S.D. Team No. P08456
CDR Packet
P08456: Lighting System for an Underwater ROV
Concept Design Review Information Packet
Team Members:
Jeremy Schiele: Project Manager
Jonathan Lent: Housing Design
Justin VanSlyke: Mounting System
Ryan Seeber: Control System
Benoit Hennekinne: Circuit Design
Table of Contents
Project Information3
Customer Requirements3
Multi-spectral Research4
Housing Design
Scalable Concept5
Interchangeable Sealing End Concept6
Mounting System Designs7
Control Method Design8
LED System
Background on High Brightness LEDs10
LED Bulb System11
LED Driver Design11
Power Supply Design13
Pugh Concept Scoring Matrix14
Project Information:
The goal of this project is to create a lighting system to be used on an underwater remotely operated vehicle (ROV). A first generation underwater ROVwas previously designed by Senior Design project P06066 and used a High Intensity Discharge (HID) lighting system with basic on/off control. That ROV platform is now owned by Hydroacoustics Inc. (HAI) and uses LED lights. This project is focused on the improvement of the HID lighting system by using lower power Light Emitting Diodes (LEDs) and providing greater control capability.
Need No. / Customer Need / Importance 1<51 / Light Intensity is Controllable / 5
2 / Provides plenty of light for camera / 5
3 / Multi-spectrum capable / 2
4 / Minimal power use & heat generation / 4
5 / Open architecture system / 3
6 / The housing can be shared with motor unit / 3
7 / Housing can be easily attached to, removed from and moved around platform / 3
8 / Mountable onto other platforms eg:RP-10 / 3
9 / Software compatible w/ thruster controls / 5
10 / Open source system / 4
11 / Survivability to required water pressure- 400 ft / 5
12 / The design is Modular / 3
13 / The design is Scalable / 3
The design’s housing is to be shared with the thruster units being designed concurrently by P08454. The light produced must be able to be dimmed from the control module, this same software package should control the thruster unit as well. LEDs will be used as the light source to minimize power consumption and prolong the unit’s battery life. A study into the implications and benefits of lighting with different spectrum lights is being conducted to determine if a second color of LEDs is required, and if so which color will provide the best light penetration underwater.
Table 1: List of Customer needs
Multi-spectral Research:
The possibility for multiple spectrum lighting is being designed into the current concept in the probable event that multiple colors of LEDs will be used to gain light penetration in varying water conditions. The principle of penetration is based on the absorbance and scattering characteristics of water and the materials that are dissolved or suspended within it. The three categories of materials found in water are 1) color dissolved organic material (CDOM), 2) chlorophyll, and 3) suspended solids. The visible spectrum of light is from 400nm to 700nm; in order the main colors: are violet, blue, green, yellow, and red. Because the ROV uses a conventional video camera that senses and shows light within the visible spectrum, we will focus on this range of light.
The absorption of light through water and these materials is dependent on wavelength and can be seen in figures 1 and 2. The CDOM is made up of mostly decomposed organic material that gives the light a yellow tint. This is because the CDOM absorbs the most light at the low end of the visible spectrum, till it dies out around 500 nm. The chlorophyll in the water also absorbs a large amount of light that is turns into its energy via photosynthesis. The absorption versus wavelength curve for the two common types of chlorophyll (a and b) are shown in figure 2. They each have two distinct spikes around 450 and 650 nm respectively. The mass absorption of the blue and red light makes the algae and other photosynthesizing organisms appear green. Suspended solids don’t absorb as much light as the first two, but do scatter more light.
The last major factor in light absorption is that characteristics of the water it’s self, which doesn’t change between fresh and salt water. The absorption due to water is almost entirely within the second half of the spectrum, in the region 500 nm and up (see figure 1).
The scattering of light linearly decreases across the visible spectrum from 400 to 700 nm. Because the blue light scatters the most and is absorbed the least by clear water, the open ocean is bright blue. The white LEDs have abroad spectrum light and provide a wide enough band of light for clear water. When there is heavy CDOM, chlorophyll, or suspended solids, a light between 550 and 600 nm will be absorbed the least, providing the best light penetration for the video camera.
Figure 1: Absorptions vs. Wavelength.
Figure 2: Absorption due to chlorophyll a and b vs. wavelength.
Housing Design:
There are two similar but unique hosing concepts currently being developed. The first is referred to as the scalable design, and as its name implies is scalable to accommodate either the light unit or motor unit for the thruster. The design is common between the two, but the size is modified for the potentially larger motor that is contained within it. Its sealing surface is gasketed and held closed with three draw latches. It would take up the minimum size required for its components, and potentially have the smallest size envelope. However, each version of the design would need its own sized o-ring, draw latches and mounting bolts. Also, the space saving possibilities of this design are dependent on the diameter and length of the motor unit and accompanying components. If the diameter of the LED chip and motor are close in size, if will no longer be efficient to produce two sizes of the housing.
Figure 3: Scalable housing concept w/ latch clasps
The alternate design uses a similar design but different technique to make it modular with the thruster motor unit. Named the Interchangeable Sealing End design, the two end uses both share a common rear portion, which has the required wiring connectors permanently attached it to. This rear portion is only as long as is required to house the light’s electrical components. For use as a light the sealing end is closed off by a gasketed lens, thus keeping a short profile and minimizing wasted space. For use as a thruster housing a longer section is mated to the sealing surface, which provides the extra volume required by the larger size of the motor and gear components. These two versions would both use the same gaskets, latches and mounting bolts.
Figure 4: Interchangeable concept with LED lens end.
Figure 5: Interchangeable concept with motor housing extension end.
Both concepts have two blind holes tapped into the housing for mounting to the ROV platform. This way the thruster can be securely hard mounted to the platform, but the light can have a piece between the housing and platform to allow adjustment, more information available on this in mounting section. The proposed materials for the housing are black-anodized aluminum to prevent electrolysis from corroding any dissimilar metals, or 316L stainless steel, which has excellent resistance to corrosion from salts even at high temperatures.
Mounting System Designs:
Besides hard-mounting the unit directly to platform, there are three concepts applicable for mounting either housing to an ROV or other platform. The first is the hinge concept, which provides two axis of adjustment via a quick release lever (y-axis) or single bolt (z-axis). That single bolt is used to attach the light to the ROV so quick adjustment or removal of the unit is possible.
Figure 6: Hinge mounting design. Note: Quick-release not shown at pivot point.
The second concept is a ball joint style concept. This provides the same two axes of adjustability, but only requiring the screw affixing the ball joint in place to be loosened. This concept is less secure when fixed in a position, but could be used to allow the lights to be positioned further off the edges of the platform so the same water between the target and ROV is not shared by the lights and the camera. Failure to prevent this can cause the particulate directly in front of the camera to be brightly illuminated, prohibiting the camera from clearly seeing the target.
Figure 7: Dual ball joint LED module mounting concept
Where the first two concepts are intended to be used primarily with the light unit, the third concept is meant for either the light or thruster unit; it is an extension for hard mounting the housing to the platform. It is meant to move the impeller away from any obstructions, like the supports that unit is attached to. This attachment method doesn’t allow for adjustability, but assures a rigid mounting when one is required.
Figure 8: Stiff arm hard-mount extension for either light of thruster module.
All three concepts utilize the same 2-bolt pattern on the housing so any can concept can be used with a single housing.
Control System Design:
Each light or motor unit will be controlled by its own individual microcontroller, which is in turn controlled by the master microcontroller on the ROV or other platform. The individual μC selected to use within the housings (for both light and thruster unit) is the Atmel AVR family of controllers. This family has multiple benefits for both ROV teams. They have many free services and applications available including: Integrated Design Environment from Atmel, C Compiler and Assembler for Windows and Linux (GNU), and simulator. Being that there are free and open source tools available, the project is more accessible and more of an open architecture system. The 8-bit AVR family of microcontrollers all use the same core, so when upgrading to a different AVR controller, the code would only need minor changes. This enables scalability when possible enlarging or miniaturizing the unit and modularity with the possibility of common parts.
Figure 9: ARV microcontroller unit
The communications to the light module will be done using the RS-485 standard, which is a common method for this type of application. It uses differential signaling – the A and B signals are checked with respect to each other instead of ground, which allows the receiver to detect signals even if there is a small voltage shift. There are only four pins and wires required to control both the light unit and thruster unit. Two of these are devoted to power, the other two are a twisted pair for data. The twisting is done to cancel out as much electromagnetic interference (EMI) from external sources as possible, i.e.: from thrusters and other electronic equipment. This cabling has the capability of being extended over long distances without losing signal because of the reduced interference and non reliance on ground. Even though when used in the ROV it will only have to span a few feet, it does not restrict the modules only be used under those size constraints. The use of RS-485 also enables up to 32 lights or thrusters to be controlled by a single microcontroller with the multi-drop or “party-line” system which sends out signals intended for specific lights or thrusters. Below is a circuit design for the conversion of RS-232 to RS-485 (figure 10). This circuit can be used to allow for a standard PC to be used as the RS-485 master computer during the testing phase or in real use applications. The circuit has been made and tested, and verified to work well.
Figure 10: RS-232 - RS-485 converter.
Source: Dr. Reddy
Background on High Brightness LED:
In recent years, high-brightness (HB) LEDs have gained prominence as the lighting source for a variety of applications. HB LEDs are rugged and reliable semiconductor devices capable of several tens of thousands of cycles—up to 100,000 hours of operation. That performance represents an operating life that is orders of magnitude longer than conventional incandescent and halogen lamps. Thus, HB-LED applications can be found in automotive lighting, public and commercial signage, and architectural lighting.
HB LEDs are PN-junction devices especially processed to produce white, red, green, and blue light when forward biased (amber and a few other colors are possible as well). As PN-junction devices, LEDs exhibit characteristics similar to those of conventional diodes, but with higher voltage drops across their junctions. Little current passes through an LED until the forward voltage reaches a required VF, which varies from 2.5V for red LEDs to about 4.5V for blue LEDs; when the VF is reached, the current increases very rapidly (as in conventional diodes).
Consequently, the designer must employ current limiting to prevent possible damage. Current limiting can be implemented with three basic methods, each with advantages and disadvantages.
The first method is to use a resistor. This method is not expensive and use only one large component but it can not control current accurately and provides high power dissipation in the resistor.
The second method is to use an active linear control, but this solution is more expensive than the simple resistive current limiter and dissipates about the same poweras a resistor limiter for the same supply voltage. Moreover it may require mechanicalheat sinking of the active pass device.
The third method is to use a switching regulator control with a control loop which regulates LED current precisely. This solution allows dimming by amplitude control or low-frequency PWM. Moreover it does not usually have a mechanical heat-sink, which saves cost and complexity.
LED Bulb System:
The bulb concept uses 6 individual LED bulbs, of which a set 3 can be controlled at a given time by a single LED driver. This 6 x radial pattern provides even lighting with up to 2 colors of LEDs (white + one other). The smaller LEDs are more efficient, providing more luminous flux per power consumed (lm/watt). The overall bulb will be able to consume up to 10.2 watts, with each individual bulb using up to 1 Ampere and 3.6 Volts. The intensity produced will be close to 600 lumens, but this intensity will cause premature failure of the LED, at a sustained current of 700 mA each bulb would give off 100 lumens, so the light unit producing 300 lm. The multiple LED design also allows for the secondary color in each module to be selected independently of the others, so many color options are possible for alternate uses on the Robotic Platform, etc.
Figure 11: This small HB-LED has been chosen: Luxeon Rebel from Lumiled by Philips.
LED Driver Design:
In order to control the brightness of a light, LED current must be adjustable and controllable.In many cases it is advantageous to dim an LED by pulsing its current at a low frequency (50Hz to 200Hz) and controlling the width of the pulses. Though the LED illuminates with the same brightness during each pulse, the eye perceives a dimming as the pulse narrows. The light spectrum, moreover, remains constant, unlike the case of dimming by amplitude modulation in which the light spectrum shifts as the LED current varies.
Figure 12: Examples of PWM Dimming from the Datasheet of the MAX16820
The LED driver used will be able to accept the full 24 V and regulate the power consumed by the LED bank. Its dimming capability will be controlled using a PWM signal from the Atmel AVR microcontroller contained within the housing. The driver must be a low cost continuous buck driver with a wide input voltage range, enabling it to be used on platforms that use larger or smaller voltage sources (batteries).
The choice of the led driver has been made with the following specifications:
-1A output at least,
-VOUT ≥ 12V,
-VIN = 24V,
-Allow PWM Dimming (dedicated input),
-Step-down (buck) regulator with efficiency as high as possible,
-Switching regulator control,
-Bulk as tiny as possible.
According to these specifications the MAX16820 from MAXIM has been chosen since it is the most efficient product for this application.
Figure 13: Schematic of implementation of LEDs & Driver.
Thedifference between VIN and VOUTis important because larger differences result in an efficiency decrease. That’s why the use of 3 LEDs in series is interesting because each LED needs a VFof 3.5V; 10.5V in all, so this application requires more power than a one led application.
Power Supply Design:
All the digital components require a 5V input. The 24V power supply must be decrease until 5V. The gap is important. Using a step-down (buck) regulator will reduce the heat dissipation and the efficiency will be better. The LM2674M-5.0 from National Semiconductor is adapted to make this power conversion.
Figure 14: Power Supply Schematic
Table 2:Pugh's Concept scoring matrix for critical subsystems
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11/7/2018