Servos or stepper


Page 1 of 2 12 LastLast
Results 1 to 12 of 18

Thread: Servos or stepper

  1. #1
    Gold Member
    Join Date
    Oct 2004
    Location
    USA
    Posts
    176
    Downloads
    0
    Uploads
    0

    Default Servos or stepper

    Okay, call me stupid but I don't know the difference between a stepper motor and a servo motor. Can someone help me understand.

    Just trying to take it all in

    Thanks,
    Allen James

    Similar Threads:


  2. #2
    Gold Member
    Join Date
    Mar 2003
    Location
    United States
    Posts
    2839
    Downloads
    0
    Uploads
    0

    Default

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

    The biggest difference is 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).

    With apologies to David Letterman, here's some "Top 10" lists I came
    up with for choosing between steppers and brush DC servos. Others can
    add more, I'm sure.

    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 resonances. Requires microstepping 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 Disadvantages:

    1) Requires "tuning" to stabilize feedback loop.
    2) Motor "runs away" when something breaks. Safety circuits required.
    3) Complex. Requires encoder.
    4) Brush wearout 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, servodrives.
    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.

    Mariss



  3. #3
    Registered
    Join Date
    Oct 2003
    Location
    Sweden
    Posts
    399
    Downloads
    0
    Uploads
    0

    Default

    Mariss, it's always a pleasure to read your posts. Thanks for sharing!

    Arvid



  4. #4
    Gold Member
    Join Date
    Oct 2004
    Location
    USA
    Posts
    176
    Downloads
    0
    Uploads
    0

    Default

    Mariss,

    Thank you for your fast and very educational reply. I appreciate the knowledge.

    One question....On the Top Ten DC Servo Disadvantages you wrote, Requires "tuning" to stabilize feedback loop. Is this done in the drive or is this done in the encoder? and the safety circuits, are these in the drive or is this something I would have to buy seperately. Like your drives, are these items in your drives?

    Okay, there was more than one question

    Thanks Again,
    Allen James



  5. #5
    Registered Torsten's Avatar
    Join Date
    Nov 2004
    Location
    U.S.A.
    Posts
    260
    Downloads
    0
    Uploads
    0

    Smile

    The difference between a stepper and a servo in simple terms is
    the fact that when not moving a stepper is held in position by current
    a servo is not.

    The full current is flowing to hold a stepper motor in position this is why
    they are rated for a certain amount of holding torque.
    As the stepper starts to rotate the torque deminishes as the speed increases.

    No current flow on a stopped servo motor its power is regulated by a error register in the driver.
    When a force is encountered like somebody trying to turn the shaft by hand the error of position on the shaft will cause a current to be send to the motor to correct the error.
    The amount of current is in direct relation to the amount of error so a small error will produce only a small currrent.
    This is like a Rubberband effect when trying to move the motor and should be taken into consideration when the resolution of a system is considered.

    With the ever moving improvements of the technical community both technologys have been working its way towards each other.
    Servos can now be controlled just like stepper systems and
    Multystepping systems with current reduction at stand still make
    them behave much more alike.

    Good Luck



  6. #6
    Gold Member
    Join Date
    Mar 2003
    Location
    United States
    Posts
    2839
    Downloads
    0
    Uploads
    0

    Default

    Tuning refers to adjusting the PID coefficients to cause a critically damped response from the motor / load when adjusting to a disturbance. Sounds complicated but it's not.

    PID stands for Proportional, Integral and Differential. The "difference" error (where you should be versus where you are) is seperated into 3 channels (PID), then recombined. You perform this algorithm unconsciously when you drive a car.

    Say you take a road trip from LA to San Francisco up I-5. Your task is to stay side-by-side next to the "command position" car on this trip. It can accelerate and stop instantly, you can't. Your gas pedal and brake (torque command) only adjusts acceleration and deceleration like in a real car.

    At the start of the trip, both of you are stopped. The "command car" instantly accelerates to 85MPH (average to slow for what you see on I-5).

    The first thing you notice is a lot of distance has opened up between you and the "command car". This is the Proportional component. You press on the gas and away we go. Your speed builds up and after a while the distance begins to close.

    Your rate of closure is the derivative or Differential component. As long as the distance to the "command car' keeps opening, you press harder on the gas. As it closes, you ease up.

    To close the distance, you have to go faster than the "command car". Otherwise you will never catch up.

    You are now getting very near the "command car". Both the seperating distance and the rate of closure decreases towards zero so they are no longer of use. You have come off of the gas enough to nearly match its speed.

    This is where the Integral component comes in. You are now side by side. You now adjust your speed based on inches of seperation. If you slightly edge into the lead, you ease off. If you slightly fall behind, you make up the difference. Rate of closure (differential) or seperating distance (Proportional) are too small to use.

    Using this method (PID), you will arrive at your destination simultaneously even though hundreds of miles and hours of travel have elapsed. You do it naturally and unconsciously.

    A mistuned servo (not enough D or too much P) by this analogy would have you overshoot the "command car", hit the brake, fall behind, over-accelerate and overshoot the "command car" again, over and over. Tough on your passengers and car (400 miles, 4 hours) and equally hard on your servo motor.

    All sorts of other stuff works with this analogy. Two things come to mind.

    1) Feed-forward compensated PID servos. This is where you are told ahead of time what the "command car" will do. You don't have to sit and watch in surprise when it suddenly takes off or changes direction; you are fore-armed with its future intentions. This somewhat makes up for time otherwise lost in catching up.

    2) S-shaped accel/decel profiles. This keeps the 2nd derivative of velocity finite, minimizing the "jerk" factor. Again, you do this naturally driving a car.

    Imagine you are cruising down a boulevard when the light up ahead changes from green to red. In is simple CNC program, you stand on your brake until you come to a stop. This would be very uncomfortable in real life.

    When you decelerate in a car, you tense your muscles to balance against the deceleration G-force until you just counteract that force. When you come to a stop, deceleration abruptly dissapears along with its G-force. Your tensing against it does not though. The result is head and body bobbing back and forth until you find the new balance. Not comfortable.

    What you actually do when coming to a stop is to tail-off on the brake pedal. Before coming to a stop you lessen the rate of deceleration (brake pedal pressure) to make it become zero as your speed approaches zero. This is an S-shaped decel curve.

    Where it matters on your CNC machine is it eliminates ringing (head bobbing) at the beginning and end of acceleration and deceleration. This decreases wear and error.

    Mariss



  7. #7
    Moderator ynneb's Avatar
    Join Date
    Feb 2004
    Location
    Oz
    Posts
    2337
    Downloads
    0
    Uploads
    0

    Default

    That was a loverly analogy Maris. I almost felt like I was in that second car.

    You finished off with the acceleration and deceleration bit. I have a very low acceleration and deceleration on my machine because if I dont my machine doesnt like it. But I have always believed that fast acceleration and deceleration is a positive when it comes to CNC machining. I have heard that you get better arcs/circles with higher acceleration/deceleration. Is this true, or have I read bad information??

    Being outside the square !!!


  8. #8
    Gold Member
    Join Date
    Oct 2004
    Location
    USA
    Posts
    176
    Downloads
    0
    Uploads
    0

    Default

    Thanks Again Mariss!! You're one smart cat

    The second time I read this it all seemed to make some sence. I am still wondering though how one makes the adjustments. Is it something you adjusts physically or maybe it's done with software? How are the errors measured or is this something you would see like oscillation. Which brings up an interesting point, oscillation seems to be what you are describing, over shooting in both directions. Back and fourth, is it?

    I have been around, mainly running, CNC machines for years. I have seen many adjustments being made over the years. I never fully understood though what they were doing or why they were doing it. Getting information from some people proves to be a very tough task. You give it freely, I appreciate it.

    Allen James

    Last edited by freak_brain; 11-15-2004 at 11:02 AM.


  9. #9
    Gold Member
    Join Date
    Oct 2004
    Location
    USA
    Posts
    176
    Downloads
    0
    Uploads
    0

    Default

    thanks Torsten

    I appreciate the information.


    Allen James



  10. #10
    Registered WoodSnarfer's Avatar
    Join Date
    Nov 2004
    Location
    United States
    Posts
    78
    Downloads
    0
    Uploads
    0

    Default

    Quote Originally Posted by Mariss Freimani
    Where it matters on your CNC machine is it eliminates ringing (head bobbing) at the beginning and end of acceleration and deceleration. This decreases wear and error.

    Mariss
    Awesome post, thanks! So, what design choices can the would-be router designer make ahead of time to minimise the chance of servo ringing? Is it simply a wise choice of motor (i.e., matched to the load & speed demands of the axis) -- or does the Gecko handle it for us -- or does Mach2 (or some other software) handle it for us?

    Thanks,
    Chris



  11. #11
    Gold Member
    Join Date
    Oct 2004
    Location
    USA
    Posts
    176
    Downloads
    0
    Uploads
    0

    Default

    woodsnarfer, that is one funny looking picture you have there!! And the name, lol




    Guess I shouldn't talk about names huh?


    Allen



  12. #12
    Registered WoodSnarfer's Avatar
    Join Date
    Nov 2004
    Location
    United States
    Posts
    78
    Downloads
    0
    Uploads
    0

    Default

    LOL! Whoever said "all the good names are taken" just lacked some imagination, didn't they?

    -Chris



Page 1 of 2 12 LastLast

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  


About CNCzone.com

    We are the largest and most active discussion forum for manufacturing industry. The site is 100% free to join and use, so join today!

Follow us on


Our Brands

Servos or stepper
Servos or stepper