When dealing with high resolution applications where positioning down to the nanometer level is required, choosing the right hardware is critical. For systems such as these, a piezoelectric motor is often selected over a conventional electromagnetic motor. Piezoelectric motors have excellent positioning accuracy, high speed capabilities, and minimal electromagnetic interference. This makes them an excellent candidate for applications where high precision and speed is required over small distances, as well as for applications in sensitive environments.
One of the downsides of piezoelectric motors however is that they are non-linear, and therefore are much more difficult to tune when compared to a standard brushless servo motor. To overcome this challenge, The Galil Applications Team utilized a Galil DMC-30019 coupled with a LEGS linear piezoelectric motor from PiezoMotor. The DMC-30019, a customized controller/driver for use with Piezo LEGS motors from Piezomotor, can easily be programmed and tuned to achieve high performance motion with accuracy down to the nanometer scale.
System Hardware
The hardware used in this demonstration system consisted of a DMC-30019, a PiezoMotor LEGS LS15 linear motor, a Micro E Systems Mercury II encoder with 5nm feedback and a PM-Bearings Precision Slide with 40mm of travel. Two sample moves were used to demonstrate the use of this system. The first was a step and settle move, and the other was a longer slew move. Each sample move was done twice, once with the system unloaded and once with a load of 225g. Tuning was done by viewing the system response to motion commands in GDK's scope tool and adjusting filter gains on the fly.
System Performance
The first move was a 0.5 mm step and settle at 5 mm/sec with an acceleration and deceleration of 300 mm/sec^2. Once tuned, the system was able to settle within +/- 5 counts in 4.73 msec. Figure 1 shows the system's performance of this step and settle move. The vertical green shaded area shows the time the system takes to settle, while the horizontal area shows the +/- 5 count error constraint. The settling time, error, and system gains used are listed in Table 1.
| KP | KI | KD | FV | Settling Time (msec) | Final Error (counts) | Final Error (nm) | ||||
| Unloaded | 4.25 | 0 | 0 | 11 | 3 | +/- 5 | +/- 25 | |||
| Loaded | 4.75 | 0 | 0 | 11.5 | 4.25 | +/- 5 | +/- 25 |
Table 1. Loaded and Unloaded Step Move System Parameters and Characteristics
The second move was made longer (5 mm at 5 mm/sec) to show the dynamic performance of the system. With the gains shown in Table 2, the system was able to maintain a following error of +/- 35 counts (175 nm) during motion in both the loaded and unloaded tests. In Figure 2 the vertical green shaded area represents the constant velocity portion of the move, while the horizontal shaded area shows the maximum amplitude of the error during this section.
| KP | KI | KD | FV | Steady State Error During Move (counts) | Steady State Error During Move (nm) | ||||||
| Unloaded | 4.75 | 0 | 0 | 14 | +/- 35 | +/- 175 | |||||
| Loaded | 4.5 | 0 | 0 | 14 | +/- 35 | +/- 175 |
Table 2. Loaded and Unloaded Slew Move System Parameters and Characteristics
Conclusion
The Galil DMC-30019 and LEGS motor from PiezoMotor can meet the high requirements of today's applications. The DMC-30019 takes care of the difficulties associated with the non-linearities and high friction that make for difficult tuning, saving development time and engineering resources. For more information, or to discuss a specific application, please contact Galil's Application Engineering Team at 1-916-626-0101 or toll free at 1-800-377-6329 (US only).