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Aerotech's Harmonic Cancellation algorithm provides performance benefits for elastomer test machine manufacturer

31 March 2011

Prescott Instruments, the UK-based specialist test instrument designer and manufacturer, recently introduced a new multi function rheometer for rubber and elastomer materials testing that uses direct-drive brushless servomotor technology for a sinusoidal motion technique that offers greater flexibility and a wider range of test conditions.

The Rheo-Line Multi-Function Rheometer can provide tests at lower frequencies to improve the study of polymer processability in addition to more traditional moving die rheometer (MDR) testing and dynamic mechanical analysis to evaluate the cured properties of materials.
The new rheometer uses a single axis frameless and brushless torque motor with a peak rating of around 20 Nm in combination with a high resolution optical encoder together with Aerotech's Soloist™ HPe integrated servo drive and motion controller. The sinusoidal motion continuously oscillates the sample under test at selected low frequencies and is delivered in combination with a fast output pattern from the Soloist that gives the Rheo-Line the time vs. angular position information that is critical for the in-phase and out-of-phase measurements recorded by the machines sophisticated sensors. The synchronisation and accuracy between motion and output needs to be extremely accurate, with a working specification in the order of a just few thousandths of a degree.

During machine development and for commissioning each new machine, Prescott Instruments engineers have benefitted from the advanced tuning tools and algorithms available with Aerotech's Dynamic Controls Toolbox. The Toolbox works in concert with the Soloists' IDE software – a Windows®-based GUI that provides powerful diagnostic, development, and analysis tools that shares a common theme with all Aerotech motion control software platforms.

The Rheo-Line's impressive performance is helped by Aerotech's Harmonic Cancellation algorithm which dramatically reduces positional errors on systems with a periodic trajectory commands or cyclic disturbances typically found in oscillating trajectories.
The full effect of Harmonic Cancellation for this application can be seen on the following screen shots taken from the Soloist's IDE software including the Dynamics Controls Toolbox. The first screen (fig 1) is an initial loop transmission test that shows a well tuned system. The cross over frequency is at 108Hz and gain and phase margin indicate a system which satisfies stability criteria. Please note that an increase in low frequency gains could adversely affect stability criteria.

The machine test required an oscillation of one degree at 33.5Hz. As can be seen on Fig 2, using the Scope utility, at this frequency there is a phase shift causing position error of almost the commanded amplitude and the Position Feedback (PosFbk) plot shows that the sine wave is far from achieving the commanded value. This phase shift is due to the large inertial load and the controllers inability to track the commanded frequency with the stable gains.

Using the Fast Fourier Transform (FFT) utility on the position feedback, Fig 3 clearly shows that the input command frequency of 33.5Hz is the dominant frequency, however the plot also indicates a less significant peak at 100.5Hz.

Using the Aerotech Dynamic Controls Toolbox, Fig 4 shows the Harmonic Cancellation selected for frequency and applied at 33.5Hz.

Looking again at the Scope, Fig 5 shows that the position error has reduced from ±0.5 degrees to less than ±0.005 degrees - an improvement of 100x original error. The phase shift is effectively eliminated at this point.

The FFT of the Position Error in Fig 6 now shows that the original 33.5Hz harmonic has been eliminated and the 3rd harmonic is now dominant (100.5Hz).

To further improve the system response, Harmonic Cancellation is now applied at 33.5Hz as well as the 3rd Harmonic of 100.5Hz (Fig 7).

The resulting position error is now reduced from ±0.5 degrees to ±0.0013 degrees (Fig 8). This gives the customer a 385x improvement in position tracking from the original error and falls well within the original specification that Aerotech was required to meet.

Aerotech's Harmonic Cancellation algorithm is extremely effective where a motion profile is continually repeated such as the sinusoidal oscillation required for the Prescott Instruments Rheo-Line. Other applications that may benefit include sensor testing of accelerometers, gyroscopes and other inertial measurement. The algorithm may also be useful for multi-axis positioning systems where a periodic or rotational motion on one axis leads to unwanted error motions on the other axes. Machine tools, tracking systems, spin stands for data storage, and semiconductor wafer processing equipment all exhibit this problem to some degree. The harmonic cancellation algorithm can adapt to such disturbances that are periodic on the position of another axis, even if the speed of that axis, and thus time-based frequency, changes.

Other Dynamic Control Toolbox tools available to improve machine accuracy and increase speed and throughput include the ETM (Enhanced Throughput Module) – a software and sensor based hardware solution that measures the unwanted motion in several directions on the machine base, sending the information to Aerotech controller which modifies acceleration/deceleration and other factors. This results in significantly improved move-and-settle time and contouring performance which in turn increases the throughput of existing and new machines, and greatly reduces the effects of frame motion on the servo system.

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