Various active vibration isolation systems (active systems) are available with different hardware, software, and overall technological capabilities. Instrument users and manufacturers must have a thorough understanding of these differences in order to select an appropriate system to support their instruments within their respective environment.
One recent differentiation made is distinguishing between serial and parallel active systems, with the AVI and TS Series coming under the Parallel active systems category. This article explains how Serial and Parallel active systems are often compared and helps to understand the factual and fictional differences.
Myth vs. Fact
Myth: Serial > Parallel Active Systems for Low Frequency Vibration Isolation Performance
Fact: Parallel > Serial Active Systems for Low Frequency Vibration Isolation Performance
A feedback loop system is the widely used and effective isolation mechanism in active systems. It comprises of a series of inertial sensors, control electronics, and actuators, which are used to dynamically attenuate ambient vibration noise.
All feedback systems that consist of inertial sensors are characterized by a low-frequency cut-off determined by the sensor’s low-frequency noise. The level of this noise is defined principally by the inertial sensor’s mass, which differs between respective active systems.
A balance is established between size and low-frequency response with respect to small form factor active systems, such as AVI and TS Series. The mass is maintained low to enable a compact construction suitable for precise compact microscopes. The compact size limits the available isolation between 0 - 1 Hz compared to active systems featuring larger-mass inertial sensors.
For the AVI Series (Figure 1), the Low-Frequency Sensor (LFS) System (Figure 2) was developed to bridge the gap of performance by providing a larger-mass inertial sensor that can operate in tandem with the AVI Series to achieve superior low-frequency vibration isolation. When coupled to the LFS System, the AVI Series delivers greater low-frequency isolation performance over any high-mass feedback systems, serial or parallel.
Figure 1. AVI-400 supporting SEM column directly
Myth: Parallel Active Systems Need to Be Tuned to the Payload
Fact: Parallel Active Systems with High Quality Feedback Loops Do Not Require Payload Tuning
There is an argument that the overall isolation of the platform is limited by parallel active systems due to the requirement of specifically tuning them to the payload being supported. This argument is true for some commercially available some parallel active systems, but is not a standard feature of all parallel active systems.
It is a function of the quality of the feedback loop system being designed for the active system. The quality of a feedback loop system is defined by the range of frequencies that can be accommodated by it and how effectively resonances in the payload are damped by it.
Figure 2. LFS system
The feedback loops are driven into oscillation due to phase changes caused by undamped resonances in the payload. Well designed feedback loops with a high-frequency response (several KHz) can effectively damp payload resonances by eliminating their oscillation within the feedback loop.
This is true in the case of AVI and TS Series platforms. In addition to having a broad-frequency feedback loop, the AVI and TS Series platforms do not require tuning to specific payload resonances. This stays true throughout its entire production history (Figure 3).
Figure 3. Resonant
Myth: Decoupling Serial Active Systems from the Payload Offers an Advantage
Fact: Decoupling Serial Active Systems from the Payload Offers a Disadvantage
Serial active systems using a larger mass tend to have a poor feedback response because many manufacturers report data to 100 Hz. This limitation forces serial active systems to decouple itself from the payload at approximately 20 Hz. As a result, passive isolation mechanisms are allowed to mitigate the higher frequencies.
Although this decoupling eliminates higher frequency payload resonances, it does not consider external vibration sources from direct coupling to the payload itself.
Decoupling the payload becomes a drawback due to the fact that the feedback loop of the active system cannot be applied to the vibration noise directly imparted to the payload (e.g. vibration noise not from the floor).
A payload held by an inertial platform and decoupled at 20 Hz as a “serial” system has reasonable isolation from floor vibrations, but the level of performance deteriorates when more external connections are made to the payload. A payload such as an electron microscope will typically be coupled to the surroundings via electrical cables, pump lines, and conduits, meaning it is indirectly coupled to the floor.
While most of these lines and cables are stiff in nature, the sum of the cable stiffness can come close to or be even higher than the stiffness of the 20 Hz decoupler. As a result, the cable and pump lines will create a mechanical bypass to the active system itself, introducing noticeable vibrations directly into the payload.
The problem of a device with indirect vibration sources being overlooked by an active system is absent for parallel active systems such as the AVI and TS Series. These systems are stiffly connected to the payload and are directly fighting the forces entering the payload from the floor as well as the surroundings. This is a unique differentiating benefit over serial active systems.
It should be noted that all active systems will be decoupled from the payload at a very high frequency, be it due to rubber feet under the payload or due to internal resonances in the payload. However, typically, this frequency will be in the range of a few hundred Hz, far distant from the 20 Hz range of serial active systems.
It is important to have an understanding about the factual differences between active isolation technologies in order to help an instrument achieve optimal vibration isolation performance for the conditions of its laboratory. Parallel active systems are more effective at detaching the payload from forces from the surroundings.
Understanding these variations and planning accordingly not only saves time and money, but also enables the end user to concentrate on their work rather than on their isolation platform.
This information has been sourced, reviewed and adapted from materials provided by Herzan LLC.
For more information on this source, please visit Herzan LLC.