SeaDAQ - ATM

Introduction

The SeaDAQ is the Keynes Controls high speed, high resolution measuring instrument with control systems capability designed to operate under water. The SeaDAQ offers synchronisation between all channels including digital I/O within the instrument and between instruments across a network. Using the STM-1 and OC-3 ATM networks a large number of high speed channels can be distributed across a network without any loss in synchonisation.

The SeaDAQ is supplied in a corrosion proof and water proof high specification grade 5 Titanium housing, as shown in Figure 1. The SeaDAQ enclosure utilises the same metallic construction in all component parts in order to prevent electro-chemical potentials forming between dissimilar components and thus prevents corrosion damage.

Features

• High Performance Grade 5 Titanium Housing
• 6 - 14V DC 1.8W Power Supply
• Operating Temperature Range - 40 to 60 Deg C
• Corrosion Proof Enclosure
• Waterproof operations to a depth > 5 Km
• 8 Full differential Inputs +/- 2V Rms Range
• 24 Bit ADC Resolution Sigma Delta Converters/Channel
• To 100 KHz/channel operations.
• 2 Digital TTL compatible Inputs
• 2 Digital Outputs
• 4 point Auto Calibration
• 155.52 Mb ATM network interface
• Fully Synchronised analogue and digital inputs within the instrument.
• Fully Synchronised operations between instruments across ATM network.
• Fully compatible to Vanilla (Industry Standard) network systems.
• Unique Serial numbers for simplified systems management.

Waterproof Connector

The SeaDAQ housing is waterproof to 10,000 psi i.e. a depth of > 5 Km. At each end of the instrument a waterproof 19 pin connector is used to supply to sensor signals and network connections. The 19 way connector is suitable for connection and disconnection underwater making the SeaDAQ easy to replace by divers in case of repair or re-deployment especially in the low light conditions underwater

The mating of the socket onto the pins forms a water tight seal with excess water being forced out of the connector as the plug is screwed tight. Also the 19 way connector has a self alignment tab mechanism, as can be seen in Figure 2, making the pins automatically align with the plug as it is screwed onto the socket. This feature makes connection and disconnection easy to undertake when deploying the instrument.

Serial Number Identification

The SeaDAQ utilises a unique serial number to identify the instrument. The Data analysis and configuration software automatically identifies the serial numbers within the data streams and uses this information to identify the instruments on a network. The unique serial number identifier is particularly useful to identify instruments on large distributed networks especially those using large or multi-channel switches where a number of network strings are combined and the distances between nodes is great.

Sample Rates

The Table 1 shows a summary of the sample rates versus number Sonet nodes and therefore channels that can be deployed on a single SMT-1 and OC3 fibre optic network. In practice the sample rate of the instruments be incremented in steps of 8 KHz from 0 to a maximum sample rate of 96 KHz/channel. In practice 100 KHz/Channel can be achieved if requested at time of order. All instruments on a network and channels within an instrument are synchronised together. The digital input level readings are are synchronised to that of the analogue readings. It is currently not possible to mix sample rates between instruments connected together on the same network.

Boot Sequence Protection

The SeaDAQ contains a boot-up ramp protection system to ensure that the correct ramp speed levels for the ATM node power supply and network intiialisation signals are maintained even at the lowest operating range of the instrument. The protection system is required to ensure that the ATM sonet node section of the instrument boots correctly at low temperatures which may occur at extreme depths of the ocean.

Analogue Data Acquisition System

The data acquisition system within the SeaDAQ consists of 8 fully differential analogue input channels offering up to 256 individual input ranges. Each channels utilises an individual 24 Bit Sigma Delta ADC technology to undertake the analogue conversion.. Each input allows for up to 16 million individual input levels ensuring that the highest dynamic range and smallest input signals are easily resolved.

Channel Synchronisation

All of the analogue inputs are synchronised within an instrument to within a time skew of 10 ns. All inputs are further synchronised across a network. The driver software can be used to correct for the time delay that occurs between Sonet nodes due to the propagation delay between the individual instruments on a network. The propagation delay between sonet nodes is typically 0.15ns/metre.

Filter Frequency Respose

Figures 3 & 4 show the normalised frequency respose for the Sigma Delta ADCs. There are slight filter characteristic differences for sample rates 0 - 50 KHz and 50 to 100 KHz. The digital filter characteristics built into the ADC cannot be physically changed i.e. the filter type adjusted. The pass band and stop band settings are sample rate following and automatically assigned to the correct value when setting the sample rate.

Figure 5 shows the pass band ripple for the Sigma Delta filter. This feature of the anti-alias retains the same shape no matter the sample rate. The pass band ripple is excellent and for the majority of applications should be considered flat.

Figure 5 - Sigma Delta Pass Band Ripple Filter Characteristics

The following image shows a typical network layout for the SeaDAQ instruments deployed on an STM-1 copper network giving a maximum distance between nodes of 100 meters and further connected back to a dedicated data logger computer via a fibre optic network and ATM switch. The part numbers shown are for the Marconni ATM network systems and are included for example purposes only.

The example above shows a typical distributed network layout for a system containing 6 x SeaDAQ instruments connected to an STM-1 network and further passed to a data logger computer system via a Fibre Optic network interface.

Power Supply

The SeaDAQ power supply utilises a switched mode 6- 14V DC 1.8W power supply to drive the electronics. As long as the power supply levels stays within bounds of the specified input voltage then the instrument will work perfectly. the power supply includes a thermal cutout which will automatically switch off the instrument if the internal temperature exceeds a preset limit.

Calibration & Auto Configuration System

The SeaDAQ utilises an automatic calibration system to maintain the accuracy of its readings. The calibration system can be run automatically from the driver software or upon demand from the user. The calibration system uses a 4 point calibration process to determine the input factors. The ranges for which calibration values are taken are 2V, 200mV, 20mV and 0V. A linear interpolation algorithm is used to provide the calibration factors.