Gibbot Board v3 Actuating Board
The Gibbot Board v3 will attempt to fix the issues in the v2 board and add additional functionality. The board will test the design of a few unverified systems. These systems include: buck converters, XBee, magnet control, encoders, I2C Communication. To prevent the board from being useless the board with be created with modular designs so that the rest of the board can be used if an untested design fails. The characteristics of the board are:
- Include a modular 48V to 24V, 24V to 12V and 12V to 5V buck converter circuit board with redundant inputs. If the buck converter does not work the module can be cut and the rest of the board powered by an external power supply.
- Include a separate modular board for the second arm of the robot. This board will have only 4 inputs to minimize the number of wires through the link, these are: 24V, GND, SDA and SCLK (or possibly RX and TX). The components on this board will include:
- dsPIC33EP512MC806
- MPU9150 9-Axis IMU
- Logic level MOSFET driver circuit for controlling the magnet at 24V SSM3K329R,LF
- Buck converter circuit from 24V to 5V for driving the encoder on the magnet and on the motor
- Two encoder inputs JST SH 5 pin
- TLV1117 Linear Rectifier to step down from 5V to 3.3V to power the dsPIC and IMU.
- Status LEDs
- Input button
- Reset Button
- Programming Interface
- Reduce the size of the motor driver by using the layout from the HIP4086 3-phase BLDC Motor Drive Demonstration Board for the MOSFET H-bridge circuit.
- Reduce the number of current sensors from three to one by measuring the overall current consumption across all 3 motor driver leads instead of a sensor to measure the current on each of the three leads.
- Use the X6BB (100mV/A sensing) version of the ACS716 Current Sensors instead of the X12CB (37 mV/A sensing) for more precise measurement.
- Use the dsPIC33EP512MC806 for more pins and functionality.
- Include a 0.1uF decoupling capacitor between each of the 3.3V and GND pins on the dsPIC as well as a 10uF ceramic through hole decoupling capacitor on at least one pair of pins.
- Add a MPU9150 9-Axis IMU.
- Modify the following components to meet current load specs:
- Add encoder inputs for magnet encoder JST SH 5 pin
- Improve the connection from the XBee to the dsPIC with hardware flow control pins (CTS and RTS).
- Include logic level MOSFETs for driving both magnets SSM3K329R,LF.
- Include holes for 4-40 stand off screws.
- Status LEDS
- On/Off Switch
- 6 IR LED outputs
- User Button
- Reset Button
- Plug in connectors instead of Screw Terminals
Maximum Height = 0.8 inches
The maximum current consumption required from the buck converters, assuming worst case buck converter efficiency, is calculated in the tables below.
| Component | Current per unit | Total Component | Value Source |
|---|---|---|---|
| 2 Encoders | 78mA x 2 | 156mA | E3 1600 CPR product page 62mA maximum no load current + 8mA x 2 maximum output current |
| Component | Current per unit | Total Component | Value Source |
|---|---|---|---|
| dsPIC | 320mA | dsPIC33EP512MC806 datasheet, absolute maximum rating current from VSS | |
| Status LEDs | 3.3mA x 6 | 20mA | Assuming 3.3V and 1k resistor |
| IMU | 10mA | MPU-9150 datasheet 3.9mA gyro + 6mA magnetometer | |
| IR LEDs | 10mA x 6 | 60 mA | 3.3V and 330 ohm resistor |
| Total | 0.410 A |
| Component | Current per unit | Total Component | Value Source |
|---|---|---|---|
| dsPIC | 320mA | dsPIC33EP512MC806 datasheet, absolute maximum rating current from VSS | |
| Status LEDs | 3.3mA x 6 | 20mA | Assuming 3.3V and 1k resistor |
| Motor Hall Effect Sensors | 5mA x 3 | 15mA | Assuming 5V and 1k resistor, worst case all three on |
| 3 Encoders | 78mA x 3 | 234mA | E3 1600 CPR product page 62mA maximum no load current + 8mA x 2 maximum output current |
| IMU | 10mA | MPU-9150 datasheet 3.9mA gyro + 6mA magnetometer | |
| XBee | 95mA | Series 1 XBee Datasheet 45mA transmit current + 50mA recieve current | |
| IR LEDs | 10mA x 6 | 60 mA | 3.3V and 330 ohm resistor |
| Total | 754mA |
| Component | Current per unit | Total Component | Value Source |
|---|---|---|---|
| HIP4086 | 40mA | 11mA VDD operating current + 2mA x 3 xHB + 2mA x 3 Bootstrap current (Experimental value < 40mA) |
| Component | Current per unit | Total Component | Value Source |
|---|---|---|---|
| Magnets | 230mA x 2 | 460mA | APW Magnets datasheet |
| Main Board 24V to 12V Converter | 40mA / 40% | 100mA | From above |
| Main Board 24V to 5V Converter | 754mA / 82% | 919mA | From above |
| Secondary Board 24V to 5V Converter | 156mA / 82% | 190mA | From above |
| Secondary Board 24V to 3.3V Converter | 410mA / 89% | 461mA | From above |
| Total | 2.130 A |
For an itemized list of issues with the v3 board that were fixed in the next iteration refer to the Gibbot v4 page.
###XBee Communication Issue### There was an issue with UART communication over XBee. The XBee would drop the occasional null character byte 0x00. We contacted Digi support center and found out that the issue was not a faulty Xbee, but a baud rate mismatch. A footnote on page 39 of the Xbee datasheet states that "The 115,200 baud rate setting is actually at 111,111 baud (-3.5% target UART speed)." After reducing the baud rate to be closer to the 111,111 value we have not noticed any issues with XBee communication.
###Voltage Spikes ###
We have observed inconsistent behavior from the control system when the motor is running. These effects were first observed with the I2C bus crashing when the motor is turned on. The voltage spike is correlated with the PWM signal to the motor as shown in Figure 1 below.
The blue line is the signal on the gate of the high side MOSFET that is being driven for the current state of the commutation sequence, it has been scaled down to 10V/division. The purple and green lines are the voltage on the 48V and 24V lines. As the image shows, there is a high amplitude voltage spike on the rising edge of the PWM signal and a slightly smaller voltage spike on the falling edge of the PWM signal.
A closeup of the voltage spikes coupled to the rising edge of the PWM signal are shown in Figure 2 below.
The purple line is 48V, the green line is 24, yellow is 12V and blue is 5V. All voltages are scaled to 5V/division and vertically offset so that their average value is centered at 0. The plot shows that the maximum peak to peak voltage spike on the 48V line is 16V. The peak to peak voltage on the 24V line is 8V.
Figure 3 below shows a closeup of the spikes coupled to the falling edge of the PWM signal.
The amplitude of the voltage spikes is much lower on the falling edge.