Fit Junior Design Essay

2309 words - 10 pages

Battery
Specifications
The main power supply for the bot will be one Traxxas 2872 LiPo battery with three cells. The design for our preliminary bot design requires a supply of 12V and last for at least one continuous trial consuming between 25 and 35A. The LiPo has a 11.6 working voltage, a peak voltage of 14.7 V, and a cutoff voltage of 8.8V. The battery is rated for 100 amps discharging rate and a capacity of 5Ah. The physical dimensions of the battery are 6.1 x 0.98 x 1.8" and weighs 6.29kg.
Internal circuitry
The LiPo battery contains an internal Power Circuit Module (PCM) that protects the battery from being damaged. This particular PCM in this LiPo battery is designed for the ...view middle of the document...

The assumption is that the Lunabot will be continuously moving its motors and actuators; however, the actuators are only active when extracting and depositing lunar regolith. The calculations are to provide a theoretical situation of constant consumption, so the Lunabot will be able to complete two trails continuously with one LiPo battery. The physical design of the Lunabot will be a modular design where the battery is interchangeable in case it needs to be swapped for a fresh one in between trials (added redundancy).
Power Consumption
Another source of points in the competition is reporting power consumption to the judges. To measure the power draw, both current and voltage will be measured. For current, a 50A 10mΩ high side shunt is placed in series with the battery and its load. A differential pair from the shunt connects to the AD8211 high side current integrated circuit (IC). The AD8211 has a linear response over a large range; however, the error grows exponentially as the voltage drop across the shunt decreases (see fig 2). The AD8211 has a fixed gain of 20. The output is connected to an Arduino Uno microcontroller platform. The wires going from the shunt to and from the AD8211 and to the Arduino Uno are shielded to reduce noise. Other extra precautions that have been taken as to reduce noise on the ADC are disconnecting the power from the ADC pins and adding a 0.1uF decoupling capacitor between the AREF pin and ground. A program was written in the Arduino IDE to sample analog pins A0 and A1, however, data throughput was less than optimal. Therefore, the code was rewritten in C which allowed a sampling rate of 5.8kSamples/sec for current and voltage each.
Voltage is measured using a simple voltage divider which consisted of a 10kΩ voltage trimmer potentiometer as the datasheet recommends the source impedance should be 10kΩ or less. To optimize the accuracy, the trimmer is set to output 1V when the battery is fully charged. The ADC reference voltage is selected the on-chip bandgap voltage reference which is 1.1V.

Figure 8: AD8211 Current Shunt Monitor
The baud rate for the ATMega328P microcontroller is set at 250,000 baud. This was chosen for two reasons. Since the clock is set at 16MHz the UART clock prescaler can only be set at powers of 2. The ADC gives accurate readings if and only if the ADC clock is set below 200 KHz. In this case, the ADC prescaler was set to 128, so that the clock ran at 125 KHz. For a 10-bit reading, it takes the ADC 13 clock cycles, thus around 9600 samples. However, each reading is 10-bits and a tag scheme must be implemented to distinguish a voltage reading from a current reading.
Current
1111 11xx xxxx xxxx

Voltage
1111 10xx xxxx xxxx
A current reading’s first 6-bits are always 0xFC while a voltage reading’s first 6-bits are 0xF8. The total number of bits is now increased to 16 bits for a single sample. To transmit this information over serial, an addition stop bit is needed which bring the total to 17...

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