snorr/ece4501s2/ viewthe power supply team went through several iterations in the design process...
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Design of the Independent-Drive All-Terrain Electric Vehicle
Power Supply Team
Rousey Johnson III
Motor Driver Team
The goal of the Power Systems project was to design and build a four-wheel independent-drive all-terrain electric vehicle. Some of the specific design features will someday include regenerative braking, power assisted steering, and a vehicle design that will be truly all-terrain. Time constraints led the class to set goals of independent four-wheel drive control, and basic acceleration and braking controls. To do this, the class divided into four teams to design and build the individual components of the system.
The job of the power supply team was to design and build a small and stable current and voltage protected power supply. The supply provides power to three system components: 24 VDC for the four motors, 12 VDC for the microprocessor, and several voltage levels for sensor power.
The source consists of two 12 V batteries wired in series. Each battery is individually connected to the power supply with fuses and switches. Fuses provide protection to the batteries from over current, and switches allow isolation from other systems during battery charging.
The motors are provided with individual 24 V posts that have back EMF protection. This prevents the reverse current from damaging the battery, microprocessor, and sensors. Each motor is fused to prevent over current above 6amps.
Voltage regulation for the microprocessor is provided via a 12 V voltage regulator. This provides constant 12 V output through the full range of expected battery voltage (13 V at maximum droop to 26.5 V at full charge). The 12 V supply to the microprocessor has in-line fuses and an on/off switching control to allow for protection from the battery during charging operations.
The sensor requirements consist of +5 VDC and 12 VDC. Positive 5 VDC is again provided with a voltage regulator. The unregulated 12 VDC is provided directly from the battery terminals.
Difficulties in the power supply design became apparent immediately. The greatest problem was that the battery source was not a constant source. The team went through several initial designs to provide constant output, and in the end turned to off-the-shelf voltage regulators to provide the necessary outputs without great increases in weight, size and complexity. The final product consists of a smaller than a shoebox power supply with all components internal, and all fuses mounted for easy replacement.
The Motor Drivers
The Motor Driver Team researched and designed an H-bridge motor driver circuit to provide independent control of the four separate motors. An H-bridge allows for both forward and reverse operation, as well as the potential for regenerative braking (recharging the batteries while slowing down). The input requirements include four independent 24V lines for each of the motors, a common ground, and a 5V line to power TTL logic. A forward/reverse switch provides a signal to determine if the motor drivers are in forward or reverse, and four independent pulse-width-modulation signals from the microprocessor determine the speed of the motors. The motor driver circuitry includes a positive and negative line for each of the four motors.
The Sensor team was assembled to provide the vehicle operator with controls, and to monitor the system for possible situations that require action to avoid damage to the system. The operator controls consist of a throttle and brake, and tachometers and current sensors will monitor the operating state of the motors.
The current sensor measures the current passing through the motor. The sensor changes its output signal when the current goes above six amps. The throttle and braking controls use potentiometers to provide the micro-controller with analog signals for vehicle control. The tachometer sensors provide a digital pulse, where the frequency is proportional to the motor speed.
The microprocessor is the component that seamlessly integrates individual circuits into a coherent system. In addition, it serves as the monitor of the system to avoid potentially damaging situations. The signals that the microprocessor monitors are:
1. Acceleration ---- Analog ------ 1 input.
2. Speed Sensors ---- Digital ------ 4 sensor inputs.
3. Current Sensors ---- Analog ------ 4 sensor inputs.
The Speed and Current sensor signals are monitored to avoid potentially damaging over-current situations. If these signals achieve certain levels, the microprocessor will temporarily shut down the problem motor to prevent damage to the circuit.
The microprocessor controls the speed of the vehicle by generating four independent pulse width modulated signals to drive the four motor drivers. The Accelerator signal from the vehicle operator is read through the analog to digital converter, and used to determine the PWM signals. Future designs will also include power-assisted steering, where the speed of the wheels on the outside of the turn is increased as the operator turns.
Each individual team successfully completed the component systems that were assigned to them. However, as with any large design project, some problems were encountered when the components were integrated. For example, the motors chosen for the vehicle do not have a starting torque high enough to move the cart. This problem resulted in some last minute changes that sent teams scrambling to adapt designs.
The project as a whole would be excellent for a multiple discipline engineering team. An Industrial or Mechanical engineering team could design a vehicle that would withstand travel over any terrain, while an Electrical Engineering team could design the control circuitry. This would result in very nice four-wheel independent-drive all-terrain electric vehicle.
The goal for the Spring 2002 Power Systems class was to turn a $750 Chancellors Small Grant into an electric four-wheel independent-drive all-terrain vehicle. To do this, the class divided into four groups, each tasked with a particular aspect of the vehicle.
The Power Supply group was created to design and build a single power supply to provide the several needed voltage levels for separate systems in the vehicle. These include a 24-volt supply for the motors, 12 and 5-volt supplies for various sensors, 12 volts for the motor drivers, and a 12-volt supply for the micro-controller. The design also incorporates protective circuitry to keep the power supply from being damaged.
The Motor Driver Team researched and designed a motor driver circuit to provide independent control of the four separate motors. The design provides for both forward and reverse operation as well as the potential for regenerative braking (recharging the batteries while slowing down). Independent pulse-width-modulation signals, one for each motor, are used to control the speed, and a forward/reverse signal is used to control the direction of rotation. These signals are generated by two other groups.
A Sensor team was assembled to provide the vehicle operator with controls, and to monitor the system for dangerous over-current situations. A throttle and brake are provided for user input, and tachometers and current sensors indicate the operating state of the motors. Each of these sensors provides a signal that must be interpreted. That is the job of the microcontroller.
The Microcontroller team implemented the control center for the vehicle using the Motorola 68HC12 microprocessor. The microprocessor provides the logic required to link the individual components of the vehicle into a single coherent system. The software interpreting the sensor and control data and creating signals to control the motors is critical to the over-all success of the project.
Improvements to the electric all-terrain vehicle include assisted steering, regenerative braking, and a truly all-terrain design. These could be implemented by an inter-disciplinary design team consisting of Industrial or Mechanical engineers along with the electrical engineers. The result could indeed be an independent-drive all-terrain electric vehicle.
Design of the Independent-Drive All-Terrain Electric Vehicle
Engineers are always looking for a challenge, and student engineers are no exception. So it comes as no surprise that the Spring 2002 Power Systems class applied for a Chancellors Small Grant and took on the design of an electric independent-drive all-terrain vehicle, and that with only half the semester to accomplish it in.
The vehicle specifications include a single voltage and current-protected power supply, four-wheel independent drive, power-assisted steering, and regenerative braking, all mounted on a frame that can carry an individual over any terrain. Perhaps a team of multiple-discipline engineers can implement the full design at a later date. In setting attainable goals for the eight-week project, the class scaled back the full design, putting p