The core of the power management system is the data-gathering module. The module uses a PIC microcontroller to measure voltages, currents and temperatures and send the results to the computer via USB.
The end result of this system is similar to the power management utility in your laptop. Just as with a laptop this is a self contained system that will not have access to external power some of the time (when there is no sunlight). We feel that because of the complexity of the multi user computer as well as the photovoltaic system we needed to design a system that not only watches battery voltage but also stores and interprets data for intelligent power decisions.
R esearch
The current sensors used are LEM FHS series sensors. They are Hall effect sensors which operate by measuring the electric field produced by the current carrying wire. The field is proportional to the current. These current sensors are isolated from the actual power path and can be turned off to conserve power. Different models of the sensors are capable of measuring anything from milliamps to over 50 amps. The biggest advantage of these sensors is scalability. A single chip can handle any range of current by clever PCB board design. We acquired several sample units from LEM and put them into small project boxes that can be used anywhere. They attach to the power management system via d-sub cables.
Temperature measurements use a small probe, which outputs a voltage proportional to the ambient temperature. We chose the national instruments LM335z. These are Zener diodes with a linear internal voltage drop proportional to the temperature of the device. Our design allows for up to 4 sensors.
Because the PIC uses a very small amount of power, it is continuously operating to ensure system safety. The PIC is connected to a small LCD screen which displays pertinent information about the condition of the battery and solar panels.
Two buttons mounted beside the screen allow a user to scroll through the information showing them battery voltage, current, and amp hours remaining. Also there will be displays for voltage and current from the solar panel. This information will be used to compute time and percent remaining before system power down. There are also three status LED's that inform the user of system power, battery good, or battery low.
The PIC communicates to the computer via USB. This is done using the FTDI UM232R. This device automatically converts the USB signal to a standard serial signal. It also registers with the computer as being a serial port. With the connection, the computer can query the PIC about the status of the battery and can shut the system down safely when the battery is low. The computer can also query the PIC to get real time data on the voltages, currents and temperatures. The information is stored in an XML file and can be easily reprocessed into any file format for analysis of the data.
Voltage Measurements
Voltage measurements require the addition of a voltage divider into the circuit because the PIC has a maximum dynamic range of 5 volts. An advantage of using the PIC to measure voltage is that the value of the resistors in the voltage divider can be accurately measured and the program can use that information to improve the accuracy of the voltage measurement.
The built-in 10-bit ADC of the PIC 18F4520 microcontroller is a cheap way to measure analog voltages. A drawback is that it is limited to measuring only 0-5VDC. By adding a voltage divider, customizing the software slightly, and using a digital IO pin as a current sink, we can efficiently and accurately measure much larger voltages.
Figure 12 - Voltage divider with selectable current sink.
The 10-bit ADC on the PIC18F4520 is implemented as an RC circuit inside of the PIC. Because of this, there are certain parameters to be aware of, all of which have been taken from the PIC18F4520 Datasheet.
By adding a voltage divider to the input pin of the PIC, we are now able to scale the input voltage. All that is left is the selection of resistor values. Starting with known values, we can select vout to be 5V, as that is vref+ on the PIC. Also, recalling from the first section, the nominal input impedance to the PIC is 2.5kΩ, and as such will be selected for R2. The node labeled vin is the high voltage input to the voltage divider, and should be selected as the highest voltage desired to be measured. For this example we will say 50V.
As can easily found by inspection, the formula for a voltage divider is:
vout = vin ∗ (R2 / (R1 + R2))
This gives us a formula, three knowns, and one unknown. Solving for R1:
R1 + R2 = vin ∗ R2 / vout
R1 = R2 ∗ ((vin / vout) – 1)
R1 = 2.5kΩ ∗ ((50V / 5V) – 1)
∴ R1 = 22.5kΩ
We can now build our new voltage divider and accurately measure voltages up to 50V! To convert the ADC result to actual voltage, the following equation is now used:
van0 = ((VDD ∗ ADC_output) / 1023) ∗ (R2 / (R1 + R2))
Control Board
This is the schematic and for the control board. It is mounted behind the control panel and connects directly to the LCD header. There are headers for connection to the status LEDs as well as the control buttons. A ribbon cable connects this board to the rest of the circuit
Figure 13 – LCD control board schematic.
Power Management Board
This circuit allows connection of up to 4 current sensors, 4 temperature probes, and 4 independent voltages. The voltage inputs have a voltage divider circuit that can be configured to accept voltages for systems running at 12, 24, or 48 volts. A USB connection allows data sharing with the computer.
Figure 14 - Power system monitor schematic.
Conclusion
Our power management system performs as it was designed to. Each channel accurately reads a voltage from the sensor and converts the voltage to a value. The PIC then uses this information and displays this information clearly on the LCD panel. Both boards are mounted in an enclosure with connectors for each sensor. The board will be mounted in the main enclosure and will allow a user to turn of the entire system with the touch of a button. The sturdy PCB design and construction will stand the test of time. The PCB board also allows for on site reprogramming if it is deemed necessary.
Figure 15 - Power system monitor and faceplate.
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