The Difference Between Stepper and Servo Motors for Robotic Fluid Dispensing
The Big Question and Great Debate
We are often asked what the difference is between stepper motors and servo motors when it comes to robotic fluid and adhesive dispensing systems. What are the pro's of each? What are the drawbacks? This article will help you understand the differences and benefits/drawbacks of each.
A Brief Overview of Stepper and Servo Motors
Stepper Motors
Stepper Motors are open loop motors that “step” through a number of pulses to create a pre-programmed rotary move. In a cartesian robot system, this becomes linear motion typically through a belt drive. Both static motor position and requested motion are determined by the magnetic force of the stator acting on the rotor. You can feel this force by pushing the axis out of position when idle.
Servo Motors
Servo Motors are closed-loop, auto-encoded motors that track live position and use this feedback to adjust actual vs. desired position. This self-correction has value in most applications. Typically, the servo drives a ball screw to create linear motion, directly or in parallel fashion. The feedback available includes position, current draw, torque, and following error range (how far from desired position). When you try to push an idle servo out of position, you will hear the motor working to hold position, and it will be more difficult to force position loss than with a stepper.
Differences Between Servo and Stepper Motors
Below is a table showing some basic differences between servo and stepper motors. This will help explain both the capabilities and utility of each type of motor.
| Attribute | Stepper | Servo |
|---|---|---|
| Payload/Holding Force | Generally lower in the same envelope | Generally higher |
| Accuracy | Comparable | Comparable |
| Repeatability | Lower | Higher, due to closed feedback |
|
Velocity + Path Repeatability |
Lower | Higher, with following control of all axes, over wider range of loads and speeds.1 |
| E-Stop Recovery | Requires homing | Can reset and continue. Strongly preferred for safety device apps.2 |
| Torque Limits | N/A | Torque can be limited. Has value for safety, PM prediction.3 |
| Manual Teaching | N/A | Can manually position and save point data, saving time vs. jog |
| Data Availability | N/A - No feedback | Variety of data available. Useful for diagnostics and optimizing the app. |
| Parameter Optimization | A few parameters can be modified | Gain, inertia, other parameters can be optimized to the app. Also some diagnostic value. |
Further Reading
When drawing a path, all axes affect velocity and position of the tool. Servo axes will give better momentary position, as all axes are within a set tolerance at any moment. No feedback with steppers, so this is not possible. Also, velocity control is better with closed loop feedback of servos.
Servo is much faster for E-Stop recovery/restart. With servos, the current position is known, so work can be continued without homing. This is especially useful with long cycle times and palletized work, as the process can be continued with minimal lost time; correct pallet restart position can be stored. With steppers, homing also causes program interrupt, requiring restart and probable removal and re-palletizing of parts, and lost production. Servo recovery/restart is also very quick after light curtain or other safety E-Stop. On systems where the operator has to reach through the curtain to load/unload parts, this is a big time-saver.
Torque (~ force) can be limited for pressing operations, and also to limit force in potential injury scenarios. Setting limits for torque has had some utility to monitor axis health; as required torque rises, typically an element of the drive is changing state, whether mechanical , motor, or amp.
Note that encoded stepper systems also have position feedback. However, encoded stepper cost is on a par with servo cost, and servo brings other feedback that has utility. So, servo is generally preferred.
