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Characterizing the Behavior of an Electronic Throttle Body using the Data Acquisition Toolbox and the Instrument Control Toolbox

by Scott Hirsch

Electronic throttle control ("drive-by-wire") has been in development since the mid-1970s, and is now being used in mass-market vehicles. An electronic throttle control system replaces the mechanical linkage between the accelerator pedal and throttle with an electronic linkage. A basic electronic throttle control system consists of an accelerator pedal, a controller module, and an electronic throttle body. The pedal is outfitted with pedal position sensors, and the throttle body contains throttle position sensors and a motor to position the throttle. The control module regulates the throttle position with feedback from the position sensors.

Electronic throttle control systems are able to tune the throttle's dynamic behavior through modification of the control law. These capabilities include driver selectable performance, altitude compensation (maintaining constant pedal response at high altitude), and reduced emissions.

To design a controller that provides this wide range of capabilities requires a thorough understanding of the open-loop dynamic response of the throttle body. To gain this understanding, the engineer will measure the throttle's response to position commands. He needs to execute a series of tests under a range of operating conditions (temperature, ambient pressure, etc.) to acquire all of the necessary information.

The results of this series of tests can be used to develop a robust numerical model of the throttle body, which can be integrated with a complete system-level simulation. A controller for the component can be designed initially with this simulation, and tested with automatically generated code for rapid prototyping. Finally, the production code for the controller can be generated from the same simulation.

The basic process demonstrated here (subsystem characterization, numerical simulation, rapid prototyping, and embedded code generation) can be applied to a wide range of system integration challenges. This article will focus on the first step, which is subsystem characterization. We will use MATLAB, along with the Data Acquisition and Instrument Control Toolboxes, to perform a series of tests of the throttle body and to analyze the results of these tests.

This demonstration uses custom electronics to interface the throttle body with the data acquisition hardware. While a similar setup may be replicated in other laboratories, this article is not intended to provide step-by-step directions. This article is intended to give readers ideas of how to incorporate the Data Acquisition and Instrument Control Toolboxes into their test applications.

Hardware setup

The electronic throttle body used for this demonstration is shown in Figure 1. The brass disk is the throttle plate; it is shown open by approximately 15°, which is its resting position. The electrical connections are shown on the right side of the photograph. The throttle actuator is a DC motor driven by a bi-directional pulse width modulated signal. Since the motor is required to operate in both directions, an H-Bridge driver is used in a production system, with a 12 V, 7.5 A power supply.

Figure 1. Electronic Throttle Body

A schematic of the experimental configuration is shown in Figure 2. The black box labeled"Relays / Data Acq. Connectors" contains the circuitry to control the throttle and to output the measurements of its position sensor. To reduce the complexity and cost of this demonstration, the Pulse Width Modulator and H-bridge are replaced with digital signals generated by a data acquisition card, a simple dual-relay circuit, and a lower current power supply. Digital open and close signals sent to this circuit are used to command the throttle's position. The throttle position sensor outputs an analog voltage that is proportional to the throttle angle.

The Data Acquisition Toolbox communicates with the throttle through a Measurement Computing Corp. DAS16/12 Data Acquisition Card. This card provides 8 double-ended analog input channels (16 single-ended) and 8 digital I/O lines. An analog input channel is used for the position measurement, and two digital I/O lines are used to open and close the throttle.

A traditional digital oscilloscope is often used when initially configuring a test. Here, we will use a Tektronix TDS 210 Oscilloscope, which can be connected to a PC through its serial or GPIB ports. The GPIB (General Purpose Interface Bus, also known by its standard as IEEE 488) communication capabilities of the Instrument Control Toolbox are used to configure the oscilloscope for testing and to import the oscilloscope's data into MATLAB for analysis. A description of how to use the serial port for this communication is provided following the description of how to use GPIB. Serial port communication is built into basic MATLAB.

Figure 2. Demonstration setup. Click on image to see enlarged view (37 k).

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