Digital Cannon Tester (1972)
The so-called Royal Netherlands Navy Guided Weapon (GW) frigates Tromp (F801) and De Ruyter (F806), with the characteristic large radar dome, were to be equipped with a 12-cm Bofors cannon. These cannons were mounted on the A and B type submarine hunters, including HMLMS Gelderland (D811) and dated back to the 1950s. The cannons were controlled by a very old analog electronic and mechanical fire control system. The actual servo control was carried out via synchros. That was a regulation with a relatively low bandwidth, which resulted in a nice smooth steering, but also with too large errors during fast target movements or high accelerations as a result of sea movements.
The new Tromp-class frigates coming into service in 1973 were to be equipped with a digital fire control (Signal type WM 25). The control of the cannon had to be powerful enough to transmit the desired control signal and the stabilisation signal for ship movements. As a result of the inaccuracies in the fire control and the digital control, unwanted noise is created. The latter demands part of the control power.
HSA (Hollandse Signaal Apparaten) suggested performing the calculation of azimuth and elevation angles at a frequency of 25 Hz. According to an experienced LEOK department head, that frequency was too low. It would lead to inaccuracies due to lag and too much noise in the control of the cannon. He argued for 100 Hz. However, that would place a much greater burden on the fire control computer supplied by HSA. This created a serious difference of opinion between Signaal and the LEOK about the implementation of the servo control.
Ultimately, it was decided to conduct research into:
- The noise to be expected from the fire control (amplitude and spectral composition)
- The required sampling frequency
- The required control level at which the signal is made up of three components: target tracking, stabilization and noise.
A sampling frequency that is too low leads to aiming errors, a frequency that is too high required too much computing power at that time. A high noise level requires a lot of steering power, which is reduces the power for target tracking and stabilisation, and can lead to excessive cannon wear. Filtering the signal suppresses the noise but also the other signal components resulting in aiming errors. Ultimately, a compromise has to be made.
Theoretical research and many calculations do not lead to agreement. Testing had to be decisive. For this purpose, a “digital cannon tester” was developed and built by LEOK. With this digital test box, the cannon could be controlled from both angles. The box, therefore, contained synchro-digital converters for measuring the position of the cannon and amplifiers for steering the cannon. The box also contained electronics to generate sinusoidal or other control signals and to add noise. Bandwidth and strength of the noise and the frequency of the digital generation of the control signal could be set. Relevant signals were recorded on an 8-channel paper recorder.
The test took place in 1972 onboard one of the old submarine hunters. Present were the Head of Quality Control of HSA, the LEOK department head and one of his employees. “It was a very special experience for me as a young employee. In the small space below deck, you stood next to the rotating ammunition drum with rattling grenades. It was quite noisy. Both experts laid their hands on the rotating and vibrating ammunition drum. For the experienced gentlemen that provided already a very good indication. Ultimately, a 100 Hz control frequency was chosen.