Mechatronic approach facilitates automotive electric motor control
In the latest generation of cars, there are many applications that require merging electronics and mechanical actuators in order to simplify the automotive electrical distribution system, improve electronics quality and reliability, and to reduce C02 emissions. Mechatronic approaches help reach those targets by dealing with the fully integrated and optimized design of an entire system, including sensors, actuators, mechanics, electronic components, and all the data processing signals required (i.e., control of the "internal system," communication with external "entities" by means of physical layers, or simply, generic, data transferring). A very simplified diagram of a typical mechatronic approach is shown below.
From safety critical applications to comfort related ones, most automotive applications may benefit from this synergistic integration, and the use of mechatronics has even increased in recent years. In fact, it is abundantly used in cars, and is found in more than 15 systems, including keyless entry, fully integrated rear-view mirrors, climate control, adaptive front lighting, ABS brakes, electric power steering, and many others.
Within the DC motor control area (window lift, steering column lock, sunroof, and smart motor connectors, etc.), mechatronic approaches are multiplying. This is because, on top of the above mentioned needs for the merging of electronics and mechanics, this approach brings additional advantages.
Traditional centralized topologies, in which the control unit is located far from the actuator, need very distant connections (wiring) and a high number of different interconnect technologies including rivets, solder, wiring, and joints. Having lengthy power connections leads to unnecessary power waste, less than optimized EMC performance (due to stray components), and an overall less compact solution. A typical mechatronics solution, in which a single electronic module has been constructed using a multi-chip module and "connectorized" for compact and simple interfacing with a motor housing, would usually have many less interconnections than the traditional solution.
A typical mechatronics implementation for a simple, low cost, DC motor control is illustrated below.
The SUPPLY signals are delivered to an LDO voltage regulator that fixes and re-distributes a 5V constant voltage for all the devices populating the PCB. Communication with the "external world" is done through the LIN (local interconnect network), which is the correct choice in all those ECUs where low speed networking is enough to properly handle the data. The internal module connections, algorithm, and driving strategy of the motor and power management are handled by an 8-bit microcontroller. Finally, there is the actuator driver that, according to the load configuration, can vary from a fully integrated silicon state solution to some simple relays.
For those applications that require a cost effective, protected and in-place design, a valuable and well-proven solution is the L99PM60J, below. A power management system IC that features one low-drop regulator, two protected low-side drivers for driving external relays (final actuators of DC motors), a direct drive for two high-side drivers, and a LIN 2.1 compliant SAE J2602 transceiver.
The device’s integrated standard serial peripheral interface (SPI) controls all operation modes and provides driver diagnostic functions. The voltage regulator (V1) is designed for very fast transient response. Its output voltage is stable with load capacitors >220 nF. This regulator provides 5V supply voltage and up to 100 mA continuous load current and is mainly intended to supply the system microcontroller. The V1 regulator is embedded in the power management and fail-safe functionality of the device and operates according to the selected operating mode.
The voltage regulator is protected against overload and over-temperature; under-voltage with NReset is also available. An external reverse current protection, a diode, has to be provided by the application circuitry to prevent the input capacitor from being discharged by negative transients or low input voltage. Current limitation of the regulator ensures fast charge of external bypass capacitors. The output voltage is stable for ceramic load capacitors >220 nF.
The device implements four different operating modes: Active, flash, V1 stand-by, VBAT stand-by. In active mode, all functions are available, and the device is controlled by the ST SPI Interface.
To program the system microcontroller, the L99PM60J can be operated in flash mode, in which the internal watchdog is disabled. In addition, the SPI Interface and low power modes are not available in flash mode. In V1 stand-by mode, to supply the microcontroller in a low power mode, the voltage regulator 1 (V1) remains active. In order to reduce the current consumption, the regulator goes into low current mode as soon as the supply current of the microcontroller goes below the ICMP current threshold. At this transition, the L99PM60J also deactivates the internal watchdog. Relay outputs and LIN Transmitter are switched off in V1-standby Mode. High-side outputs remain in the configuration programmed prior to the standby command. A cyclic contact supply (for cyclic monitoring of external contacts) can be activated by the SPI and using the direct drive input (DRV).
In VBAT stand-by mode, the voltage regulator, relay outputs, and LIN transmitter are switched off. High-side outputs remain in the configuration programmed prior to the stand-by command, and the current consumption of the L99PM60J is reduced to a minimum level.
To enhance application ruggedness, the L99PM60J may enter different fail safe modes in case of watchdog/V1/SPI failures or if the device triggers a thermal shutdown. In fail safe mode, the L99PM60J returns to a default state with all outputs turned off, and the condition is indicated to the remaining system.
The conditions during fails safe mode are as follows:
● All outputs are turned off.
● All control registers are set to default values. Write operations to control registers are blocked until the fail safe condition is cleared.
● LIN transmitter and SPI remain on.
● Corresponding failure bits in status registers are set.
● FSO bit (bit 0 global status register) is set.
The device provides a total of two high-side outputs: Out1,2, (7Ω typical at 25C) to drive, for example, LEDs or Hall sensors. The high-side outputs are protected against common automotive failures: over-voltage, under-voltage, over-temperature, and over-current. In case each of those events occurs, the drivers switch off. The according status bit is latched and can be read and optionally cleared by the SPI. The drivers remain off until the status is cleared. There is the also the possibility to exclude drivers from a switch-off in case of under-voltage (by setting a relevant bit in a control register).
Open-load conditions and a design example
In the case of an open-load condition, the according status register is latched. The status can be read and optionally cleared by the SPI. The high-side driver is not switched off. In the case of a fail-safe condition, the high-side drivers are switched off. The control bits are set to default values (except OUT1/FSO if it is used as a high-side output).
The outputs, REL1, REL2 (RDSon = 2Ω typical at 25C), are especially designed to drive relay loads. Typical relays used have the following characteristics:
Relay type 1: ON-state: R = 160Ω typical +10%, L = 300 mH: Off-state: 240 mH
Relay type 2: ON-state: R = 220Ω typical +10 %, L = 420 mH: Off-state: 330 mH
The outputs provide an active output Zener clamping (44V typical) feature for the demagnetization of the relay coil, even though a load dump condition exists.
The low-side drivers switch off in case of:
● VS over-voltage and under-voltage (can be masked by SPI)
Low-side protection works in the same way as high-side outputs.
Digital diagnosis features are provided by the SPI and include:
● V1 reset threshold programmable
● Over-temperature including pre-warning
● Open-load separately for each OUT1,2
● Overload status separately for each output stage
● VS-supply over-voltage/under-voltage
● V1-fail bit
● V1 under-voltage
● Chip reset bit (start from power-on reset)
● Number of unsuccessful V1 restarts after thermal shutdown
● Number of sequential watchdog failures
● LIN diagnosis (permanent recessive/dominant, dominant TxD)
● Device state (wake-up from V1stby or Vbatstby)
● Forced Vbatstby after WD-fail, forced Vbatstby after over-temperature
● Watchdog timer state (diagnosis of watchdog)
● Fail safe status
● SPI communication error
An application diagram for a mechatronic window lift, based on L99PM60J adoption, is shown below.
It is useful to highlight that there is no microprocessor core within the device. Thus the L99PM60J is particularly useful for those applications where the use of a particular microcontroller is dictated by its presence on the approved BoM list of the car manufacturer–and a new microprocessor, for timing constraints or any other reason, is not allowed.
Packaging technology has evolved toward a surface mount approach even for low- to medium-power applications. System miniaturization requires thinner packages with an improved ratio between silicon die area and package footprint. The reduction of interconnections among chips, while simplifying the design, also decreases the assembly cost, increases system density, and improves reliability of the system. The device comes with a compact power package, the PowerSSO-16, with exposed pad technology. Such a package offers very good thermal performance that is useful if the adopted PCB is, for example, a simple FR4, double layer instead of a more powerful four layer PCB plus vias.
Thus, such a mechatronic control architecture can simplify the electrical distribution system, improve quality and reliability—and lead to reduced power consumption and emissions.
About the author
Giovanni Luca Torrisi joined STMicroelectronics in 1995. As an IC designer, he worked in the field of power electronics (designing electronic ignitions and AC/AC converters for Halogen Lamps); in these areas, he holds several patents. In 2002 he joined the technical marketing team and now leads a group, covering USA and Japan areas, responsible for automotive body products for ST.
This story appeared first on Automotive Desingline courtesy of EE Times USA