In order for this machine to work properly, it must satisfy several design requirements. These requirements were established based upon the environmental conditions to which the machine will be subjected and the overall problems the project aims to solve. The design requirements are divided up into three sections: requirements for the hardware, requirements for the software, and requirements for the system as a whole. A block diagram of the UDC is shown in Figure 3.

Figure 3: Hand-drawn Block Diagram of the UDC

 

3.1 Entire System

The machine must be able to autonomously rise one foot, stay still for one minute, then rise one foot again and repeat this process 130 times per day. The final design must also be completely waterproof since the Underwater Data Collector will be at the bottom of Ballston Lake for several years. The machine will be submerged underwater, so when designing the electrical interfaces it will be ideal to keep them in one or two waterproof capsules. The more capsules present increases the risk that one will leak.

The machine must also be able to operate in varying degrees of temperature with the coldest being 40 degrees Fahrenheit. The system must be able to operate during all four seasons. The lowest temperature of the lake in the winter is 40 degrees Fahrenheit. The device must be able to be manipulated without destroying the parts inside. If the battery dies it must be replaced, preferably without destroying the rest of the system. There must be a cable attached to the base of the machine that will allow it to be removed from the water if necessary. When the machine needs to be removed from the bottom of the lake the easiest way will be to have a cable attached to the base of the machine that can be used to pull it up from the bottom of the lake onto a boat.

When choosing components I focused on cost and availability. For example, if one of my components broke I did not want it to set me back weeks waiting for a new one to arrive. Instead, I chose components that are standard values that can be easily acquired. The budget for the machine during fall and winter term 2018 is $500. The machine must be built before the end of winter term 2018 in order to qualify for Capstone.

 

3.2 Battery and Charging Station

First and foremost the battery must be able to last a minimum of five years and the system must be able to run for a minimum of three hours before requiring charging. The machine will be collecting data for ten years and it will be difficult to replace the battery. When the battery needs to be replaced a boat must be rented, so it is ideal to have the battery last at least five years. The charging station for the battery must also be able to slowly trickle charge the battery. The lead acid battery must be trickle charged since it is only in use approximately three hours a day and dormant the rest.

 

3.3 Communication Wires

The communication wires must be able to transmit data from the Underwater Data Collector to a source on shore. They also must be appropriate for the temperature and pressure of the south basin. The data collected by the machine must be able to be transmitted so that it can be analyzed. They must be able to work in a temperature 40 degrees Fahrenheit and a pressure of 4 atmospheres. The wires must also have two-way communication and will utilize RS422 in order to use differential signaling to avoid noise.

 

3.4 Motor and Encoder

         The motor must have a kill switch/override program in case of an emergency. In case the machine gets stuck on something in the lake (animal, waste, etc.) and draws too much current it must be able to override the original program and turn itself off until the problem can be addressed. If the machine did not have this function it could suffer irreparable harm. The motor is a RS550 with a no load RPM of 19,300. This motor is combined with a RS-540 gearbox, which reduces the RPM by a factor of 256, resulting in a RPM of approximately 75. However, for the machine to rotate at the proper speed it needs to rotate at roughly 1 RPM. I implemented PWM into the system in order to achieve this reduction.

 

3.5 Floating Sensor

The floating sensor must be able to measure redox, salinity, and pH within 5% of their specifications from the data sheet. The floating sensor that was previously bought came with a data sheet with specifications about redox, salinity, and pH readings. The sensor will need to be sent back to the company to be recalibrated, which is under the warranty. The company will also ensure the sensor is still working properly.

 

3.6 Cable Drum

The cable drum must be able to rotate at a rate of 1-5 rpm or less under all torque conditions. In order for the sensors to remain calibrated the cable drum must rotate at a speed less than 5 rpm. If it rotates faster the sensor will become uncalibrated, and then the entire machine will need to be pulled out of the lake.

 

3.7 Microcontroller and Software

The machine must be able to autonomously rise one foot, stay still for one minute, then rise one foot again and repeat this process 130 times per day. The Geology Department has requested that the machine takes a set of measurements every foot. The machine takes approximately 20 seconds to stabilize and approximately 20 seconds to take data. Furthermore, there is an extra 20 seconds in case one of these processes takes longer than expected. The lake is 130 feet deep, so this process must be repeated 130 times per day. It will need to complete this process autonomously.

 

3.8 H Bridge

The H bridge will be used for motor speed control. It must be able to rotate the motor clockwise and counter-clockwise. It must be integrated with the PWM so that if the machine is going too slow or fast, it will self correct. The H bridge must also be able to handle a motor stall current of 50 A.