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Project Specs
#Year Milestones
- 200 W (50 V @ 4 A) power source attached to the wall.
- 10 A max discharge for 24 V motor
- 48 V/25 A H-bridge
#Sensors
| Names | Verification Tests | Status |
|---|---|---|
| 3 Rotary Encoders | Detect +/- 90 deg accurately, verified with plots | Completed on 2 of 3 encoders |
| 2 9-axis sensors | Accelerometer accurately detects downward slope, Gyro shows angular velocity detected by encoders | Sensor not installed |
| 1 Current sensors | Reading is stable at constant current consumption, current reading agrees with external multimeter | Testing not completed |
| 2 Battery managers | Battery voltage readings change in line with expected charging curve | Battery system not completed |
| Vision system | Vision system detects robot correctly at measured increments along board, Frames per second test | Completed |
Sensors should be able to detect:
- maximum angular velocity of 9.3 rad/s
- 25 Amps max through the motor (the chip we currently have has a pin-compatible 25 A version.)
- 1/4 degree resolution of rotary sensors
- speeds of 200 rpm coming out of/going into encoders/decoders
- 4g max accelerations
For encoders we will be using the 1800 CPR E3 encoder from US Digital for the magnets and EC60 flat from Maxon Motor.
The current design uses Tenergy LiFePO4 18650 3.2V 1100mAh Power Cell (Max: 25A) cell batteries, but our design should accommodate a variety of 18650 cells with different capacities and discharge/charge rates. The ideal cell battery is the A123 18650 cell, but they are not easy to come by.
#Actuators
| Name | Test | Status |
|---|---|---|
| 2 Magnets | Magnets turn on and off with PIC control | Completed |
| BLDC Motor | Motor can be commutated | Completed |
- Accel x2 (450 uA x2) -- SPI bus + 2 CS
- Gyro x2 (7 mA x2) -- SPI bus + 2 CS
- Magnets x2 (230 mA x2) -- 2 DIO
- H-bridge x3 -- 6 PWM
- Current sensor (11 mA x3) -- 3 AI
- BM (15 mA x3) -- SPI bus + 3 CS
- BC (450 uA x1) -- 3 DIO (ENC, /FLT, /CHRG; could also read IBMON & IIMON => 2 AI, not included in budget)
- Xbee (50 mA x1) -- 1 UART w/ CTS/RTS (essentially 2 UART modules)
- Encoders x3 (450 uA x3) -- SPI bus + 3 CS
- Status LEDs x4 -- 4 DIO pins
- SPI - 3 pins (CLK/MISO/MOSI) + 10 CS (DIO)
- DIO - 9 (4 LEDs + 2 Magnets + 3 BC)
- AI - 3
- UART - 4 pins
- PWM - 6 pins
35 total pins + 10 additional = 45
- 27 V < VWALL < 48 V, IWALL > 5 A
The LTC4000 battery charger has 4 regulation loops for
These feedback loops require the selection of a resistor voltage divider circuit so that the relevant values above are not exceeded. Unfortunately, we will need to fix these values depending on the specific batteries being charged. The following example is based on the a 48 V battery pack composed of 15 Tenergy LiFePO4 18650 3.2V 1100mAh Power Cell (Max: 25A) cell batteries. These batteries have the following features
- Standard charging method: CC/CV = 0.55 A/3.6 V
- Max charge current = 1.5 A
- Charge termination when current = 20 mA +/- 5 mA
- Charging temperature of a cell is between -10 Celsius and 70 Celsius.
The input current is based on the power supply, which has to supply > 270 W (54 V * 5 A) of power into the system.
While discharging the microcontroller must make sure that
- The voltage does not fall below 2 V per cell or 30 V (15 cells * 2 V) for the pack
- Discharge current does not exceed 25 A
- Temperature of the cells are between -20 degrees Celsius and 70 degrees Celsius.
We should have enough of a safety factor built-in to make sure the robot shuts off when nearing these absolute ratings; the system should be designed so that the magnets can always be safely turned on.
- Data logger