By Dave Elliott, Altera Corp.
An automobile’s dashboard serves as the center for consolidating all information pertaining to the safety and management of the vehicle, and displays the information for the driver. In today’s digital age, a vehicle’s instrumentation system must monitor all key functions and can even personalize the information needed by the driver. Industry requirements have led to many semiconductor solutions for the automotive industry ranging from ASSPs to full-custom devices. These may be fixed-function solutions with restricted or lacking product development options that designers need for operational flexibility. In comparison, a scalable solution can support multiple similar applications in a complete vehicle product line without any cost overhead. This type of custom solution enables more features at a lower price.This article provides an overview of an innovative architecture using CPLD to completely eliminate the use of microcontrollers and their drivers, thereby providing a low-cost, low-power solution for digital dashboard clusters. The analog dashboard solution (ADS) presented here effectively implements a digital car network and exploits the advantages of the digital world.
DASHBOARD CLUSTER SOLUTIONS
Traditionally, the real-time outputs from instruments such as odometers, etc. were obtained mechanically and displayed using analog drivers. However, with the digitization of these data inputs, stepper motors and LEDs replaced meters and gauges. Expensive microcontrollers were used to process and display the digital output. ASSPs next appeared, leading to high non-recurring expenses (NRE), which limited upgradeability and improvements. The product life span and support for different product lines were also major factors favoring the inexpensive programmable alternative.
Stepper motors used in pointer-dial type displays convert electrical pulses into discrete mechanical movements. The stepper motor spindle rotates in discrete step increments when electrical control pulses are applied in the proper sequence. A digital instrument cluster usually employs stepper motors to emulate the performance and visual efficacy of an analog dial and pointer display, and simultaneously provides the excellent positional accuracy demanded in a digital setting. Micro-stepping these motors becomes necessary to achieve smooth and non-quantitized pointer motion. In addition, the number of measured samples that can be broadcast from the vehicle sensors to their respective meters is limited by the bandwidth of the digital link between them. The samples for each quantity displayed in the instrument cluster are made available after defined time intervals. A scheme for overcoming the absence of continuous information to the display becomes increasingly necessary in such clusters to ensure that the pointer motion makes up for the time when no data is available to the meter from the sensors. These challenges increase the processing power required and thereby the cost of a digital dashboard system, which deters their use in vehicles owing to the unfavorable price/performance ratio.
CPLD-BASED DASHBOARD CLUSTER CONTROLLER
The limitations of expensive solutions can be easily overcome by using a CPLD. The ADS offers customers more flexibility during design cycles as iterations are simply a matter of updating or changing the programming file. Additionally, it is possible to add new features or upgrade products that already are in the field, thereby deploying effective technology while meeting specific user and product requirements. Using the same basic system, slight modifications that support different devices in new product lines are easily incorporated.
A CPLD-based ADS that enables product developers and manufacturers to choose from several devices for their needs and without concern about semiconductor component obsolescence is available. It features a low-cost selling point and supports higher-end features plus a wide scope for future expansion. The architecture uses Altera’s MAX II CPLDs and includes six modules: the serial sensor data unit, the main motion control and arithmetic unit, and four PWM generators. The serial sensor data unit receives input from the sensors; the main motion control and arithmetic unit performs the required calculations; and the PWM generators provide the appropriate control signals to all phases of the stepper motor, taking in commands from the main module (Figure 1).
To fully describe this ADS architecture, the functions of different blocks and how micro-stepping is performed for the stepper motor that drives the pointer dials in a low-bandwidth vehicle data network must be understood. Because sensor data is not continuously required for all the displays in the cluster, the system holds the pointer positions when data is not available. For smooth action, the move commands sent to the pointers are a current deflection function yielding no jerky pointer step changes.
PWM GENERATORS
Four PWM generator modules drive the pointer-dial stepper motors to indicate data from different sensors (Figure 2).
Micro-stepping is used to drive the motors, creating smooth motor movement. In micro-stepping, the resulting magnetic field developed by the motor is created at positions that are not in line with either of the field coils, but at some angle to them. Thus the holding torque can be developed at more positions, allowing the rotor to be held in space between the two extreme field coil axes. When a field coil is energized, the magnetic flux produced is proportional to the current flowing through it. If both field coils are excited, the resulting direction of the current, and hence the resulting magnetic field is obtained by the vector sum of the currents in the two windings. Therefore, if the current in the stepper motor windings is changed incrementally, such that a set of equally spaced positions of the resultant magnetic field is created, the stepping resolution of the motor can be increased. Using this principle, the step of a stepper motor is divided into micro-steps over which the shaft is actually moved (Figure 3).
The four PWM modules keep a constant supply of PWM pulses of a fixed duty cycle value to the stepper motor, thus making the pointer move at a particular speed and keeping the four stepper motors in constant motion. They receive move commands plus direction input from the motion control units and apply appropriate voltages across the windings to move each motor one micro-step in the desired direction.
MOTION CONTROL AND ARITHMETIC UNIT
The delta generator is the main component in the motion control and arithmetic unit. This module receives the target deflection for each of the four stepper motors from the sensors. When the target deflection is received, two signals are sent to the respective PWM modules. One signal, the trigger pulse, changes the current duty cycle value to the next value in the PWM module. So when the range to be spanned is large, the pulses are sent at a high rate and the PWM module steps through the various duty cycle values rapidly, thus turning the stepper motor at a high speed. When the pointer reaches the target deflection, the trigger pulses to the PWM module to stop while the PWM module supplies pulses of a constant duty cycle value that effectively holds the shaft in position. The other signal notifies the PWM module which direction the pointer should move.
The delta generator has second important function, where the data representing sample values is sent intermittently. If this information is represented directly on the stepper motor, the pointer produces jerky movements. To avoid such movement, the delta generator uses an update number referred to as delta δ. Each time a new target deflection value is received, the generator recalculates δ for that particular pointer. This incremental number δ is then added to the current deflection of the pointer at a rate much higher than a new target deflection is received. During the entire operation, the unit’s current deflection is continuously monitored. When it changes, a command to move the stepper motor by one micro-step in the required direction is sent. If the target deflection is greater than the current deflection, the direction is clockwise, if smaller, it is counterclockwise.
Thus in the interval between the receipt of two target deflection values, the pointer is moved at a faster rate, through smaller steps, to reach the target, thus resulting in a smooth movement. It extrapolates the value appropriately to cover the gaps and keep the motion smooth irrespective of the input variations.
SENSOR DATA INPUT UNIT
The sensor data input unit uses an SPI interface that supports communication with slow peripheral devices that are accessed intermittently. These devices communicate using a master/slave relationship, where the master initiates the data frame. When the master generates a clock and selects a slave device, data may be transferred in either or both directions simultaneously. The SPI slave module has been implemented at the back of the CPLD for system input. Sensor data is ported to the arithmetic and motion control unit and is the target deflection of a particular sensor or new LED data. The sensor data is sent along with the target address, identifying its source.
The CPLD-based ADS can be scaled up easily for even more accuracy and higher-end functionality on different platforms. It can support different assembly topologies for different classes of vehicles with minimal changes in programming and almost no additional expenditure, as the development time decreases exponentially. The ADS can be coupled with common, robust automotive digital data networks for instant induction into the manufacturing process, thus providing CPLD flexibility to easily implement add-on functionalities.
CONCLUSION
Using a low-cost, low-logic density CPLD, a complex ADS can be achieved, thus overcoming the shortcomings of traditional dashboard solutions. Because re-programmability is the inherent advantage of the ADS, the re-usability of the design means additional solutions can be developed quickly using the growing library of available IP and cores. Reconfiguration is easy, and newly developed products reach the customer faster. NREs can be recovered easily because of long product life, and manufacturers can prolong the lifecycle of an already-developed product without new NRE investments.
About the author
Dave Elliott is a senior automotive marketing manager in Altera’s consumer and automotive business unit. He has more than 25 years of semiconductor marketing and sales experience, including the past 10 years, where he has specifically focused on the automotive market. Prior to joining Altera in 2005, Elliott spent five years at Xilinx as a senior corporate account manager in its automotive unit and was Xilinx’s global supplier business manager at Avent for three years. Prior to Avent, Elliott spent 15 years at National Semiconductor in various sales and marketing roles.
