Reaction Force Compensation
Accelerated linear drive movements lead to noticeable vibrations in the frame to which the drives are attached. These are generally undesirable because they cause inaccuracies in the machine or even damage. The vibrations are also transmitted to the environment via the frame and therefore interfere with other machines. Particularly for high-precision applications, which commonly use air-guided systems, these interferences cannot be tolerated. This can result in long settling times, which significantly reduce throughput, e.g. for automatic exposure processes.
There are several options to reduce or compensate for vibrations depending on the application. Adding a large weight to the frame is the easiest way to reduce these vibrations. However, for the vibrations to be reduced to an acceptable level, the weight required quickly reaches several thousand kgs. This approach is not possible due to restrictions in the installation space or maximum permissible weight of the machine. Alternative measures therefore need to be taken.
Figure 1: Illustration for a reaction force compensation using a second drive.
Figure 2: Vibration at the granite base with and without reaction force compensation (RKK) in a sample system. Details: mass basis: 200 kg, moving payload: 7 kg, movement: sinusoidal oscillation with A = 0.7 mm and f 50 Hz.
One solution that AeroLas has successfully implemented, is to use a second drive to compensate for disturbances caused by the operational movement in a linear short-hub application used for the OLED display production. This type of cancellation of disruptive vibrations is also called reaction force compensation (RKK). In the compensation drive, a mass similar to the payload is moved with an identical acceleration profile, but in the opposite direction (Figure 1). The two drives must be aligned with one another in the direction of movement, and the compensation drive and utility drive must be synchronised in their movement. Depending on the movement profile, the cancellation of vibrations can be fine-tuned by adapting the path amplitude of the compensation axis.
Figure 2 shows the effect of reaction force compensation using a measurement technology. A utility and compensation drive were screwed onto a granite base with a weight of approximately 200 kg, similar to that shown in Figure 1. The moving utility mass weighs 7 kg and the air-guided slide oscillates at a frequency of 50 Hz with an amplitude of 0.7 mm. The vibrations of the granite were measured using a sensitive acceleration sensor mounted on the granite base in the direction of travel of the utility drive. The compensating drive was only active on the “with RKK“ curve, and its amplitude was adjusted to minimise vibrations on the granite. Without RKK, the vibrations on the granite are 100 times greater (logarithmic scale!) than with RKK and are clearly noticeable.