Using Virtual Instruments to Design an Automotive Air Conditioning Simulation System for Fiat

Author(s):
Stefano Prastaro - REM automazioni - Ufficio Commerciale

Industry:
Automotive

Products:
CAN, Signal Conditioning, PXI/CompactPCI, LabWindows/CVI

The Challenge:
Providing Turin Centro Ricerche Fiat with a thermodynamic test bench to simulate an automotive air conditioning system in various climate conditions and under different motor spin regimes.

The Solution:
Developing an electronic control system using a hardware platform with NI AT-MIO and SCXI hardware for regulation and data acquisition applications, as well as NI LabWindows™/CVI software for logical control, measurement, and analysis tasks.

Multiple Engineering Features
At REM Automazioni, we designed an automotive air conditioning simulation system for Fiat with features including independent control of nine proportional integral derivative (PID) control loops such as dragging motors, pump and fan flow rates, heaters, and cooling valves; maximum flexibility in test-parameter setup; advanced data analysis with custom algorithms; and a reference table to assist with engineer activities.

Developing Hardware with Three Functional Blocks
To reproduce real “on car” operating conditions for air conditioning systems, we developed hardware consisting of three functional blocks: a water condenser – a variable-flow, cold-water circuit that simulates the car radiator; an evaporator – a solid state relay (SSR) -controlled boiler that reproduces a thermal load in the car cockpit; and a refrigerator circuit – a 30 KW AC-3 servo motor with a frequency inverter that drags an automotive compressor with a magnetic clutch.

The compressor ranges from zero to 6,000 rpm with high-acceleration dynamics. A stepper motor driven valve regulates the Freon flow rate. We achieved thermal exchange between different stages using a brazed plates heat exchanger water glycol solution rather than R-134a Freon gas. Also, we put the circuit components through electrical transduced probes to gain full control of any parameter needed to regulate loops and system mathematical characterization.

We based our hardware control system on an NI AT-MIO board with an NI SCXI-1000 chassis with 32 analog inputs including temperature, magnetic and Coriolis flow meters, pressure transmitters, torque meters, and angular speed of compressor dragging motors.

We used multiloop regulation tasks based on NI SCXI-1124 modules with six analog outputs to control an SSR power modulation of water heaters, a three-way motorized cooling valve, motor inverters for compressor dragging, and condenser and refrigerator circuit pumps.

We completely submitted test bench digital controls including manual commands, actuators, and lamps to the SCXI system using the NI SCXI-1162 and SCXI-1163 optically isolated digital input modules. We implemented safety controls using electromechanical circuits and dynamic oversight of software and hardware systems that included a watchdog tool with automatic power-off command in case of fault. Using NI products in this system design, we eliminated the need for programmable logic controllers (PLCs).

Centro Ricerche Fiat engineers rely on accurate temperature measurements to deliver quality test results, which depend heavily on complex calculations closely related to the acquired signals. Therefore, we selected NI SCXI as our high-performance signal conditioning platform because each analog-input channel has its own instrumentation amplifier, a 2 Hz lowpass filter, and a precision cold-junction compensation sensor.

A Variety of Software
We designed the system software to execute a preliminary stabilizing step of thermal circuits followed by a simulated motor speed cycle during the car run phase. During the test phase, the software plots all of the data on trend and XY graphs and saves it to external database files.

The first step implements closed-loop selection for any stage and relevant setpoint setting. Altogether, the system uses nine control strategies, returning to five simultaneously operating PID loops. Because the system is so complex, we used the ANSI C programming language and the LabWindows/CVI PID Control Toolkit for software development to give Centro Ricerche Fiat engineers freedom in bench setup and use.

We based the simulation phase on a compressor motor speed variation during an adjustable time cycle. During the cycle, the system maintains constant operating conditions for the circuits. We compelled high-precision control of program execution time with the exact reproduction of the motor speed profile using a multithreaded structure with different scan times for elaboration charge and task priority.

Next, we converted PC/SCXI hardware architecture using an NI PXI-1011 combination chassis. With this upgrade, we maintained SCXI signal conditioning modules using an NI PXI-8176  embedded controller as an automotive CAN bus interface module.

User Interface with Supervisor Controls
We developed the user interface main panel with supervisor controls including analog measurement, PID loop state, and compressor motor speed. A text message panel shows any system event and signals alarms and anomalies by underlining the text in red.

The bench engineer can use three independent strip chart panels to switch on or off any analog measurement curves. This function was very useful for analyzing physical transient phenomena during testing.

We designed the comments recorder for Centro Ricerche Fiat engineers to use as a specific text input panel to write free messages or observations into a test report file, as well as to locate specific events during offline data analysis.

We completed the user interface with a series of service panels such as I/O configuration, advanced hardware diagnostic, and alarms and log file management panels.

Using LabWindows/CVI, we can reuse ANSI C code and drastically reduce software development time so the Centro Ricerche Fiat programming staff can concentrate on developing bench control features.

For more information, contact
Stefano Prastaro
REM automazioni – Ufficio Commerciale
Tel.: +39 011-6498689
Fax: +39 011-6499253
stefano.prastaro@remautomazioni.it
REM snc di Visca Gian Franco e Roberto
22, strada Serene - 10028 Trofarello TO, Italy

 

The mark LabWindows is used under a license from Microsoft Corporation. Windows is a registered trademark of Microsoft Corporation in the United States and other countries.

 

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