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Category — Servo Motors

Driving a Ballscrew – Servo vs. Steppers

Excellent example of Servo vs. Stepper vs. Feed Speeds. This is by John Dammeyer, developer of the Electronic Lead Screw Project: Electronic Lead Screw Main Page

…now I did some experiments as a result of this posting subject. I found if I turned the handle for moving the carriage at what was a pretty normal speed it went 10” in about 4 seconds. That means 10 inches in 4 seconds is 2.5 ips or 150 ipm.

If your goal is to replace the lead screw with a ball screw and still be able to traverse as quickly as the rack and pinion while maintaining knowledge of the position to the nearest 0.0005” you need to consider how you drive this lead screw and the pitch of the screw.

Let’s look at a stepper based solution first.
1. Max RPM with a stepper and still decent torque is about 700 RPM.
a. Use 600 RPM to be conservative (10 RPS).
b. 10 uSteps per step results in 2000 steps per rev x 10 RPS is 20,000 steps per second. (Gecko Driver)
c. Resolution is 2000 steps per rev but accuracy is still only to the nearest ½ step so 400 steps per rev.
2. With 400 steps per rev and a target of 0.0005” accuracy a pitch of 0.2” or 5 TPI is required.
3. With 10 RPS we’re moving 0.2”/Rev x 10 Revs/Sec = 2”/second (2 IPS). However, your target is 2.5 IPS or better.
4. This system won’t move the carriage quite as fast as you normally would with the handle.

Now a Servo.
1. Max RPM is usually 3000 RPM and torque is good all the way to the top.
2. Aiming for 750 RPM means a 4:1 belt reduction.
a. 4:1 on the belt though results in 4x the encoder resolution.
b. With a 100 line encoder in quadrature you get 400 lines per rev x 4:1 or 16,000 steps per rev.
c. Accuracy is still 16,000 steps per rev.
3. Stay with a 4 TPI ball screw and 0.2” pitch we resolve (and position) down to 0.0000125” per step.
a. But at 20,000 steps per second from the ELS you can really only turn the lead screw 1.25 turns per second
b. That’s 0.2”/rev x 1.25 revs/sec = 0.25”/second (0.25 IPS). A factor of 10 too slow.
4. I think Gecko has a step multiplier that turns the servo 10 lines for every step in.
a. Now we’re at 2.5 IPS which was the goal.
So: A DC servo Motor with 100 line encoder on a 4 TPI ball screw results in a position resolution and accuracy of 0.000125” with a top speed of 2.5 IPS and a step rate input of 20,000 steps per second.

Please correct my math if I’ve made a mistake.

From: E-LeadScrew@yahoogroups.com [mailto:E-LeadScrew@yahoogroups.com]
Sent: March-19-16 8:29 AM
To: E-LeadScrew@yahoogroups.com
Subject: [E-LeadScrew] Re: Ball screw question

Though I am not an engineer it looks to me that the stock design of the
leadscrew on this particular lathe looks to be at fault. The leadscrew has a
slot that runs down the entire length of the screw itself that is used to
power the cross feed. The screw was turned and then the slot was milled.
This left many “teeth” that in a relatively short amount of time “cut” away
the half nut. The half nut itself just pushes forward on a lever. There is
no “backing ” at all to support the half nut and in order for it to track a
considerable about of force is applied to keep the half nut engaged.

I doubt it is the design’s fault. Thousands of lathes use this exact
system to drive the carriage and cross slide without and wear issues. The
half nuts stay engage without any extra pressure also. I am wondering if
someone ground the bed and didn’t take the extra time to align the lead
screw with the carriage? I can assure you something else is going on with
your lathe other than the keyway down the center of the lead screw.
Rick in WA State

[Non-text portions of this message have been removed]

__._,_.___

Posted by: “John Dammeyer” <johnd@autoartisans.com>


Reply via web post Reply to sender Reply to group Start a New Topic Messages in this topic (12)
EXACTLY !

I use a 10.000 count encoder on the servo, and 1:2 belt drive, and 5 mm 
pitch screw.
=> 20.000 counts/rev at screw.
=> 4000 steps/mm.
= 0.25 microns.

This is pretty much the ideal setup.

However,
3000 rpm = 50 revs/sec.
50 revs x 10.000 counts = 500.000 kHz.

So, I must have a hardware timing engine able to pulse at 500 kHz 
(Cslabs CSMIO-IP-S).

End result.
The servo will spin upto 3000 rpm in approx 0.03 secs (20 ms no-load, 
and 20-30 ms with the 100 kg carriage).
This is far too fast, huge wear on belts and screw and ways, and very 
much more than needed.
So, I limit it in sw to something same.

Speed.
At 1500 rpm at screw, or 25 revs/sec
=> 25 x 5 = 125 mm (==5") per sec.
Movement on z axis is approx 400 mm max.
So, end to end is 3.2 secs.
This is far too fast, so I software limit it to something sane.

Generally, the tools are only about 100 mm from the workpiece, and in 
less than a second the lathe has the potential to crash.
Thats why I normally keep the speeds way down.

The 0.25 micron step size results in real-worl, actual, resolution of 
approx 1 micron, or a bit better.
Hope is to get 0.5 microns, and may (or may not) need ground ballscrews.

Engineering theory and lots of real world examples says 0.5 microns is 
routine, given a sufficiently rigid system.
Thats why I use a very think, 32 mm, ballscrew, because it has very high 
rigidity of 54 kgf/um.

(High precision optronics screws with 0.25 mm rise can position to 0.5 
micron accuracy and several manufacturers quote similar accuracies.
Available from thorlabs, about 70€ each.
Its the kind of thing I will be making).

Accuracy is not, at the moment, 0.5 microns, but in theory and practice 
I will probably get there.
Plan is to add glass scales, and secondary feedback to the CSMIO controller.

At the moment, its possible to do incremental movements to 1 micron.

This means I can make gages, with steps, of 1 micron.
By then measuring the gage, one of the steps will be 49.999 mm.

This will then be what I make bore for, to mount high precision bearings.
(7210AC-DUP-P2. Yes P2 or ABEC 9).

The z axis started working last tuesday, after 300 work hours (this time 
round), and 12 years of development and tries.
Alignment was really hard.

Last TS bracket (not really needed) will hopefully be finished today.

Very Big Grin.
Gonna go make bracket.



On 19/03/2016 17:18, 'John Dammeyer' johnd@autoartisans.com 
[E-LeadScrew] wrote:
Now a Servo.
1. Max RPM is usually 3000 RPM and torque is good all the way to the
top.
2. Aiming for 750 RPM means a 4:1 belt reduction.
a. 4:1 on the belt though results in 4x the encoder resolution.
b. With a 100 line encoder in quadrature you get 400 lines per rev x
4:1 or 16,000 steps per rev.
c. Accuracy is still 16,000 steps per rev.
3. Stay with a 4 TPI ball screw and 0.2” pitch we resolve (and
position) down to 0.0000125” per step.
a. But at 20,000 steps per second from the ELS you can really only
turn the lead screw 1.25 turns per second
b. That’s 0.2”/rev x 1.25 revs/sec = 0.25”/second (0.25 IPS). A
factor of 10 too slow.
4. I think Gecko has a step multiplier that turns the servo 10 lines
for every step in.
a. Now we’re at 2.5 IPS which was the goal.

So: A DC servo Motor with 100 line encoder on a 4 TPI ball screw 
results in
a position resolution and accuracy of 0.000125” with a top speed of 
2.5 IPS
and a step rate input of 20,000 steps per second.

Please correct my math if I’ve made a mistake.

John Dammeyer
— -hanermo (cnc designs) ———————————— ———————————— ————————————

I think its a fact that microstepping, where you are actually stopping at the microsteps (i.e. using them for resolution) definitely is less torque, simply because the motor cannot hold that position accurately against load. In a dynamic situation however, lots of factors come into play. For example, if the axis is in constant motion the only torque required is to overcome friction and cutting loads, there is little torque required to provide acceleration. Unless the motor is being operated close to its torque limit (at that speed/volt/current combo) then microstepping should have little impact. The general rule I have used is in the spreadsheet is that the motor should provide 3x required dynamic torque at the maximum speed.

Large motors have high inductance so the torque drops off very fast with speed – the corner speed of those motors is 240rpm. I don’t know how big your axis are, but I’m guessing its going to be around 1 – 1.2m? With your 10mm pitch screws 2.5m/min = 250rpm, so that is close to optimal (and 1/8 stepping = 6664steps/sec) and it looks OK at cutting speeds, but its very marginal at 10m/min rapids and that is where you may have lost steps (=27000steps/sec). You need to reduce rapids to 7m/min but it should be OK at 1/4 or 1/8 stepping.

March 19, 2016   Comments Off on Driving a Ballscrew – Servo vs. Steppers

Couplers

From the ever popular CNCZone:

Al_The_Man and Crevice Reamer:

CR:

Only 3 types of couplers offer true zero backlash connection:

Solid. (which can cause problems unless PERFECTLY aligned, Helical, or Oldham.  Helicals are basically solid couplers with some alignment flexibility. Make sure you get the clamping type.

Al:

If by Lovejoy you mean spider type then it depends on what material and fit is used for the spacer, the next best is Oldham which is a form of zero backlash spider style.
There is also Disc type zero backlash, if using Helical for any kind of stepper or servo torque application then stainless should be used, aluminum does not stand up to repeated flexing, especially with any slight misalignment.
Either the Lovejoy or Misumi sites will show all the different kinds they offer.

October 19, 2015   Comments Off on Couplers

Dual Closed Loop Control with KFLOP

The Dynomotion KFLOP controller can control 8 motor axis (either as 4 dual/slaved or 8 independent), complete with 8 dedicated hardware encoder channels (while any generic input could be used in theory for encoding,  I understand the general I/O speed is much less than the dedicated lanes).

Placing rotary encoders on stepper or BLDC/BDC/AC motors can feed information on the motor position back to KFLOP, which will then apply corrective action to the drivers to accurately place the motors exactly where they were commanded to be (See Dynomotion – Closed Loop Stepper Control)

Unfortunately, this does nothing to compensate for pitch error in the screws, lash, or other purely linear mechanical elements. The solution to these is to add linear encoders in addition to the rotary; measuring the actual position of the linear elements and sending that feedback to KFLOP.

With a stepper system, one might be able to utilize linear encoders alone, and achieve good results. The stepper motor design itself imparts built in feedback: when the motor is commanded to assume a position, the drive pulse forces the rotor to assume the commanded angle via the two phases. When the motor overcomes the initial frictional opposing forces, the torque is naturally reduced. A stepper motor can be though of as a spring and mass system. The rotor is the mass, and as torque is applied, the magnetic force acts like the spring, storing energy and releasing it (this is also the reason for mid-band resonance in a stepper system). If the load attempts to move the motor out of that position, the increased magnetic field will “push” it back to the commanded position; this force does vary depending on where the motor is (full step and half step are stronger than microstep positions), as the magnetic torque follows a sine pattern.

Having said all that, the “bandwidth” (ability to react in short time frames) with linear encoders alone is said to be rather low/slow.  Enter the Dual Loop setup: using the rotary encoders for velocity control of the motors, and the “outer loop” (linear encoders) for motion/positional control (see Dynomotion – Dual Closed Loop).

Multiplexing Encoders

Unfortunately, encoders #0-4 occupy the same JP connector as that used to connect to a KSTEP (which then uses those pins for multiplexing KSTEP I/O). This is perfectly acceptable for the majority of stepper driven systems (being open loop) and the JP5 connector can be utilized for single loop motor or linear encoders (using the #4-7 encoder channels).

Fortunately, KFLOP can be programmed to multiplex the encoder inputs over to alternative connectors. If the multiplex bit is asserted, the dedicated inputs shift from their normal pin assignments on JP5 and JP7 (respectively) to the first 8 inputs on JP4 and JP6, respectively (4/2012 firmware changesMux’d Encoders Discussion).

The first 8 Inputs on JP4 are Encoders 0-3 A/B signals (when mux’ed). The first 8 Inputs on JP6 are Encoders 4-7 A/B signals (when mux’ed).

Reference MUX program:

 // Mux encoder inputs from KFLOP JP7 & JP5 to JP4 and JP6

FPGAW(ENC_NOISE_FILTER_ADD) = ENC_0_3_JP4 + ENC_4_7_JP6 + ENC_NOISE_FILTER_DEFAULT_VAL;

Mux = 0                                                              Mux = 1
Signal       I/O Bit  Pin                             I/O Bit    Pin
ENC0A        0                 JP7 – 7                       16             JP4 – 5
ENC0B        1                  JP7 – 8                       17             JP4 – 6
ENC1A         2                 JP7 – 9                       18             JP4 – 7
ENC1B         3                 JP7 – 10                     19             JP4 – 10
ENC2A        4                 JP7 – 11                      20             JP4 – 11
ENC2B        5                 JP7 – 12                      21             JP4 – 12
ENC3A        6                 JP7 – 13                      22            JP4 – 13
ENC3B        7                 JP7 – 14                      23            JP4 – 14

ENC4A        36               JP5 – 1                        26            JP6 – 5
ENC4B        37               JP5 – 2                        27            JP6 – 6
ENC5A        38               JP5 – 3                        28           JP6 – 7
ENC5B        39               JP5 – 4                        29           JP6 – 10
ENC6A        40               JP5 – 5                        30           JP6 – 11
ENC6B        41              JP5 – 6                          31           JP6 – 12
ENC7A        42               JP5 – 7                         32          JP6 – 13
ENC7B        43               JP5 – 8                         33          JP6 – 14

Note that JP6 and JP4 are 3.3V only. Driving them above 3.8V will not do good things to KFLOP’s integrated circuits (JP5 is 5V tolerant by design).

 

KMOTION Dual Loop Programming

KMotion (the C program loader) must be programmed to join the two servo control loops together, forming an “inner” and “outer” loop. Dynomotion has an example program to do this:

#include "KMotionDef.h"

// Creates dual feeback loops for cases such as rotary motor encoder feedback
// with linear scale encoder feedback.  Two KFLOP Servo Axes are required,
// one for each loop.  Output of the outer loop is applied as a velocity
// to the inner loop 

main()
{
    for (;;)  // loop forever
    {
        WaitNextTimeSlice();
        ch0->Dest += ch2->Output;  // move ch0 at speed of ch2 output
    }
}

This uses two KFLOP Axes channels per set, allowing 4 combined axis. The “FOR” loop can be added to the KFLOP initialization C program as well.

Examples of the settings for KFLOP axes:

Set Inner Loop Axis as:
—————————-
Input Mode = Encoder
Input Channel = Rotary Encoder
Output Mode = DAC Servo

Set Outer Loop Axis as:
—————————-
Input Mode = Encoder
Input Channel = Linear Encoder
Output Mode = No Output

Each loop can be tuned separately:

– Tune the inner loop axis for optimal performance based on the rotary encoders

– Tune the outer positional loop for optimal performance based on the linear encoders.

The outer (positional) loop is a higher level control, so it’s output should drive the inner (velocity) loop. example: ch2->Dest +=ch3->Output;

 

Multiplexing Encoders vs. Input Loss

Multiplexing the encoders to JP4 and JP6 means the previously utilized general I/O connections can only move to JP5 and other connectors (JP7 will still use the I/O pins for multiplexing KSTEP’s inputs). This results in a net loss of available inputs that may be non-trivial to regain.

The Velocity Loop in External Drives

A simpler solution is to fit servo drives that accept encoder feedback and close the velocity/torque loop in the drives, leaving KFLOP to handle the linear encoder inputs, which forms the position loop. The drives will then make certain the motors respond to where they should, and the KFLOP will correct for positional errors via the linear scales. Less control by KFLOP, easier wiring and installation.

Pros and Cons

If the level of mechanical compliance/accuracy of the system is already acceptable, adding the (possibly) significant cost and complexity of closed loop control may not be warranted. Conversely, if the mechanicals are so sloppy a strong wind or quake can shift positions, additional feedback will not make for quality performance.

Closing the control loop around linear scales includes more “bad” mechanical things inside, making the loop more difficult to control. Lash for example, crossing a axis point (think the four quadrants of a circle) will cause considerable havoc as the servo system chases the lash around for that split moment the affected axis is unloaded. Always best to remove lash rather than attempt to correct for it. In addition to Backlash, there is Stiction (static (sticking) vs dynamic friction), and Compliance (elastic modulus). Poor machine design can render expensive zero backlash ballscrews useless. As examples:

  • Inherent design (Box Ways, Dovetail Ways) have relatively large amounts of stiction, which is offset by the proportionally greater load bearing surface area, compared to linear guides.
  • Ballscrew classes can have significant lead error (According to Thompson Ballscrews), a Transport Class 7 rolled Ballscrew can have a permissible variation of 900um in 2500um (0.035 inches in 98), and up to 0.18mm of lash (0.007″).
  • Thermal expansion. On a large machine, the growth of the ballscrew can be significant. (11.7 ppm/° Celsius). As an exaggerated example, a 1219mm long (48 inch) ballscrew, during a 50 degree Celcius rise in temperature, will “grow” 0.73152mm (.0288 inches).
  • Rigidity (stiffness, opposite of compliance) of the unit: floppy drive belts, worn or “sloppy” gearboxes, torsion and bend loads in the structure, flexible couplers

As an example of such from Tom Kerekes of Dynomotion (on a post from CNCZone):

 

 …So for example say the linear encoder has a small error that the servo wants to correct. This will usually cause the servo to ramp up the motor torque. Eventually the motor will have enough torque to break stiction and begin accelerating. But the linear scales may not yet indicate any thing has moved because of the system’s backlash/compliance. So the Servo will continue to ramp up torque further. Eventually when the backlash/compliance is taken out the Axes will move and the Linear scales will report the motion. But at that point the motor may be at such a velocity that it is difficult to avoid a significant overshoot. Most systems like this can be made to work and be stable but only with very low feedback gains (torque is ramped up very very gradually) resulting in poor dynamic performance (errors are not connected quickly and therefore are allowed to grow to larger values).

 

February 24, 2015   Comments Off on Dual Closed Loop Control with KFLOP

More Stepper and Servo Wisdom

Again, from CNCZONE:

Q.) What are the advantages/disadvantages between steppers and servos?

A.) Step motors and servo motors service similar applications, ones where precise positioning and speed are required.

The biggest difference is that steppers are operated “open loop”. This means there is no feedback required from the motor. You send a step pulse to the drive and take on faith it will be executed. Seems like a problem but it’s not.

If you have a quartz watch with hour and minute hands, then you have a step motor on your wrist. The electronics generates 1 step pulse per second, driving a 60 step per revolution motor which turns at 1 RPM. It keeps nearly perfect time. Any errors are due entirely to the electronics timing accuracy (quartz crystal oscillator).

Top Ten Stepper Advantages:

1) Stable. Can drive a wide range of frictional and inertial loads.
2) Needs no feedback. The motor is also the position transducer.
3) Inexpensive relative to other motion control systems.
4) Standardized frame size and performance.
5) Plug and play. Easy to setup and use.
6) Safe. If anything breaks, the motor stops.
7) Long life. Bearings are the only wear-out mechanism.
8) Excellent low speed torque. Can drive many loads without gearing.
9) Excellent repeatability. Returns to the same location accurately.
10) Overload safe. Motor cannot be damaged by mechanical overload.

Top Ten DC Servo Advantages:

1) High output power relative to motor size and weight.
2) Encoder determines accuracy and resolution.
3) High efficiency. Can approach 90% at light loads.
4) High torque to inertia ratio. Can rapidly accelerate loads.
5) Has “reserve” power. 2-3 times continuous power for short periods.
6) Has “reserve” torque. 5-10 times rated torque for short periods.
7) Motor stays cool. Current draw proportional to load.
8) Usable high speed torque. Maintains rated torque to 90% of NL RPM
9) Audibly quiet at high speeds.
10) Resonance and vibration free operation.

Top Ten Stepper Disadvantages:

1) Low efficiency. Motor draws substantial power regardless of load.
2) Torque drops rapidly with speed (torque is the inverse of speed).
3) Low accuracy. 1:200 at full load, 1:2000 at light loads.
4) Prone to resonance. Requires micro-stepping to move smoothly.
5) No feedback to indicate missed steps.
6) Low torque to inertia ratio. Cannot accelerate loads very rapidly.
7) Motor gets very hot in high performance configurations.
8) Motor will not “pick up” after momentary overload.
9) Motor is audibly very noisy at moderate to high speeds.
10) Low output power for size and weight.

Top Ten DC Servo (brush type) Disadvantages (besides higher relative cost):

1) Requires “tuning” to stabilize feedback loop.
2) Motor “runs away” when something breaks. Safety circuits required.
3) Complex. Requires encoder.
4) Brush wear limits life to 2,000 hrs. Service is then required.
5) Peak torque is limited to a 1% duty cycle.
6) Motor can be damaged by sustained overload.
7) Bewildering choice of motors, encoders, servo drives.
8) Power supply current 10 times average to use peak torque. See (5).
9) Motor develops peak power at higher speeds. Gearing often required.
10) Poor motor cooling. Ventilated motors are easily contaminated.

Q.) Should I use servos or steppers in my machine?

A.) If you are designing a machine and you get to motors, the first thing you should do is calculate the power you need. Never buy a motor (stepper or servo) first and then figure out if it will fit what you need.

Motors are motors. They couple power to your mechanism and power is what makes things happen. The choice of a motor comes after you know what’s needed.

Power is velocity times force or torque times RPM. It doesn’t matter if the motors are steppers, servos or a gerbil in a spinning squirrel cage at the start.

To separate what motor need (neglect the gerbil), is the power your mechanism needs.

Rule #1: If you need 100 Watts or less, use a step motor. If you need 200 Watts or more, you must use a servo. In between, either will do.

So, how do you figure the power you need?

Method 1: You have a plasma table, wood router or some other low work-load mechanism. You have a clear idea of how many IPM you want but you’re not sure of what force you want at that speed.

Pick the weight of the heaviest item you are pushing around. If it weighs 40lbs, use 40lbs. multiply it by the IPM you want. Say that’s 1,000 IPM. Divide the result by the magic number “531”. The answer is 75.3 Watts so use a step motor.

Eq: Watts = IPM * Lbs / 531

Method 2: You have a Bridgeport CNC conversion you are doing. The machine has a 5 TPI screw and you need a work feed rate of 120 IPM. 120 IPM on a 5TPI screw 5 * 120 or 600 RPM.

How about force? Not a clue? Use your machinist’s experience on a manual machine. The hand crank is about 6″ inches in diameter. How much force would you place on the hand crank before you figure you’re not doing something right? I hear about 10 lbs.

10 lbs is 160 oz, 160 oz on the end of a 3″ moment-arm (6″ diameter, remember?) is 480 in-oz (3 times 160) of torque on the leadscrew.

The equation for rotary power is: Watts = in-oz * RPM / 1351

For this example, Watts = 480 in-oz * 600 RPM / 1351 or 213 Watts.

213 Watts is servo territory. You have to use a servo motor to get that, about a NEMA-34 one.

 

Steelspinner:

Something else to keep in mind with the stepper motors is that bigger isn’t always better. You have to look at the phase inductance. Higher induction results in a faster torque curve drop off (lower usable rpms). Generally the smallest motor that will work for the needed torque with the lowest phase inductance will perform better that larger overkill motors. The inductance that you should be looking for will be below 2.5mh ideally below 1.8mh. With inductance higher than this the usable rpm range drops drastically.

Don’t forget voltage. The higher voltage you run the higher the usable rpms. Ideally you should run your motors at: V=32 x sqrt(motor inductance). This usually ends up being higher than most drivers can handle, unless you get high voltage drivers. This allows the best motor performance without over heating. Stepper very rarely run at full amperage and only for very short period of time. Driver current is important but correct voltage will give much more noteworthy real world performance.

Watch out for high torque for size (ie. nema 23 motors with >400 in/oz), they generally have a very low torque drop off, rpm wise, due to high inductance. They are fine for slow speed operation but anything above about 150-200 rpm have almost no torque unless you run them at very high voltage. High voltage drivers can get pretty expensive. Also, larger motors have a larger rotor inertia. This means slower response to speed and direction changes: meaning higher voltage for the same performance.

IMHO, you should look at wattage needs of your machine. (Watts = IPM * Lbs / 531) Lbs is the weight of the heaviest thing you have to move, including cutting force. If your system needs 100 or less watts use a stepper motor. This usually falls in the nema 17 to mid sized nema 23’s. Watts between 100 and 200 can be a stepper motor or a servo. The usable motors would be mid to large nema 23’s. Higher than 200 watts USE SERVOS. Large steppers generally have very poor performance (for the reasons I said above).

 

SCzEngrgGroup

Microstepping beyond 10X is of virtually no value. First, no benchtop machine is going to benefit from more than 10X micro-stepping, because it’s already far beyond the mechanical resolution of the machine itself. If you’re running direct drive steppers with 5-pitch screws, 10X microstepping gives a nominal (NOT actual) step size of 0.0001″. Flex and vibration in the machine will FAR exceed that. Second, micro-stepping does not produce equal-sized steps, and the higher the ratio, the greater the % error will be, so microstepping beyond a relatively low ratio does absolutely nothing to increase resolution or accuracy.

 

 

The primary function of microstepping is to provide smoother motion, NOT to increase resolution. The ultimate resolution, and accuracy, of any CNC machine is FAR more a function of the mechanical characteristics of the machine than it is the drive system. In the real world, it is quire difficult, and VERY expensive, to even get close to +/-0.001″ true accuracy and resolution. It is done on large commercial machine by FAR more expensive components, a FAR more massive (stiffer, with better damping) machine structure, and more complex software, which compensates for some of the many remaining sources of error and inaccuracy. Having a drive that is theoretically capable of high resolution will do nothing to improve the overall system accuracy or resolution, unless the many errors contributed by the many other parts of the machine are all already below the error contributed by that drive. On these machines, you typically have ballscrews with an accuracy of perhaps +/-0.003″ per foot. Flex of the machine itself can be several thousandths of an inch. Thermal expansion can contribute that much more. You always have stiction, and backlash, which can completely consume small movements. So what does reducing the step size from 0.0001″ to 0.00001″ actually accomplish? Now, you can try to address all these deficiencies, but a single high-precision ballscrew will cost more than you paid for the whole the machine, and the other parts you’d need to swap out won’t be much cheaper.

 photomankc

Microstepping is for smoothness of operation. Take a big ratchet and give it 8 notches per turn and turn it fast. It’s going to make a hell of a lot of noise and vibrate a ton. Take that same ratchet and give it 48 smaller notches and it will be much less noisy and feel smoother as it moves at the possible expense of the amount of torque it can hold against. It’s easy to hear and feel the difference in a microstep driven motor and a full or half-step driven motor. But they may not actually move the table until a couple of microsteps are taken because they lack the full torque to move the load.

I microstep at 8 so 1600 steps per rotation and 5 rotations per inch or 8000 steps per inch, but I only count on the machine being able to position within 0.001″. In most cases it *can* position to 0.0005″ but there is no way in hell it’s good to 0.000125″ as the microstepsper would have you think.

 

November 27, 2014   Comments Off on More Stepper and Servo Wisdom