Qinetiq - Modal analysis and balancing of "human centrifuge" 

• Customer: Qinetiq 
• Project: Control and balancing of "human centrifuge" 
• Execution date: Q1 2021 

About Qinetiq 

The Belgian branch of Qinetiq designs and builds "human centrifuges". These devices are used in aerospace training and research. By means of a rotating movement they generate artificial gravity for testing and training purposes. 

The project 

Qinetiq did a redesign of their centrifuges. A number of assumptions were made when performing the FEA (Finite Element Analysis). ACD was engaged for: 

1. Verification of the FEA 
2. Balancing of the centrifuge 
3. Control of the bearing

The structure was nudged with a measured force. The response of the structure was measured. In this way we know how the structure responds, i.e. how much the structure vibrates at a certain frequency per applied Newton. From this, 10 modes of motion were distilled. These frequencies were between 1.8 Hz and 100 Hz. An attenuation was determined for each motion shape. Some examples: Compared to the FEA, there was a minimal difference on some motion forms. With the measured data, the theoretical model could be further refined for future calculations. 

Balancing 

Given the large mass and low speed, the forces do not cause high vibration of the static part. As a result, standard on-site balancing methods cannot be used. 

This challenge was overcome by working with eddy-current probes. These probes look at the shaft. The motion of the shaft in the bearing is measured. Based on this movement (µm), a balancing was performed. 

The machine was measured and from the figure below, the amplitude and phase relative to the tacho signal was calculated:

After balancing we see: 

We still see a residual movement of +/- 20µm of the shaft in the bearing. This is a minimal movement, especially if we take into account a vertical top-heavy setup. The centrifuge was balanced in empty condition. When the (test) person takes his/her seat his/her weight will still have to be compensated.

Bearing inspection 

On the static part, a "classical" vibration measurement was performed to get an idea of the condition of the bearings. The measured signal was analyzed and compared with known fault images of the bearings present (NU2222ECP, NU 226 ECJ and a 29322E). No anomalies were found.

Results 

1. The FEA was verified by modal analysis. The data allowed the FEA model to be fine-tuned to the practice. 
2. The balancing clearly decreased the motion of the shaft in the bearing. 
3. The bearing was checked and approved for further use.