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Techno Linear Motion Catalog 41 Stepper or Servo Cost Comparison In general, stepper motor systems tend to be less expensive than servo motor systems.  Stepper and servo systems often become comparable in price when the stepper system uses motors larger than NEMA23 or when micro-stepping is used. Servo motors in the NEMA23 frame size tend to be 10% to 30% more expensive than similar stepper systems.  Brushless servo motor systems tend to be 50% to 100% more. Stepper motors are brushless.  They experience little or no wear, and are virtually maintenance-free. For a given screw pitch, typical four phase stepper motors can produce 200 full steps, 400 half steps, and up to 25,000 micro steps per revolution.  It is significant to note that since the stepper motor is open  loop,  it  does  not  necessarily  achieve  the  desired  location, especiallyunderload.Particularlypoorpositionalaccuracycanresult when using microstepping, which is primarily useful for smoothness of motion. Steppers have very poor torque characteristics at higher speeds.  This condition is improved somewhat by microstepping, however, unless the stepper is used in a closed loop mode, it does not usually perform as well as a servo. Stepper  motors  are  almost  always  used  in  an  open  loop configuration.  If used in a closed loop, they typically become as, if not more, expensive than servo motor systems.  The open loop nature of stepper motors is their principal drawback. Commands  are  issued  to  move  prescribed  amounts,  and barring  unforeseen  circumstances,  the  motor  moves  the amounts prescribed.  In rare cases, resonances or unexpected forces can cause a stepper motor to lose steps or stall out. Although rare, this is an ever present possibility. Brush-type servomotorsrequireachangeofbrushes,typically, every 5,000 hours.  Like steppers, brushless servo motors have virtually no servicing requirements. Servo  motor  resolution  depends  upon  the  encoder  used. Typical encoders produce 2,000 to 4,000 pulses per revolution, and  encoders  with  up  to  10,000  pulses  per  revolution  are available.  Since servos, which are closed loop, can and do achieve  the  available  resolution,  they  are  able  to  maintain positional accuracy. Servos can produce speeds and powers two to four times that of similarily sized steppers. This improvement is a direct result of the closed loop (i.e., constant position feedback), which allows for higher speed and greater reliability.  The closed loop nature of the servo also allows such a system to better utilize peak torque capabilities. By nature, servo motors have constant positional feedback. The positional feedback is used to correct any discrepancy between  a  desired  and  an  actual  position.    This  constant corrective action results in faster speeds (up to three times the throughput),  and  increased  power  (up  to  three  times  the torque) at high speeds.  The closed loop nature of the servo also  ensures  that  stalling  cannot  occur  unless  there  is  an immovable object in the path. Choosing Between Stepper and Servo Motors In  general,  we  recommend  stepper  systems  for  cost  sensitive  applications  requiring  low-to-moderate  volume  production capabilities.  Servo systems are recommended for high-speed, high-volume, high-reliability applications.  A typical Techno servo gantry system sells for about $2,700 more than a stepper system.  Servos can perform high-speed continuous motion reliably, making them particularly superior in three-dimensional contouring applications.  We have found time reductions of up to 80% on some applications.  The continuous motion also results in better finish quality without the fine faceting that is found with stepper systems.  In addition, the servo's reliable high-speed continuous motion can reduce the possibility of scorching and melting when working with woods and plastics. Servo Technology Stepper Technology Reliability and Maintenance Resolution and Accuracy Speed and Power Closed Loop Vs. Open Loop Choosing a System