INDUSTRY Insight
Harsh Self Organizing Sensors
Spacecraft
Cars
Field Robots
Small Systems
Large Systems
Video Service Toys
Cell Phones
Telecom Systems
Friendly Figure 1
Mobile systems are very diverse in their application and their operating conditions, often resulting in vastly different requirements for power management.
Remote control functions allow a supply to be remotely controlled and monitored. This includes coordinating the action of multiple supplies, setting voltage and current limits and shutdown sequencing. Remote reporting functions include reporting variations in current, voltage and temperature as well as calculated values like efficiency, power and power factor. Self-correcting functions include temperature compensation of the voltage reference, calibrated output values, temperature-driven current limits and current-driven feedback selection (hysteresis limits). Generally they also include charging features like battery charging temperature and voltage monitoring and battery charging management. Supply sharing features are those features that allow a supply to be shared between two pieces of electronics. This can reduce weight and components if the features never work simultaneously. This would include turning each application on and off as well as managing the power supply to feature mapping to have overall control. Safety features that are generally included are a watchdog timer, brown-out control, low voltage detection and an oscillator fault for the control processor. Other safety features would include careful monitoring of battery temperature, voltage and current with automatic shutdown.
24
February 2008
Diagnostics are a main feature that may or may not be included depending upon the application. Big networked applications require this while smaller applications do not.
System Architectures
Today, all of these architectures look very similar with the exception of various hybrid vehicles. This basic architecture is shown in Figure 2. The charging subsystem is either integral in the case of harsh environments, or detachable in the case of more friendly environments. Friendly environments are powered off-line and harsh environments are powered with a variety of different sources that can travel with them. By raising the bus voltage under careful control, the charging system can reverse the flow of energy from the batteries and recharge them. This is a complex and demanding task that depends upon the type of batteries, discharge levels, cell voltages, charge current, charge voltage and a variety of other factors. For that reason, charging subsystems of this nature are always run by a microcontroller. The demanding communications requirements and sequencing also require a real-time operating system to manage complexity. The battery subsystem is generally quite simple. In some cases, special pro-
tection must be included to avoid degassing or venting of batteries and voltage surges (regeneration for example). Large choppers are required in these cases to reduce voltage and protect the batteries against damage. Sometimes a secondary cell is included as a backup. This allows the bias supplies to be run from a secondary source and provides greater reliability through redundancy and guaranteed system control. The power buses can be redundant or not and can offer separate buses for bias supplies or not. There is a great deal of flexibility here depending upon the application. The communications bus or buses can use a range of protocols and wiring. Common options include: CAN, I2C, TCP/IP, UDP/IP, asynchronous serial I/O, SPI, USB and others for larger systems. Smaller systems use direct control of submodule supplies via the microcontroller. Larger systems achieve a greater degree of reliability by providing distributed operation and not directly coupling all the power modules. Overall, one microcontroller is required to run one power module due to sensing and other limitations. Multiple power modules require multiple microcontrollers—each module with one controller and each with a separate communication capability. Multiple master operation is a good idea to avoid a single point of failure.
Real-Time OS Software Architecture
In a power management system, each component needs a microcontroller and each microcontroller may or may not use a real-time operating system (RTOS) to provide the different software features, run the PID loop to control the power supply and provide communication facilities. An RTOS has a few major advantages that make it superior for developing a power management product, particularly if you are developing multiple management systems for different applications. The main advantages are: • modular approach, which eliminates retesting and rework • independent off-the-shelf communication servers • off-the-shelf timer support • integrated development tools • synchronization ability