Aluminium Extrusion Router Build

[JAZZCNC]

By closed loop stepper system I assume you mean a digital drive and a stepper motor with encoder feedback?

Yes exactly that.
There are four main advantages to servo over a Closed-loop stepper. #1 Speed #2 torque #3 Smoothness #4 Resolution
But what you really have to do is examine the differences and ask your self-do i really need to spend 4 to 5x more per axis for percentage difference they give on a machine of this size for what your going to do with it.?
Lets look at differences.
#1 Speed.
The difference between 3 phase Closed Stepper is 1500rpm vs 3000rpm. So twice the speed but In real world terms this mostly applies to Rapid speed only because cutting speeds are mostly dictated by tool and material not the motors.
1500rpm with 10mm pitch will give 15,000mm Rapid speeds and easily allow 7-9000mm/min cutting speeds in most materials.
Now 15,000mm/min covers 250mm per second so with travel of 1000mm your end to end in 4secs. Now obvioulsy got to factor in accelration/deceleration etc but you see even with stepper motors your not speed limited.

#2 Torque. This is where servo's really win over Steppers because they have full torque rating at Max rpm and upto 3x torque rating for period of few secs if required.
Stepper system on the other hand only gets close to full torque rating when stood still then starts to drop away as rpm increase untill point when completely losses torque and motor stalls at point when motor gets saturated. Gets little deep as to why but usable rpm range for stepper is 1200-1500rpm dependant on setup/voltage. I know from expereince 3 phase closed loop motors tend allow 1500rpm max.

Now if your trying to move heavy Mass at high speed or accelerate/de-accelerate at very high speed then Constant torque of servo is what you need. However on Small/Medium router with relatively light/Medium weight(upto75Kg) gantry then correctly sized Stepper system which is tuned correctly will give more than good enough performance.

#3 Smoothness
No doubt Servo's are smoother than steppers, provided they are tuned and setup correctly. However again unless doing very high detailed 3D type work or work that requires very high quality surface finishes then the difference isn't really enough to justify spending 4x the cost.

#4 Resolution
This basicly means smallest travel amount. The Encoder fitted on servo dictates resolution and to large degree smoothness of the motor.
The higher the Encoder count the smaller the travel distance and smoother the action of the motor. It also dictates to some degree the motor speed when combined with pulse rate. Higher the count the more pulses are required to reach rated rpm.! . . . .But for now lets just think about resolution.
Typical servo will use 2500CPR encoder CPR stands for counts per rev. However this is little missleading because of how they work and in actually fact the encoder will typicly be quadrature encoder meaning uses 2 channels A/B which are 90deg out of phase and because it reads the leading and trailing edge of each channel this becomes x4 hence quadrature.
So total counts per Rev is 10,000 or 0.036deg of movement per Encoder count or if connected to 10mm pitch screw becomes 0.001mm travel per Encoder count.

Stepper motor on the other hand works differently in that you set number of steps per rev which then get broken down into micro steps by drive. Typical stepper will use 1.8degree or 200 steps per rev which then each step gets broken down into Micro steps depending on setting within the drive.
Closed loop steppers come in 2 flavours 2 phase or 3 phase. 3 Phase stepper gives finer resolution because the steps are less typicly 0.9deg or 1.2deg.
Typical Micro step to apply is 1600 so I'll work on lower resolution 1.8degree stepper to calculate. 360deg /1600 =0.225deg or with 10mm pitch 0.006mm travel per Micro step.
Now this is little false because the truth is some loss will occur to fact difficult for motor to actually resolve to this degree. So if we take worst case scenario and use 200 steps which is called full stepping the smallest travel amount becomes 0.05mm. In real world use you'll easily achieve 800Ms giving 0.0125mm.
Closed loop systems use Hybrid setup which works between Stepper and Servo using lower count encoder typicly 1000CPR (x4) so when combined with 3 phase 1.2deg motor gives 0.0025mm with 10mm pitch.

So has you can see in terms of actuall travel distance both give more than enough resolution the main difference of servos higher encoder count being motor smoothness. This does however come at cost of requiring more powerful pulse engine, which isn't problem with cslabs anyway but can be for other lesser cards.

So the real point of this lecture being is it really worth paying 4 to 5 x per axis more for the extra speed you won't use, Torque you won't need, resolution you can't use and with the only real viable contender being slightly smoother movement.?

Then factor into the mix the potential complexitys of setting up servos, which these days is much simpler. However they don't tolerate poor setup or sloppy electrics and require higher degree of eletrical work to avoid potential problems which can affect them. Ie Electrical interference.
This add's to expense because will require line filters etc and highe quality cables which stepper system will happily run without.

Stepper systems in general are bullet proof when sized and installed/setup correctly.

My advise is save the Money and spend in other areas. Ie Spindle or better ballscrews.
You'll probaly save £300 per axis so that's £1200 which would go nicely towards an ATC spindle.!!