.

Good luck

Offline Tef9

*
  •  89 89
    • View Profile
Re: Servo Basics
« Reply #9 on: January 20, 2013, 03:51:43 PM »
Hi Hood,

Ahh so the motors are capable of 5000mm/m but only by useing an external device like smooth stepper?  Without an external device if I use 100KHz / 2000 * 60 this gives a figure of 3000mm/m maximum or am I missing somthing?

Thanks,

Andy