Vision measurements are based on the idea of dynamically measuring the elongation of the balance as a function of time. Three aspects are important to guarantee the requested specifications
- Center of rotation: The center of rotation of the balance wheel must be measured with an accuracy of less than 30 microns
- Geometric: Known marks must be visible on the balance wheel. They make it possible to detect the angular position of the balance on each image, and thus define the elongation curve
- Temporal: For an adequate measurement of walking at the maximum speed of the balance wheel, the sampling frequency must be sufficiently high and precise.
Achieving the above objectives involves two successive measurement modes: the first must allow viewing of the entire balance (Ø 12 mm maximum) with a limited image acquisition speed. The second should allow the acquisition of a smaller field of vision but at a high rate. It is important to note that, for the system to be able to acquire and plot an elongation curve, the partial field of vision must allow permanent monitoring of the landmarks. In the case of the AP 3120 caliber, the weights are used as a benchmark. The selected vision system allows acquisition of images with a frequency linked to the vertical resolution of the image. To measure the walking and amplitude characteristics, it is not necessary to process all the images of the alternation. Eight acquisition windows for two oscillations (see figure below) are defined and positioned in anticipation of the position of the pendulum
Since without an exhaust there is no acoustic signal, the amplitude, walking and speed measurements are made only by vision.
The operation is slightly different from the measurement in maintained mode, the detection of zero speed images is identical but the method of calculating the amplitude is different. The images corresponding to the turning points of the balance (moment when the balance stops and starts again in the other direction) and the images corresponding to the moments when the speed is maximum are recorded and once the balance is stationary the VideoBalisometer analyzes the recorded images in order to plot the damping curve, the walking curve and the max speed curve.
More technical information is available in the act of the chronometry congress
A motorized system (see Figure 13) allows the assembly to be moved automatically to carry out all the desired measurements, in the following positions
- CH / CB / VH / VD / VB / VG + 45 ° intermediate positions
the measurements are carried out without operator intervention, both for the maintained mode and for the free mode.
- No special preparation of the balance wheel by marking, as long as it includes markers
- No lifting angle to know
- Very reliable gait and amplitude measurement
- Measurement at all small amplitudes in free and maintained mode
- Comprehensive depreciation measurement
- Analysis of disturbances of the regulating organ due to the exhaust
- Amplitude measured and not calculated
The VideoBalisometer is state-of-the-art equipment intended mainly for watchmaking laboratories, so that they can carry out precise measurements on the regulating organ and analyze the influences of all types of exhaust. It can also be used to verify the measurements made with other types of equipment.
Developments are planned such as
- Management of specific pendulums
- Increase in measurement rate (all oscillations)
- Synchronization of SMEV with vision
- Calculation of the acoustic amplitude in direct mode
- Benchmark measurement by vision
Audemars Piguet and qmt presented the VideoBalisometer. The documents are available below