P08201: RP10 Drive Platform 2nd Generation
/public/

EESite

Table of Contents

Design Information

Motor Controller (H-Bridge)

public/HBridgePCB.JPG

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).

H-Bridge

H-Bridge

PWM Logic Board

public/PWMLogicBoard.JPG
In order to have the potential to run and steer four or more drive wheels, without multiplexing PWM signals, the microprocessor would need to support at least eight PWM output signals since there would be a total of eight motors. However, even with eight channels, the motion would be limited to just one direction, such as always driving forward or turning all the wheels clockwise, unless multiplexing is used.

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 Monitor

public/BatteryMonitorPCB.JPG

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.

Transistor Network

Transistor Network

The visual battery health indication method uses six of the transistor networks shown to the right. Two have red LEDs, two have amber LEDs, and two have green LEDs. When the battery is fully charged all six of the LEDs will be lit, indicating that the robot can be operated safely. As the battery charge decreases the voltage supplied by the batteries will decrease and the LEDs will turn off sequentially from green to amber to red. When the red LEDs are the only remaining the user should cease operation of the robot as the battery charge is low and operation is no longer safe. The board layout for the battery monitor board is included in figure 3.7 and shows the layout of the LEDs as D2 through D7, red to green respectively. The circuit operates using a voltage divider to divide the battery voltage into several different smaller voltages that are linearly related to the battery voltage. Each of these voltages change as the charge on the battery changes and can be used to control the gate to source voltage of an N-channel MOSFET (metal oxide semiconductor field effect transistor). The transistor network, shown in figure 3.6, behaves like a voltage controlled switch. Many of these switches can be cascaded for each of the different divider voltages and will turn on or off sequentially with battery voltage. When the NMOS (N-Channel MOSFET) transistor turns off due to battery voltage decrease, the voltage on the gate of the PMOS (P-Channel MOSFET) is pulled up to 5 volts which turns off the PMOS transistor and the LED that it drives.

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.

Battery Monitor

Battery Monitor

Battery Monitors

Battery Monitors

public/blank.JPG

Power Distribution

public/PowerBoard.JPG

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.

Regulator

Regulator

The 24 volt supplies were achieved by using two 12 volt batteries in series. Additional batteries can be connected in parallel to these batteries to create two 12V battery banks if more power is required or if longer battery life is desired. Separate 12 volt outputs, one from each battery bank, are used for the battery monitor boards. The 12 volt output from the first battery bank was used as an input to a regulator circuit to achieve a 9 volt output. Two more regulator circuits were used to achieve a 5 volt output from the 9 volt input and a 3 volt output from the 5 volt input. The regulator circuit used is shown to the right. The resistors in this circuit are changed to drive the output voltage of the circuit to the desired value, 9, 5 or 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.

Power Distribution

Power Distribution

Electrical Components

Electronics

Electronics

Electronics

Electronics

Electronics

Electronics

Motor Controller

Battery Monitor

Power Distribution

Wiring and Connections

Reused Parts

Strengths

Areas for Improvement


Home | Electrical Repository