Table of Contents
Motor Controller (H-Bridge)
Each motor on the platform is controlled from its own separate, dedicated motor controller. The motor controller consists of an H-Bridge circuit which is driven by a pulse width modulated (PWM) signal. The PWM signal is used to control each of the switches in the H-Bridge circuit.
The H-Bridge is a switching circuit which switches the current through the motor at a rate up to several kHz. The duty cycle of the PWM signal dictates the duty cycle of the H-Bridge. Varying the duty cycle proportionally controls the average voltage across the DC motor, thus varying the speed and torque produced by the motor.
For this application, four MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) were used to construct the H-Bridge Circuit. Typically, commercial H-Bridges are built using four N-type MOSFETs. The drawback to this design is that the top two MOSFETs require a higher voltage than the source voltage to turn on, so the PWM signal must be conditioned, or stepped up. This requires the use of a separate drive circuit to boost the PWM voltage levels to the actual H-Bridge circuit. In order to simplify the design, and to make the unit more robust, the H-Bridge designed in this project consists of two P-type, and two N-type MOSFETS. The two are used in conjunction with one another in typical CMOS fashion, with the NMOS connected to ground, while the PMOS is connected to the source voltage.
The PWM signal controls the high power NMOS transistors directly, and the high power PMOS transistors are controlled with a separate directional signal which is run through a smaller NMOS. The H-Bridge is controlled by two PWM, forward and reverse, and a Forward / Reverse input. Zero VDC indicates forward, and 5VDC indicates reverse. In addition to these signals for each motor, there is one additional enable signal for the drive motor. This signal controls a DPDT (Double Pole, Double Throw) relay which when energized, connects the H-Bridge output to the drive motor. When the relay is de-energized, the motor is then switched and connected to a load resistor which reliably and predictably brakes the motor until it comes to a complete stop.
Each motor control circuit board consists of two H-Bridge circuits, so each board controls a complete motor module (one steer and one drive motor).
PWM Logic Board
To allow all eight of the motors to be run independently of each other, a PWM Logic Board is used to control each H-Bridge Motor Control Board. The inputs for this circuit are four PWM signals, two direction signals, and one enable signal, and the outputs are eight PWM channels, forward, reverse, left, right and drive enable signals.
Battery health monitoring is critical when operating a semi autonomous robotic platform because low battery voltage may cause unexpected operation or even loss of control. To prevent this, two methods of battery monitoring were employed. The first method provides visual indication of battery health to the robot operator, and the second method allows the processor controlling the motors to monitor the battery level and automatically shutdown the drive motors when it detects that the battery is too low for safe operation.
The second battery monitoring method uses an analog to digital converter on the microprocessor that is used to control the robot. This is important because this processor is the one that is responsible for the operation of the robot in a safe and consistent manor. In order to perform this measurement the 24 volt output from the batteries is scaled to 5 volts which can be measured directly by the analog to digital converter. When the total battery voltage drops below a safe operating value the microprocessor will stop robot operation and signal the user that the batteries need to be changed.
The 12V batteries from the previous project were reused to power the robotic platform. Different voltages are required to power different systems throughout the robot so a power board was designed to take a 12V input and supply various output connectors with the required voltages for all of the components: the drive and steer motors for the robot require 24V, the battery monitors require two separate 12V supplies, the Freescale microcontroller requires 9V, the logic circuits requires 5V, and the wireless modules require 3 volts.
Fuses were used on this board to ensure that too much current cannot be drawn from any part of the robot. Capacitors were also used to reduce any ripple in the output voltages.
Wiring and Connections
- Battery Monitor
- Accurately Displays Battery Information
- Minimal Current Draw
- Colorful LEDs allow for quick visual indication
- Robust Design, easy to use
- Simple 2pin connection
- Oversized for application, minimal heat loss, high efficiency
- Easy to troubleshoot with multiple status LEDs
- Modular and stackable design
- Easy to connect all inputs, outputs and power cables
- Pull down input resistors prevent damage to device should a cable break
- PWM Logic Board
- Simple design
- Pull down input resistors allows proper operation when not utilizing all inputs
- Power Distribution Board
- Oversized traces allow for large current draw and minimal voltage drop
- All circuits are fused
- Easy indication of blow fuses, LEDs indicate where power is present
- Simple fuse replacement
- Easy to connect devices to any voltage level on board
- Flexable input from 2 to 4 batteries
Areas for Improvement
- Battery Monitor PCB
- Add Diode to prevent damage due to switching power leads to circuit board
- Relay coil terminals A1 and A2 are reversed. The relay used, DigiKey part PB1059-ND has a polarized coil. Currently the relay must be mounted on the bottom of the board to work properly. The pad spacing could also be redone as the relay is a bit smaller than the current pad spacing is in the board layout files.
- Decrease size of PCB
- Protect against reversed power connections
- PWM Logic Board
- Add current limiting resistors on all IC output pins to prevent damage to chips due to short circuit conditions.
- Decrease size of PCB by using Surface Mount Technology Integrated Circuits
- Protect against reversed power connections
- Power Board
- Use voltage specific connectors to avoid confusion in output connectors. Currently all outputs for 3V, 5V, 9V and 12V use the same type of 2 pin connector. We suggest using a separate connector for each voltage level so that a 5V connector can not be plugged into a different voltage level.
- Mount all fuses on top of board, this may require making the board larger or using smaller fuses.
- Add battery charing port that a standard 12VDC battery charger can be wired to. This would charge all 4 batteries when the power switch is turned off. Use a 6PDT or 8PDT switch to do this.
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