In this interview, Jeff Roblee, VP of Technology at Precitech, and Igor Kordunsky, Director of R&D at TMC, talk about the integration of TMC’s active vibration cancellation technology into Precitech’s diamond turning machines, and how this can help improve accuracy and performance in high-precision machining applications.
WS: What benefits does active vibration cancellation bring to users of diamond turning machines?
JR: The active vibration cancellation provided by the PEPS II-VX system brings vibration cancellation capabilities to passive, pneumatic isolators for the first time. In most environments, this results in less vibration making it into the machining envelope. The end result of this for our customers is better surface finish on their parts.
Additionally, there are benefits to removing the mechanical linkage used on passive vibration cancellation systems, which can potentially transmit floor vibrations around the isolators.
WS: How does the PEPS II-VX system improve over the TMC passive isolation systems already embedded into Precitech machines?
JR: All vibration isolators have a fundamental resonant frequency. Isolators actually amplify vibrations that are near to this frequency, instead of attenuating them. Generally, the lower the value of this resonant frequency, the better the isolator.
TMC MaxDamp isolators are excellent in this regard, with a resonant frequency lower than 2 Hz. PEPS II-VX provides the first low cost solution for this universal issue by providing active vibration cancellation in the critical frequency range of 1.4 to 5 Hz.
WS: What role does precision leveling play in the diamond turning process?
JR: A standard isolator uses a mechanical linkage and a mechanical valve to keep the machine level, whereas PEPS II-VX uses an electronic position sensor and an electronically controlled valve. This system levels much faster and more accurately, and it eliminates mechanical contacts. This more rapid leveling permits much more rapid motion of the machine slides without any degradation in accuracy.
Maintaining a precise level is critical for an ultra-precision motion platform, because it keeps the axes of the machine fixed relative to the vector of gravity. Structural deformations occur when the machine is out of level, and this creates motion errors at the tool.
L: PEPS® II-VX installed on a Precitech Nanoform® X
R: PEPS® II-VX installed on a Precitech Freeform® L
WS: Specifically what diamond turning processes will see the most benefit from precision leveling?
JR: The precision leveling portion of the PEPS II-VX system allows our customers the ability to cut slow tool servo parts faster than they could without the system, without compromising the form accuracy.
This has the clear advantage of increasing productivity, but also reduces thermal effects, which are increased with longer cutting times. Thermal effects are well regarded as the single largest source of error in diamond turning. The benefit can also be realized in other processes which involve rapid slide motion, such as horizontal ruling.
WS: Does this type of system require a high level of tuning and optimization to obtain the best results? How does Precitech help customers get the most out of PEPS II-VX?
JR: Precitech’s field service engineers are already experts on servo loop tuning – they have had additional training on how to tune the PEPS II-VX system by working with the experts at TMC. Also, we can use the remote access capabilities, which are included on all our machines, for our engineers in Keene, NH, to provide tuning support or optimize the performance of the system if required.
WS: What is the technology behind the VX active vibration cancellation system?
IK: The VX technology is an extension of TMC’s standard PEPS II technology, which processes signals from non-contact position sensors (called "proximity" sensors), and uses that information to control electronic valves.
This maintains static pressure in the isolators, allowing the payload to float and be kept level.
VX technology uses additional velocity sensors to control the pressure change in the isolators. These additional sensors can detect changes in velocity before there is any change in position, so the correcting response comes faster than from position sensors alone. This allows for the suppression of the natural resonance peak of air isolators.
In summary, we are able to quickly change the pressure in the air isolators so that the isolators act as a vertical actuator to cancel vibration on the payload in the vertical axis.
WS: What are the main practical advantages of VX technology compared to passive vibration cancellation systems typically used on diamond turning machines?
IK: There are a number of advantages. Firstly, non-contact proximity sensors eliminate noise transferred from the floor to payload via the leveling arm of mechanical valves.
In addition, the velocity sensors included in the VX option allow for the suppression of amplification at isolator's resonance frequency, which mechanical valves cannot do.
WS: How does the PEPS system accomplish precision leveling?
IK: Proximity sensors are substituted for the mechanical height control valves. These proximity sensors provide a signal proportional to the distance between sensor and payload. The proximity sensor signals are compared to the desired floating height, and the difference processed by PID (Proportional - Integral - Differential) control loops. The control loops create signals for four electronic proportional flow valves (one valve per isolator).
The valves have a variable inflation orifice, and thus a variable inflation rate – as the orifice opens wider, the isolator inflates faster. The pneumatic path also has a fixed size deflation orifice, resulting in a constant deflation rate.
To raise the payload, the valve opens to make the inflation rate faster than the constant deflation rate. Reducing the inflation rate below the deflation rate causes the pressure to decrease in the isolator, and payload goes down. When the payload is floating at the desired height, the inflation rate is equal to the deflation rate.
The PEPS II-VX vibration cancellation system
Because of this non-contact, electronic method of measuring distance, the repeatability of the position measurements is in the single micron range. Also, because of the constant deflation rate, the electronic valves do not work in a two-mode open-closed fashion, but operate within the very linear, precise region of operation.
This allows both the position of the payload to be controlled within a single micron in steady state operation, and fast correction response when the diamond turning machine changes position or vibration is detected from the floor.
WS: What were the unique challenges associated with adapting PEPS II-VX for use on Precitech diamond turning machines?
IK: The physical dimensions of the machine dictated the positioning of the isolators. The footprint of the isolators forms a rectangle with the center of mass of the machine higher than the typical PEPS II-VX applications. This causes less stability in the front-back direction and better stability in left-right direction.
The challenge was to tune the system for the front-back direction to achieve fast response without causing an oscillation.
WS: Where do you see this technology advancing in the next few years to further improve its performance in ultra-precision machining?
IK: TMC is working on a new type of isolator with more natural damping. This will allow us to increase the suppression of machine rocking in front-back and left-right directions (pitch and roll) while in operation. More suppression means less rotation relative to the gravity vector, thus minimizing the errors associated with it, as well as better vibration isolation from the floor.
WS: How did Precitech and TMC engineers work together to integrate this system into the Precitech line of diamond turning machines? Any challenges?
IK: Historically PEPS II has been used with payloads with moving X-Y stages. The stage acceleration and velocity were much larger in those applications than what we experienced with the Precitech machine.
The requirements for previous applications were to achieve fast settling time, and allow some oscillation after the stage motion ends. For Precitech machines that tuning did not work well at first. The system experienced too much oscillation, which made the quality of the surface worse. We used the FFT capability of Precitech’s machine to fine tune the PEPS II parameters. This allowed us to achieve the lowest noise and avoid oscillation, increasing the quality of the surface.
WS: Is the PEPS II-VX technology available for retrofit onto the existing fleet of Precitech machines in the field, or is it only available on new machines?
JR: PEPS II-VX is available on all of Precitech’s new machines and is also field retrofittable. The only requirement is that machines must have already TMC MaxDamp passive isolators in order to be retrofitted with PEPS II-VX.
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