Industry News Display
2021-01-19 Control Engineering Plant Engineering
Testing a direct drive and geared rotary servomotor shows there are some significant differences between the two when actually put into practice.
By: Dakota Miller and Bryan Knight
- A geared servomotor competes head-to-head with a direct drive servomotor using the same settings and features.
- Many of the results might seem minor on the surface, but they can be quite significant for applications that require absolute precision.
- Direct drive servomotors are more expensive, but they make up for it in performance and overall payback.
A geared servomotor competes head-to-head with a direct drive servomotor using the same load, motion profile and mounting fixture, simulating a real-world application to determine if the performance advantages of the direct drive system outweigh the higher initial cost. (In part 1, the case for decreasing backlash and increasing torsional stiffness was presented.)
For this test, the following evaluation criteria were considered:
- Positioning accuracy
- Settling time
- Cycle time
- Machine cost and payback time
- Design complexity.
For this test, a servo system was created representative of an indexing table application where the table has high rotational inertia. The load inertia, root mean squared (RMS) torque, and speed requirements were chosen such that a gearmotor and direct drive servomotor would both operate near their rated limits. To measure the positioning accuracy and cycle time of this simulated “machine,” an external encoder was affixed to the load to precisely measure the position of the load itself. This external encoder was not used in closing the position loop, but only as an independent measuring tool for the test. Performance differences were quantified using the data gathered from both the motor encoders and load side ring encoder feedback.
Before designing the actual test hardware, a gearmotor and a direct drive motor were chosen for comparison. These motors were selected as the 50:1 gear reduction gives the gearmotor comparable torque, speed and overall size to the direct drive motor, as seen in Table 1. Based on these characteristics, it is no stretch of the imagination that both of these motors could be competing against one another to control an axis on a new machine.