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Topic: Stepper vs Servo (Read 1 time) previous topic - next topic

Lavan

Hi there!  I never used the motors before so I need to know, What is the difference between Stepper and Servo motors and where we use them.  For making POV(persistence of vision) what kind of motor I need to use and what should be the minimum RPM of the motor is needed?

kf2qd

Okay - What kind of servo are you talking about?

Most of the time on this forum a servo is referring to a hobby servo like used in RC aircraft and such.

If you are referring to a servo motor as used for motion control, it is a different animal.

Both Stepper Motors and Servo Motors can be purchased with a driver. For a stepper they generally take one of two input formats. Most common is Step/Direction and the other option is DirPos/DirNeg. Servo Motors have come in two styles - DC Brush type and more recently Brushless. The older DC Brush type motors had an analog input, typically -10V - +10V with the polarity controlling direction and voltage controlling speed. Newer Brushless controllers generally have  two modes of operation - Step/Direction or Analog. Using the Step/Direction control option the Brushless Servo can be used in the same manner as a stepper motor.

Differences - A stepper motor has a limit on speed. As the stepper runs faster the torque falls. A stepper can be used with low ratio gearboxes/belt drives. A servo motor on the other hand has a better torque curve and is able to be run faster. 3000 - 5000 RPM is typical. Some motors are rated at 3000 RPM, but lightly loaded will perform nicely t 5000 RPM. Some Brushless drives have a selection of 2 motors - one design for speed and another design for more torque and less top speed. Brushless motors can also be configured as stand-alone drives where they are either configured to run at a specific RPM or a combinatin of inputs can be on/off and those patterns can select specific speeds. Because the brushless motor has a built in encoder the drive is able to control the RPM very closely in this mode.

Given a choice - If I could afford them I would rather use Brushless Servos, but they generally cost more, as much as 2 times, but they are also available in much higher power ratings. Brushless servos are generally rated in WATTS. 30, 50, 100 watt units are generally available to run on single phase 120VA and 200, 400, 800 watt units are available to run on 240VAC single or three phase.

PWM from an Arduino is probably not of high enough resolution to run a servo in analog mode. Though a Arduino could control a servo through the serial port.

crispy

For general stepper vs servo questions I would humbly suggest googling, there are plenty of explanations out there.

Very quick summary:
Stepper motors are used for things that need to rotate continuously with starts and stops.  They have very precise position control.  However the control system is "open loop control" (google) so if the motor skips steps you lose all positional accuracy.  They require a dedicated stepper motor driver (the Pololu A4988 is a popular choice).  A stepper motor typically has 4 wires, to drive it you need to generate a train of pulses.  The stepper motor driver takes care of this for you.

Servos (the hobby type) are a very small motor with built-in gear reduction and built-in closed loop position feedback.  Closed loop position feedback means they will return to their set position if they are "pushed" out of position.  They are also much simpler to drive from an Arduino - you just need 5V, GND, and a signal pin.  The built-in Servo.h library makes it even easier from a software perspective.  Servos are typically not able to rotate continuously (there are some types that can) - they have a set range (90 to 180 degrees) over which they can rotate back and forth.

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For making POV(persistence of vision) what kind of motor I need to use and what should be the minimum RPM of the motor is needed?


I assume you mean that you will have a head that spins in circles.  Your rpm is going to depend on your intended refresh rate.  If your refresh rate is high you could do 1 of 2 things: use a stepper motor with a gearbox to increase the output speed, or use a conventional DC motor and control the rpm carefully.  It might be cheaper to use a DC motor with a rotary encoder to read the actual refresh rate, as opposed to finding a stepper motor and a gearbox with the right ratio.  Most stepper gearboxes I see are gear down boxes and you would need a gear *up* box.

Lavan

Thank you both for your detailed explanation, you answered what I need!!!

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