Positioning Capabilities of Ultrasonic Motors

While conventional positioning systems convert the rotary motion of a motor into linear motion using a spindle, PILine® positioning systems are based on a special direct drive. This completely eliminates backlash and play and enhances the response time and positioning accuracy. Furthermore, they offer high-resolution positioning and fast step-and-settle performance.

At the same time, ultraslow motion can be realized with constant velocities down to a few nm per second. This white paper focuses on giving you an overview of the possibilities, and shows you how to make the most of your PILine^® linear stages specifically for your application.

2.  Operating Principle of PILine^® Positioning Systems

PILine® positioning systems are based on ultrasonic piezomotors that are capable of direct-driven linear motion. A piezoelectric actuator is preloaded against a runner using a coupling element (see Fig. 1).

Velocity and resolution are not the only criteria for precision motion control applications. Due to the preload and the ceramic / ceramic drive principle, the ultrasonic motor acts like a brake when not energized. This feature is decisive for applications where long term stability is crucial, for example in super-resolution microscopy and imaging. Tests with ultrasonic motors in an optical trapping set-up show significantly better stability than screw driven stages, especially over long periods of time.

More information is available in the paper below.

When targeting a position, the inbuilt profile generator of the PILine® controller (e.g. C-867) creates a velocity profile for the motor, which consists of three regions (see Fig. 3): (A) acceleration, (B) constant velocity, and (C) deceleration and settling.

In this region, the stage has reached its constant maximum velocity.

The constant velocity region can be reduced by increasing the stage velocity(parameter 0x49).

In some cases, especially when covering short distances, the stage may go directly from acceleration (region A) to deceleration (region C), without reaching the maximum velocity. If so, try increasing the acceleration (0xB) and deceleration (0xC) parameters.

In this region, the motor decelerates as it approaches the target position.

The deceleration region can be minimized by

• increasing the deceleration parameter
• adjusting the integral term of PID set 2
• increasing window enter of PID set 0

Increasing the deceleration parameter (0xC) is similar to increasing the acceleration in region A, as explained in chapter 3.1.

Faster deceleration can also be obtained by increasing the integral term of the second PID set (parameter 0x422). This increases the velocity of pulling the stage into the settling window (PID set 0), as depicted in Fig. 8.

When particularly accurate positioning is required, reservations on positioning speed have to be taken into account. Higher accuracy can be obtained by using a smaller settling window; i.e., by reducing the window enter 0 (0x406) and window exit 0 (0x407)parameters.

The achievable positioning accuracy is limited by the grating period of the scale, the accuracy of the sensor, and the interpolation factor of the sensor electronics. For higher accuracy consider acquiring:

• stages with finer grating periods
• controllers with internal interpolation

As mentioned in chapter 2, a sensor is used (see Fig. 1; also referred to as encoder) to determine the position and velocity of the stage. In PILine® positioning systems, optical and magnetic incremental and absolute sensors can be used depending on the required accuracy, energy consumption and cost efficiency. In most cases, optical incremental sensors are used. These sensors determine the distance to a given reference point by recording a periodic pattern (referred to as grating) on a scale.

The grating periods utilized range from a few to several tens of micrometers. Using two photodiodes with a 90° phase shift, two sinusoidal signals are generated allowing for detection of the direction of motion. These signals are then processed by an interpolation circuit which splits each period into several equally-spaced pulses. The final resolution corresponds to that of the grating period divided by the interpolation factor.

Using a PILine® stage with a state-of-the-art PIOne (PI Optical Nano Encoder) sensor with a sensor signal period of 0.5 μm and an interpolation factor of >1,000, sub-nm resolution can be achieved.

Typically, interpolators with interpolation factors of 256 to 8192 are integrated directly into the stage electronics. Some PILine® stages have a switch for bypassing this inbuilt interpolator. In this case, the output changes from internally interpolated A/B counts to raw sine/cosine signals, which can be processed further by an external interpolator.

The new PILine® C-867.1U controller features an integrated interpolator with an interpolation factor of up to 20,000, enabling manifold increasing the resolution of your existing stages. Applications that require precise positioning as well as very slow-motion benefit from this gain in resolution (refer to Fig. 10).

 PILine® motors feature a broad dynamic velocity range of 10 nm/s to > 100 mm/s, which can be subdivided into three characteristic ranges:

• Ultraslow motion (10 nm/s to 10 μm/s)
• Slow motion (10 μm/s to 1 mm/s)
• Fast motion (> 1 mm/s)

The peculiarities and challenges of each of those velocity ranges will be discussed in the following subchapters.

5.1  Ultraslow Motion (10 nm/s to 10 μm/s)

PILine® controllers also feature a regulating circuit for automatic excitation frequency adjustment, which may interfere with the PID regulation. Before beginning with optimization of the P-term, make sure that the automatic frequency search (0x52) is switched off. Furthermore, a slight increase of the output frequency (0x51) can prove to be beneficial when driving slowly.

5.2  Slow Motion (10 μm/s to 1 mm/s)

Typical applications for this velocity range include triggered image capturing or laser-cutting cells.

A rattling noise, created by a periodic coupling mode switching of the coupling element, may occur in this speed range. The noise might seem to be annoying; however, the cause of this is not harmful to the motor. It can be eliminated by driving the motor with a secondary phase using the motor offset parameter (0x6F), as explained in chapter 5.1. Using a secondary phase also reduces the position error as well as the required motor output.

5.3  Fast Motion (>1 mm/s)

This velocity range is mostly used for fast step-and-settle applications. Typical use cases are positioning lenses in a beam path or shutter applications. Here, the main requirement is fast and accurate positioning; the shape of the trajectory plays a subordinate role.

In most instances, the default settings of the controller can be adopted without the need for time-consuming customization. Furthermore, the use of two-phase actuation (motor offset) is not required and might in fact lead to slower final velocities and less forward force.

6.  Driving Along a Trajectory with Minimum Position Deviation

When minimum position error is required, the P-term of the active PID parameter set has to be adjusted according to the current velocity of the stage. The I- and D-term do not need to be changed; however, decreasing them might be beneficial in some cases. Fig. 12 shows the empirically determined P-terms of an exemplary PILine® stage, for which the minimum following error is obtained.