Four application specific integrated circuits (ASICs) that provide sensing, actuation, and power conversion capabilities for distributed control in a high-temperature (over 200 °C) environment were developed. The four ASICs are combined with a digital signal processor (DSP) to create a distributed control node. Patented circuit design techniques facilitate fabrication in a conventional 0.5-micron bulk complimentary metal oxide semiconductor (CMOS) foundry process.
ASIC#1, the HHT104 dual 4-channel, 16-bit instrumentation IC, is capable of digitizing up to eight differential sensor inputs with up to 16 bits of variable resolution. This part is typically used to measure linear variable displacement transformers (LVDTs), thermocouples, resistance temperature detectors (RTDs), and pressure sensors. ASIC#2, the HHT212 dual 12-bit bipolar/unipolar current driver IC, includes full bridge drivers to control torque motors and other small loads requiring precise current control. ASIC#3, the HHT250 quad complementary switch with PWM control IC, may be used with solenoids, motors, relays, and other switched or static loads. ASIC#4, the HHT300 quad switch-mode power supply controller, can operate over a wide input supply range, and is intended for microprocessor and sensor applications requiring several supply voltages. The high-temperature actuator controller development system includes the interface IC chipset as well as a DSP, CAN bus transceiver, and high-temperature oscillator.
High-reliability IC layout techniques are used to improve device reliability at elevated temperatures. Conservative design rules emphasizing redundant vias and short, wide traces are used to reduce the rate of failure due to electromigration. Depletion guard structures, plentiful substrate contacts, and integrated catch diodes help to prevent latch-up conditions at elevated temperatures.
The ICs were designed to be incorporated within the semi-autonomous node (SAN) structure shown in Figure 1. The node architecture, shown within the yellow block, is notionally connected to several input and output devices. The architecture also shows two CAN bus interfaces that are intended to enable communication between the node and the rest of the distributed control architecture, which incorporates the SANs within a ‘master-slave’ architecture. Ring and star architectures are possible as well. The SAN is configured with dual CAN transceivers to enable both inner and outer loop communications.
The selection of a CAN bus was driven principally by the availability of high-temperature- capable hardware to implement the protocol. As other hardware implementations become available, different protocols could be employed. One possibility being considered is the use of a time triggered Ethernet for the outer loop with a time triggered protocol for the inner loop.
The low-voltage process used in these ICs is extended to high-voltage operation (up to 22V) using devices with thick gate oxides and unique geometries. The ICs use circuit topologies that minimize the number of high-voltage nodes required, and over-voltage protection clamps are placed on input pins to prevent damage. Electrostatic-dissipative (ESD) structures are also used on input pins.
The controller board of the development system is shown in Figure 2 with the core high-temperature components installed. The PCB is fabricated using a high-temperature polyimide laminate and is rated for continuous operation beyond 170 °C. The PCB finish relies on an electroless nickel, immersion gold plating process to prevent oxidation of the copper traces, which would reduce the PCB lifetime. The board is physically large (approximately 15 cm by 25 cm) to facilitate initial benchtop testing and software development, but it may also be operated in a high-temperature environment. Each IC is powered independently, and numerous jumpers and test points expedite the development process, including configuration of the controller for a particular actuator. Ultimately, the IC packaging and circuit board would be miniaturized to fit the application.
An application example using a LVDT and a torque motor demonstrated that an actuator controller can be implemented with few additional discrete components. Initial oven testing at 170 °C demonstrated system operation up to 900 hours without any apparent degradation in device function or current draw. Oven testing will continue until device failures are noted so that reliability estimates at higher temperatures may be performed.
This work was done by Steve Majerus, Daniel Howe, and Walt Merrill of Scientific Monitoring Inc.; Steven Garverick of Case Western Reserve University; and Kenneth Semega of the U.S. Air Force Research Laboratory. The full technical paper on this technology is available for purchase through SAE International at http://papers.sae.org/2012-01-2210/ .