Initial Background Research
This project covers many different aspects of engineering, manufacturing, and regulation. Based on the level-one requirements, this device will require simplex satellite data transmission, miniscule power requirements, and analog to digital conversion of the loop sensor data. In order to accomplish this, FCC regulations regarding wireless transmission need to be understood and I2C specifications have to be followed. In addition, safety and ease of manufacturing will need to be considered along with the end users economic concerns.
Satellite Communication Networks
There are two major commercial satellite communication network in the marketplace. These are Global Star and Iridium. Iridium offers a short burst data service and has modems to go along with it. Multiple hobby level development boards exist for short messaging on the Iridium network. This is due to Iridium having multiple simplex, voice, and data modems available. However, Geoforce’s existing framework requires Global Star. Therefore, Iridium network compatible solutions will not be investigated further.
The specific transmitter used by Geoforce to transmit on the Global Star network is the MYTE designed by Sypes Canyon Communication. This device can carry a 72-bit data payload and can accept commands via and I2C interface at 100 Kbaud. With this device, communication with the Global Star network occurs on 4 different channels centered between 1611.25 and 1618.75 MHz.
I2C Specification
I2C (pronounced eye-squared-see) is a two wire, serial communication interface. The MYTE transmitter defines this as the protocol used to issue commands and input a data payload. It was created by Phillip (now NXP Semiconductors) and has become an industry standard, addressable interface. No fee is required to license an I2C device; however, there is a fee to register a slave address with NXP.
I2C follows a master/slave protocol. With the standards set forth by Phillip, the master devices drives the clock, and the slave device transmits data. Because the master expects data at specific times in the clock cycle, the slave can hold the clock low until a transmission is ready. This is a technique called clock stretching. In addition, I2C is capable of being “bit-banged” if the microcontroller in question does not have built in I2C functionality.
Microcontrollers
The main concerns of this project is power consumption. In order to satisfy this level one requirement, the microcontroller must draw as little current as possible during its sleep mode. Multiple companies such as Silicon Labs and Microchip provide preexisting, low power solutions. Silicon Labs provides microcontroller solutions that include onboard I2C and 10/12-bit ADCs while retaining a 300 nA current draw at sleep with a clock interrupt and only 150 uA active current. As a theoretical estimate with a single 1000 mAH battery, the Silicon Labs microcontroller could run in its sleep mode for 380 years. This far exceeds the 5 year requirement.
Analog to Digital Conversion
Analog to digital converters (ADCs) are built based on a variety of configurations. There are three architectures that are the most common: flash, sigma-delta, and dual slope. Flash ADCs operate via parallel conversion using a series of op-amp comparators. They are very fast; however, they only offer low, are expensive to manufacture, and consume a relatively large amount of power. Sigma-delta ADCs offer high resolution while still being inexpensive to manufacture. They attain high resolution by oversampling. However, this causes slower operation when compared to the clock. Dual slope ADCs use a time averaging technique to reduce noise. This results in a high degree of accuracy but is slow due to the averaging. In addition, high precision external components are necessary to ensure an accurate conversion.
Current Loop Sensors
Current loop sensors are simple devices modelled after pneumatic controls and are often used in factories where different air pressures indicate different operating conditions. Controlled pressures, usually from 3 psi to 15 psi, were maintained throughout the system and monitored in a central control room. The benefit of this range of pressures was that the reaction of the system was near instantaneous with good resolution and control. Measurements outside the range could indicate a number of problems with the reporting system as well as with the equipment being monitored.
During the 1950s, when electronics became more popular and easier to produce, many of the pneumatic designs were replaced with very similar, cheaper, and faster current loop systems. In addition, the current loop designs did away with bulky air lines, power-consuming air compressors, and added the ability to perform algorithms based on the input/output.
Current Loops allow for robust and simple data transmission, with current being a transmission medium which is resistant to electromagnetic interference (EMI) and accurate even with poor impedance matching.
Power Supply
Most important 1st Level Design Parameter is the life of the product, power-wise. The device is required to operate for approximately 5 years on a set number of provided batteries. Each of the battery cells averages about 3.6V, with a very sharp power drop at the end of their life.
The overall design must be able to work with a very small amount of power since the bulk of the battery power supplied will be for the 24V supplied to the external sensor. The system should be off (or in a low-power state) unless taking a measurement. This will help to limit the amount of power consumed and help to increase the life expectancy of the system.
Many options already exist to help manage the available power, since many devices used today are designed to operate on minimal power. The options to consider include step-up “boost” DC-DC converters, minimal resistance sources (to minimize the loss due to heat), and highly efficient components such as low power op-amps, microcontrollers, and high efficiency chips.
FCC Regulations
Because Geoforce transmits wirelessly in the United States, it must follow Federal Communication Commission (FCC) guidelines. Currently, Global Star is authorized for the 1610-1618.75 MHz band and the 2483.5-2500 MHz band. Of these, the GT1 transmits on the first band.
The requirements for this project do not require transmitting data wirelessly. Most of the design will be based around a hard-wired, twisted pair, there will be no need to concern with regulations such as FCC or interference causing device regulations from other parts of the world. However, the design must be able to handle any interference picked up on the connecting wires. This will be solved by the placement decision for the LP filter which will be used to help control any noise introduced into the system by the environment.
Safety Issues
Most of the safety concerns associated with the sensor input side of the coupling device will be for the safety of the device. Since the only ‘exposed’ part will be the twisted pair, which operates at ~24V and typically 4mA-20mA of DC current, and where the wires connect to the device, this is the only (limited) concern for human exposure. Exposure would only occur if the wires were stripped (exposed) and the person came into direct contact with them or a directly coupled conducting surface. Assuming the typical human body is seen as a 2Mohm resistance, the current through the body would be 12uA (where 1mA is the ‘threshold’ of human sensation).
Manufacturing Concerns
Loop sensors are produced en masse for a multitude of applications, therefore, there will be no need to be concerned with the availability of these devices. Also, the sensor will be provided by the end user, with no attention given by the coupling device aside from connections.
While the coupling device will be a novel design, there should be no new electrical components designed for the overall device, aside from calculations for resistance, capacitance, etc. Therefore any chips or individual components needed will be ‘off-the-shelf’. In other words, manufacture of the end design should be relatively simple, not time-consuming, and cheap.
The only custom manufactured part of the coupling device will be the enclosure. Due to the locations where the device will be deployed, the enclosure will need to be rugged and easily mounted on any surface.
Device Economics
Geoforce’s products are primarily intended for use as asset trackers for the oil and gas industry. Currently, Geoforce’s clients pay about $10 per month for service in addition to $0.10 per data transmission. The addition of the sensor data would not increase the data charge from Global Star. This is because Geoforce’s asset trackers already include a data payload.
One end goal (1st Level Design Parameter) is that the mass produced product cost no more than US$50 per unit for a production run of 10,000 units. Ideally, Geoforce would like to see a production cost of only US$20. This would ensure that the complete end package (transmitter and coupler) would be affordable to large oil and gas companies as well small private companies needing remote sensing capabilities.