Design of the Independent-Drive All-Terrain Electric Vehicle
Power Systems
Spring, 2002
Power Supply Team
Robert Ardren
Charlie Fox
Rousey Johnson III
Matt Leines
Eric Viken
Motor Driver Team
Dave Melcher
Nick Hietala
Paul Olson
Dave Thorsvik
Sensors Team
Jerod Wendt
Jonathan Schroepfer
Andy Witzke
Joe Kluenenberg
Microprocessor Team
Swagato Bhatta
Khaled Ejaz
Andy Mosenden
Mike Carlson
Jeff Green
Management Team
Eric Nordgren
Matt Lund
Instructor
Scott Norr
2
Executive Summary
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.
Power Supply
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 Sensors
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
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:
- Acceleration ---- Analog ------1 input.
- Speed Sensors ---- Digital ------4 sensor inputs.
- 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.
Conclusion
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.
Abstract
The goal for the Spring 2002 Power Systems class was to turn a $750 Chancellor’s 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.
2
Design of the Independent-Drive All-Terrain Electric Vehicle
Introduction
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 Chancellor’s 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 power-assisted steering and regenerative braking on hold.
The goals for the project did include designing the power supply and independent drive, with accelerator and braking controls. To do this, the class divided into four teams, each assigned an aspect of the project to work on. The Power Supply team was created to design a single power supply for the vehicle, providing several voltage levels, and incorporating protective circuitry to keep the supply from being damaged. The Motor Driver group was to design a circuit to provide control of the motors, including both forward and reverse operation. The Sensors team was created to provide controls for the vehicle consisting of accelerator and braking signals, as well as sensors to monitor the over-all system. The Microprocessor group was in charge of bringing all of the components together into a single coherent system, using the Motorola 68hc12 microprocessor.
The Power Supply
Overview and Goals
The power supply has the job of providing multiple outputs with regulated and protected power that will prevent over current and over voltage to any individual component of the vehicle.
The power supply must provide:
1) Four fused 24 VDC connections for the motors.
2) Regulated 12 VDC for the microprocessor.
3) Regulated 5 VDC for the sensors.
4) Unregulated ±12 VDC for sensors provided directly from the battery.
Power Supply Design
The power supply team went through several iterations in the design process including simple voltage dividers with feedback loops for voltage control and building advanced power supplies (Lines, 1991). In both cases, the lack of a constant supply voltage due to expected battery droop prevented the use of simple locally generated designs.
The final design settled on the use of 5 V and 12 V voltage regulators to provide the needed range of constant output. The specification sheets for these regulators (Motorola MC7800 Series, 1996) explain that the output will be constant if the input voltage is at least 2 V above expected output. In testing, the 5 V voltage regulator locks in at 5.004 V when it’s input is above 5.75 volts. The 12 V voltage regulator provides constant output when the input is above 13.25 volts. The input to the 5 V voltage regulator and the microprocessor is from the 12 V regulator output, which ensures that the sensors and microprocessor will continue to be powered down to 7 V of battery voltage. At 7 V, the microprocessor will still be able to function, even thought the motors will not be operable.
Motor Supply
The motors are rated for 24 volts and 6 amps. Reverse voltage and current could be as much as four times the motor rating, requiring protection for up to 96V/24A provided for each motor. The 24 VDC supplied by the battery flows through a diode, which prevents back current. This is done to protect the batteries from over current. More over current protection is provided with fuses and over voltage protection is provided by parallel zener diodes.
Microprocessor supply
The 68HC12 Motorola power supply has internal voltage regulation. Any voltage above 6 VDC will ensure the 5 VDC necessary for proper operation of the microprocessor and its components (M68HC12B Family, 2000). The 12 V voltage regulator can provide up to 3 A (Motorola MC7800 Series, 1996), which decreases as the input voltage to the voltage regulator is reduced. The total current needed by the microprocessor and the sensor units is not expected to exceed 200 mA, ensuring that throughout the range of the voltage regulator, proper voltages and currents are provided.
Sensor supply
The sensor team requires 2 different source voltages. One source needs to be a constant at +5 VDC with minimum losses due to droop of the batteries, which is provided by a 5V voltage regulator. In Addition, ±12 VDC is needed for operational amplifier power. This power can drop to ±8 VDC without affecting the circuit. The current design provides ± 12 VDC unregulated from the battery terminals (Figure 1).
Power Supply Operational Testing
No-load testing of the power supply was conducted from 5.4 V to 28 V input using the input connections on the case as two separate 12 volt batteries simulated by DC power supplies in the Power Systems lab. The design tested within the specification parameters. Above a 7 V DC supply, the +5 V output is stable to drive sensors (Figure 3). With at least a 13.5 V DC supply, all of the system outputs meet design parameters.
Load testing was conducting using 5 kW resistors connected to each motor while operating the power supply from 13 V to 26 V. Current was monitored at the output of the 12 V voltage regulator with a 250kW resistor as the simulated load. Current was also monitored to the motor loads. Motor load current was 4.5 A from 20 V to 26.5 V, and decreased as the source dropped below 20 V. Voltage and current at the output of the 12 V voltage regulator was stable at source voltages above 13.5 V, and reduced linearly below 13.5 V. These values fit within the design specifications.
Design Limitations
Due to cost constraints, several higher power items were left out from the overall design. The loss of these components will limit the range of motor operation. A DC circuit breaker in the system prior to the solid-state relay was not included. This would provide greater circuit protection. Space has been provided within the power supply box for this future add-on. The power rating of the Zener diodes and the associated inductors on the motor supply lines are limited, again due to cost. This will prevent rapid transitions of the motors from forward to reverse and ensures that any changes in speed must be done slowly to avoid damaging the power supply.