Computer history: Analogue and hybrid computing at the LEOK


Analogue and hybrid computing


Electronic analogue computers were popular in the period from 1945 to the end of the 1980s. They were used to solve differential equations and to carry out simulations of physical systems. Many simulations were technical. Physical, electrical, and chemical systems and processes were important fields of application. Especially, companies from the aerospace and defence industries used electronic analogue computers. But even a company such as the Dutch Gasunie purchased a (hybrid) analogue computer at the end of the 1970s. With hybrid computing, the links between the analogue computing elements and the input values were controlled by a digital computer, for example, a DEC PDP or NOVA Eclipse. In the end, analogue computing became “extinct” and was replaced by digital computing and simulation. The reasons were the increasing costs of digital computers, the flexibility of digital computers, and the limited accuracy of analogue computers.

Analogue calculation can also be done with mechanical and pneumatic systems. A very simple example of a mechanical calculation aid is the slide rule. Numbers on the rule are indicated on a logarithmic basis. By adding logarithms -by sliding the slide rule- the result of multiplication can be read. Another example is the fire control on the cruisers HNLMS Tromp and HNLMS De Ruyter. These were partly made up of mechanical components such as gears and camoids (discs with a circumference with bends according to a certain function). Below, we will discuss electronic analogue and hybrid computing only.

The basic element of analogue computers is the operational amplifier, abbreviated to OpAmp. By adding input signals and feedback output signals via resistors, capacitors and coils, additions, multiplications and integrations can be realised. The OpAmps and the passive components are connected using a so-called patchboard. When simulating or calculating a mathematical (differential) equation, the mathematical parameters are represented by an electrical voltage. The development of simulated functions was made visible on an X-Y plotter (course Y as a function of X) or on a multichannel paper recorder (more signals as a function of time were made visible in parallel).

Analogue computers at the LEOK

The first analogue computer at the LEOK was probably purchased in the early 1960s. It was an Applied Dynamics system with 64 operational amplifiers using electronic tubes. When the calculator was used, it produced a lot of heat. Later, still in the 60s, the system was expanded with another 32 operational amplifiers. Those, however, were already built with transistor technology.

Around 1978, the old and not very stable analogue computer was replaced. The analogue computer was ‘handed over’ to the Royal Military Academy (KMA) in Breda. The system was used there for some time for teaching purposes and simulations by the Technical Department.

Patchpanel of an analog Applied Dynamics computer (later version than in use at the LEOK)
The patch panel of an analogue Applied Dynamics computer (later version than in use at the LEOK)
[Photo by Joost Rekveld –]
The new system (1978) was a hybrid computer system manufactured by Electronic Associates Incorporation (EAI), Princeton, New Jersey (US). The hybrid computer comprised an analogue system and a (digital) PDP-11 computer. The function of the digital computer was mainly to take care of the numerical settings (i.e. the setting of potentiometers) and to quickly change them to repeat the simulations of a system with different initial states or parameters.
Two LEOK employees followed a course at the manufacturer in New Jersey to operate the system.

EAI 680 series system [courtesy Computermuseum UvA, Netherlands]
EAI 680 series system [courtesy Computermuseum UvA, Netherlands]
During the merger of the LEOK and Physics Laboratory TNO in 1985, the hybrid computer was moved to TNO-FEL, The Hague. The hybrid system was still used to a limited extent for the Mk37 torpedo improvement and the UDB projects. Almost all simulations were, however, carried out on digital computers from that time. After several years (about 1992) the hybrid computer was removed.

Applications of analogue computing

Within the defence domain, analogue computers are often used for applications in aviation, such as simulations of missile systems. Some examples of the use of the analogue computers at the LEOK were:

  • simulating control systems,
  • simulation of missile-target interaction (also torpedo-target interaction) and studying various homing laws,
  • calculating the mirror effect of tracking radars in a dynamic situation,
  • visualising calculated antenna diagrams,
  • showing the working of an Electronic Warfare (EW) system.

Two large, characteristic projects were:

  • the Seacat blind guidance project on the first analogue computer ;
  • the project MK37 torpedo improvement on the first analogue computer which was continued on the hybrid computer system.

Both projects -described below in more detail- were examples of simulations in which parts of the hardware system to be investigated were connected to the analogue computer to be able to perform a closed-loop simulation. The dynamic interactions of an air target or missile (respectively a sailing target and a torpedo) were simulated on the analogue computer connected to the electronic subsystem of respectively the missile and torpedo. This possibility of linking simulation and a hardware (sub-)system made analogue and hybrid computers attractive.

M44 Seacat blind guidance project

In the 1960s, the Van Speyk-class frigates were built according to English design (Leander class). The six frigates were operational from 1967 to 1989, after which they were sold to Indonesia. As weapons, they had, among other weaponry, a 4.5″ cannon and two Seacat launchers, each with four missiles for air defence against enemy aircraft. The Seacat was developed by Short Brothers and Harland in Northern Ireland. Every launcher was equipped with an aiming device with fire control, the M44 fire control system manufactured by Hollandse Signaalapparaten (HSA).

The Seacat operator role was very complex in the original English version of the Seacat. The operator had to turn the aiming device with his feet while simultaneously aiming a viewer with his hands at the target. He had to bring the fired Seacat into the radar bundle with an optical system and keep it aligned with the target. The installation then sent radio signals to adjust the Seacat towards the target. This proved to be very difficult for the operators. It required a lot of training.

That is why the project “Seacat blind guidance using the M44 fire control” was started in 1964. After a preliminary study in 1964, a design study followed in 1965. The radar of the M44 could independently track an air target. That is why electronics could be developed at LEOK to allow the Seacat to automatically follow the M44 and adjust the Seacat flight path.
In 1966 and 1967, a real-time simulator was developed using the analogue computer to simulate the target and the Seacat. Developed new electronics could be connected to this model. The operation and performance of the electronics could thus be tested.
In 1968, the prototype of the blind guidance was tested on the HMS Van Galen (F803) using 12 “live firings”. The final design was made between 1969 and 1970. Transfer to HSA for production and installation in their M44 fire control followed. In 1973, another series of “live firings” followed with the new M44 on the HMS Van Galen (F803).
This development turned out to be a major operational improvement for the Royal Netherlands Navy.

MK37 torpedo improvement project

Around 1970, the Royal Netherlands Navy purchased new torpedoes for its four 3-cylinder and its two Zwaardvis-class submarines. As part of a modernisation program, the submarines were equipped with new fire control equipment (type Mk17) made by HSA. The entire system including sensors, fire control and weapons (both the old British Mk 8 and the American Mk37 mod 2 torpedoes for the Royal Navy) was evaluated in 1970 when the LEOK collected and analysed the data.

The Mk37 mod 2 torpedo was propelled by an electric motor using energy from a large battery. The advantage of electrically driven torpedoes is that they are relatively quiet. The disadvantages, when compared with fuel engine-based torpedos, are the limited available energy resulting in a lower speed and shorter range.

Therefore, the Royal Netherlands Navy decided to convert some of these torpedoes into the Mk37C by replacing the battery and electric motor with a fuel tank and a combustion engine. This resulted in a higher speed and a larger range. The disadvantage, however, was an increased noise level due to the engine (self-noise) and the higher speed (flow noise). This led to more noise in the acoustic system to detect and track the target.

MK 37 mod 2 - photo Navy museum, Den Helder
MK 37 mod 2 – photo Navy museum, Den Helder

In 1975, the new torpedo was evaluated by the LEOK in combination with the LWS-30 sonar developed by Van der Heem. The evaluation showed that the Mk37C torpedo did not function properly in all cases. Because of the larger own-generated noise levels, detecting a target (hostile ship or submarine) was less easy. Often the target was lost in the tracking phase.

The US Navy did not use the Mk37 torpedo. However, the Norwegian and Canadian navies did. Therefore, the Royal Netherlands Navy could not approach the US Navy to investigate and resolve the problems. Instead, a collaboration with Norway and Canada was started. A research assignment was granted to the LEOK. The Mk37 torpedo improvement project included a large number of activities, including:

  • research on the electronics of the torpedo,
  • theoretical calculations of the possible performance of the acoustic system in an active and passive mode,
  • participation in seagoing trials,
  • analysis of shots with exercise torpedoes, among other things by reading the recorded data from a 14-channel film recorder,
  • installation of an amplifier and an electromagnetic tape recorder in practice torpedoes for the recording of the acoustic signals,
  • advise on the exercise programs together with representatives of the Royal Netherlands Navy, and consultations with the manufacturer of the torpedoes (first Northrop, later Honeywell who took over the program),

Parallel to the research at the LEOK, the Physics Laboratory TNO (PhL) advised on the flow noise and designed a new nose for the torpedo. This development resulted in a drastic reduction in self-noise.

This project made much use of the analogue, later hybrid, computer. On the computer, the paths of the target and the torpedo were simulated in three dimensions. From the distance between the target and the torpedo and the direction of the torpedo to the target, the strength of the target signal to be received by the torpedo was calculated including the directional information.

The torpedo was equipped with a transducer split into four quadrants. This allowed the direction of the incoming signal to be determined in the horizontal and the vertical plane (similar to the mono-pulse system for a tracking radar). The signal receiver and the control electronics were included in the simulation, but the transducer was not. The calculated four input signals were converted to signals on the operating frequency of the sonar system via a separately developed amplifier. These signals were presented to the electronics of the signal receiver of the torpedo. This receiver then produced the rudder commands to steer the torpedo. The rudder commands formed the input for the model on the analogue computer. In this way, a closed-loop system was created which could be analysed.

The simulations gained much better insight into the functioning of the acoustic system and the electronics of the torpedo. It became clear why the torpedo did not function properly in some situations. Proposed improvements to the electronics could be studied in the simulations and then applied in practice.
Suggested improvements were accepted and applied by the manufacturer, usually after thorough discussions. This led to new versions of the torpedo, the Mk37D and later the MK37E. A

A short article on the modernisation of the MK37 torpedo appeared in the International Defense Review in 1983. That article is based on data from Honeywell and mentions the role of the LEOK.

Hybrid computer at TNO-IZF

The TNO Institute for Human Factor Research in Soesterberg also used a hybrid computer installation in the mid-1970s. This consisted of a DEC PDP 15.40 with a 32K words, 18 bits core memory of in combination with a Hitachi-240 analog system. The hybrid system was used in the major simulators for:

  1. Calculating a mathematical model of vehicles or ships. Actions by test subjects in a mock-up are fed to a system of motion equations that describe the behaviour of the vehicle or ship in terms of translations and rotations. The hybrid computer was used mainly for vehicle simulation; ship movements are slower and could already be calculated digitally at that time.
  2. Calculating control algorithms for the control of servo motors (x, y, z (height) and small rotations around the vertical axis).
  3. Monitoring the entire system.
  4. Calculating and passing on information to the subjects in the mock-up via pointing instruments or other components.
  5. Collecting data from the experiment for later off-line processing.