NI Tutorial 2974

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Achieve Flexibility in Your AutomotiveDynamometer Applications

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Overview

This paper describes a flexible platform for dynamometer tests that you can build using National Instruments products.

Table of Contents

Introduction

Automotive Dynamometer Applications

Dynamometer Technologies

Example Applications

Integrated Control System

Conclusions

Contact Information

Appendix A Common Test Parameters

Appendix B Relating Torque to Horsepower

Introduction

Executive Summary

This paper describes a flexible platform for dynamometer tests that you can build using National Instruments products.

Design and validation applications have widely varying objectives, and especially so when using a dynamometer to test motors, engines, or vehicles. However, most dynamometer test solutions

suffer from limitations related to:

Manual operation

Integration of control, measurement, and operator interface functions

Channel expansion capacity

Compatibility with new signal types and protocols

Compatibility with third-party devices

This paper discusses tools that overcome these obstacles and describes solutions based on the National Instruments platform.

For automotive engineers and system integrators who build test cells for measuring engine and chassis performance, the National Instruments Automotive Test Platform consisting of LabVIEW

software and hardware is a platform for building real-time control and measurement systems. Unlike proprietary solutions, users can build fully customized applications with the Automotive TPXI

Platform, incorporating a wide variety of National Instruments and third-party components integrated as a single system.

While the examples in this paper focus on power-train test applications, the technologies and approaches can also be used to test the safety, durability, and performance characteristics of other

components such as hoses, brakes, belts, or electric motors.

About National Instruments

National Instruments is an industry leader in PC-based data acquisition and control products. With National Instruments hardware and software, engineers and scientists can automate test

procedures, data collection, and analysis, and can present the results in an easy-to-understand manner.

Customers in many different industry and research settings use National Instruments products to create a wide variety automated test and measurement systems using methods that result in

significantly higher productivity and lower development and maintenance costs.

Automotive Dynamometer Applications

Common applications for automotive dynamometers include:

Vehicle inspection and maintenance

Chassis design and validation

Engine design and validation

Drive train design and validation

Design and Validation Test

Design and validation testing has a broader set of objectives, including:

Measuring performance factors, such as horsepower, acceleration, and mileage

Life-cycle and durability testing

Verifying design conformance to emissions and safety standards or product specifications

Measuring noise and vibration

Ultimately, the desired product of dynamometer testing is meaningful data that can be consistently generated without risking injury to personnel or damage to the test system or the device under

test. This goal drives several key requirements for the test system:

Reliability for operator safety and data integrity

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Repeatability for consistent results

Flexibility to accommodate different models or levels of assembly

A dynamometer with these characteristics can be based on an automated control system that is expandable and configurable to meet the needs of a variety of test applications. The following

sections examine an approach to building such a control system using National Instruments products.

Dynamometer Technologies

A dynamometer is an energy-absorbing device capable of applying a controlled load to a test article. The load is applied as torque (rotating force) to an engine shaft or to the wheel of a vehicle.

There are several braking technologies that dynamometers use to generate a controlled load, including:

Inertial A large spinning mass provides a load that is proportional to acceleration. Torque can be calculated from the acceleration rate. Average torque can be derived from the time that it take

to accelerate the mass to a given rpm. Water Brake The load is produced by a water pump. An impeller connected to the shaft mechanically forces the water through the pump. A valve fitted in the output line of the pump regulates

the backpressure and resulting load.

Eddy Current The load is generated by eddy currents induced in a rotating metallic disk immersed in a magnetic field.

AC or DC Motor The load is created by an electric motor. The electric motor can also serve as a drive motor to generate torque.

Hydraulic The load is created by smooth disc power elements that absorb power by viscous shear

Each of these technologies can be classified according to cost, power, speed, dynamic response, control stability, internal inertia, and other characteristics. Eddy current and electric motor

dynamometers are best suited for automotive testing because of their responsiveness and power/torque capacity at high speeds. Motor dynamometers provide the best control response, while

eddy current dynamometers can handle higher power at higher speeds.

Many dynamometers have built-in sensors that measure torque and speed, which can be used to calculate power (see Appendix B). One recent trend is to extend torque measurements to includ

torsional vibrations generated by pistons or other mechanical components.

Some dynamometers (particularly AC and DC motor dynamometers) have motoring capability, which means that they can drive shaft with a positive load in addition to generating a braking load

Operating Modes

Dynamometers can be operated in either open-loop or closed-loop control modes.

Open-Loop Mode

In open-loop mode, the dynamometer control is set to a percentage of available dynamometer output or load. In this mode, the resulting load is independent of throttle position, rpm, or vehicle

speed. While it is possible to manually operate a dynamometer in open-loop mode, computer automation enhances the reliability and repeatability of test results.

Closed-Loop Mode

In closed-loop mode, the load is referenced to a feedback signal defined by the test procedure. For example, in constant-speed mode, the user can set the speed at which to hold the vehicle. An

increase in throttle position is counteracted by an increase in load, preventing the vehicle from exceeding the selected speed point. Several speed points may be programmed versus time so the

operator can slowly step the vehicle through the speed range of the engine while monitoring engine parameters and their relation to torque output.

Another example of closed-loop operation is terrain simulation, in which the load is varied under computer control to simulate hills, turns, or other driving scenarios.

Continuous load adjustments cannot always be implemented consistently with manual methods, so an automated control system is essential for test procedures that require closed-loop control.

Example Applications

Lets consider the control system requirements for two typical dynamometer applications.

Chassis

You are probably familiar with chassis dynamometers, which measure vehicle performance using torque and speed measurements taken from roller drums that load the vehicle drive train through

contact with the wheels

The chassis dynamometer shown here uses a control system to automate the test procedure while making measurements and logging them for postanalysis. In some configurations, the control

system drives the vehicle directly, using throttle and brake actuators. In other applications, it will provide a drivers aid to guide the actions of a human driver during the test.

To maximize reliability, the control system should be built using real-time software tools. Simply put, real-time software can perform automated tasks within a predictable response time. Because

Microsoft Windows and other popular operating systems are not predictable in this sense, they are a poor choice for controlling high-performance dynamometer test systems.

Because of graphics and networking capabilities, however, PCs are often used as the operator console. Having a separate computer also prevents operator actions from affecting the


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responsiveness of the real-time system.

Creating the software application can be one of the most difficult tasks in building an integrated control system. Most real-time software tools require someone to develop and maintain special

software libraries for handling real-time I/O interfaces and communications between the real-time system and the operator interface. Turnkey system vendors can provide a solution that that they

have developed, but such solutions are often based on proprietary hardware and closed software tools that are expensive and offer limited expansion capability.

To provide for future expandability, the control system should be built using software tools that are inexpensive, easy to use, and compatible with a wide variety of sensors and systems. In a later

section, we will see how National Instruments products achieve this goal.

Engine

Now, lets look at an engine test application, where the engine is tested by itself (that is, out of the vehicle), with the dynamometer typically connected directly to the engine drive shaft at the

flywheel. The test cell controller manipulates throttle position and other engine inputs. In this case, the dynamometer has its own controller, which communicates with the cell controller through a

connection.serial

Courtesy of Mustang Dynamometer

The diagram shows a typical engine test cell, with separate pieces of equipment that emulate fuel, coolant, oil, electrical, and other vehicle systems that are normally connected to the engine.

These systems may also have their own parameters (such as temperature, pressure, and flow) that the cell controller must manage. The test cell usually includes safety interlocks and other

facility-related interfaces that are also managed by the cell controller.

While this diagram does not illustrate them, the engine is often instrumented with sensors that provide information on internal operating conditions. In production environments, the engine may als

supply the test system with information about itself through bar codes or embedded flash memory.

The integrated control system configuration shown above is typical of systems delivered by systems integrators using National Instruments products. Notice that, unlike other approaches, the

real-time control system and signal I/O connections are all within the test cell. Long, bulky cable runs through the test cell bulkhead are replaced with a single network connection, which reduces

installation and debugging time while improving operational reliability and common-mode noise rejection.

Drive Train

In a drive train application, the objective is to measure the performance of the transmission, differential, and/or other drive train components. A drive motor applies a controlled torque at the input t

the assembly under test.

Integrated Control System

In an automated dynamometer test system, the control system provides the following functions:

Operator interface

Data logging

Data acquisition

Engine/vehicle control

Simulated driving conditions control

Development Tools

The following sections describe the National Instruments tools and approaches that you can use to build a test cell system platform. These tools are available directly from National Instruments.

LabVIEW Real-Time Module

The Module is based on the industry-standard graphical programming tools that you may already be using. Using built-in LabVIEW libraries, you can quickly progrLabVIEW Real-Time LabVIEW

hardware to acquire data, implement control loops, and communicate with instruments and devices supplied by other vendors. You can also perform time or frequency-domain analysis on data a

is acquired and immediately display the results to an operator.

With the Module, National Instruments has extended the simplicity of the graphical programming environment so that users can develop and deploy real-timeLabVIEW Real-Time LabVIEW

applications without in-depth knowledge of real-time techniques or nonstandard computer systems. With the Module, you can download critical tasks to a real-time processoLabVIEW Real-Time

where they execute in a deterministic (i.e. Windows-free) environment. The operator interface runs on a separate PC, but operates with the real-time software as a single integrated application.

The PC and real-time processor communicate through a high-speed Ethernet connection.
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In a dynamometer application, the operator interface is used to configure and control the real-time control system. The real-time control system makes measurements and logs them to disk, while

generating outputs that control the dynamometer, the engine, and other devices. The real-time system also executes test profile sequences that the user defines using the operator interface.

Note that this approach uses a single, integrated system that is much more cost-effective than alternative solutions that require separate systems for some of these functions.

Modular PXI and SCXI Hardware

Because for 20 years has been such a popular industrial computer bus for real-time systems, many dynamometer control systems are still built using or technology. Recently,VME VME VXI

however, the next-generation CompactPCI bus standard has been rapidly adopted in industrial applications.

National Instruments has helped to define , which extends CompactPCI for measurement and control applications. The configuration that you see below has an embedded real-time processoPXI

and a mix of data acquisition and signal conditioning modules. Using a variety of chassis configuration and expansion options, it is easy to build systems with more slots, so the system chanPXI

count can be expanded effortlessly.

Low-Cost Options

A high-performance control system is not required or appropriate in all situations. For customers that need less capability, options available from National Instruments include

LabVIEW Real-TimeModule running on modulesFieldPoint

LabVIEW Real-TimeModule running on PC-based or PXI-based data acquisition devices

Running all control and acquisition processes under Microsoft Windows

You can learn more about real time controllers, data acquisition and signal conditioning at , and .http://www.ni.com/rt http://ni.com/measurements http://www.ni.com/scxi

Signal I/O

In the process of specifying your control system, you must consider all of the parameters that it will need to handle.

This is not an exhaustive list, but it summarizes most of the parameter types commonly encountered in dynamometer applications.

The measurement and control I/O for these signals can be implemented in several ways. Two commonly used approaches are to connect sensors and actuators directly to the cell controller or to

devices that perform a specific task (such as the AC dynamometer controller in the engine test example). As a result, the data may be communicated to the control system in the form of simple

voltages or currents or through communication protocols such as , , orRS-232 CAN GPIB.

This variety of signal interfaces creates another potential problem in building the control system. While it may be easy to inventory the I/O requirements for sensors and equipment that you are

using today, it is much more difficult to predict the parameter types or channel counts that you will need a year from now. Proprietary solutions can limit your options, particularly with respect to

special sensors, third-party instruments such as emissions analyzers, or even expanding the existing system to include additional I/O channels.
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Here is a sampling of sensors and signal types that can be measured with National Instruments modules. While you probably do not need all of these options now, they are available if youPXI

need them in the future. Native , and communications provide compatibility with most third-party instruments and subsystems.GPIB RS-232 RS-422/485

Certain sensor and signal types require conditioning before they are connected to the control system. National Instruments modules provide excitation, isolation, multiplexing, and other sigSCXI

conditioning features required when using the sensor and signal types shown below.

Conclusions

In this paper, we have reviewed some of the requirements that drive the need for flexible dynamometer test.

We have seen how the LabVIEW Real-Time Module can be combined with PXI and SCXI hardware modules to create an integrated control solution with the flexibility to meet your changing

requirements. With the PXI platform, users can build fully customized applications using a wide variety of National Instruments and third-party components, integrated as a single system.

Contact Information

This document is only a brief introduction to the tools and products offered by National Instruments for dynamometer test. If you have questions or would like to schedule a discussion of your

specific needs, please contact us at:

Rick Ary

National Instruments

11500 N. Mopac Expwy.

Austin, TX 78759, USA

(512) 683-6800

[email protected]

See Also:

For more information regarding building your dynamometer or finding a systems integrator with a National Instruments-based system.

For more information on National Instruments products.

Appendix A Common Test Parameters

Summary

Control values may include:

Variable Engineering Units Device

Throttle, brake position % Actuator

Loading or driving

torquelb ft, N m, % Dynamometer

Directly measured values may include:

VariableEngineering

UnitsSensor

Force lb, N Load cell

Torque lb ft, N m Torque meter

Rotation speed rpm Tachometer

Flow liters/s Flow meter

Pressures psi, psig, psia Pressure sensor Temperatures C, F Thermocouple, RTD

Emissions ppm, % Electrochemical, Infrared

Vibration g Accelerometer

Noise Pa Microphone

Calculated values may include:

Variable Engineering Units Derived from

Torque ft lb, N m Force, moment arm

Power hp, W Torque, rpm

Speed mph, km/h Drum rpm, diameter

Acceleration/Deceleration m/s , ft/s2 2 Speed

Braking force N Speed, vehicle mass
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Fuel Efficiency mpg Speed/Flow or Distance/Total

consumption

Measurement and Control Terminology

Torque and Force

Torque is "twisting force". It is typically measured using a dynamometer connected to the engine flywheel or to roller drums driven by the wheels of the vehicle.

Under actual driving conditions, the primary forces that affect a vehicle are:

Driving force (torque)

Braking force

Aerodynamic drag

Gravity (on slopes)

Some test procedures require the dynamometer to simulate these forces.

Horsepower

Horsepower is a measure of an engines power output. Horsepower can be calculated as follows:

HORSEPOWER = TORQUE (lb ft/s) x rpm / 5252

So by measuring rpm and torque, you can calculate power output.

Note: For an in-depth explanation of torque and horsepower, see Appendix B, Relating Torque to Horsepower.

Vehicle Speed and rpm

Vehicle speed and rpm are used to calculate in horsepower and acceleration. Measured parameter values are commonly plotted against speed or rpm for the purpose of display or analysis.

Temperature and Pressure

Temperature and pressure conditions have a direct affect on engine and vehicle performance. Temperature and pressure parameters may be measured in the following locations:

Ambient (barometric pressure)

Oil

Coolant

Manifold

Fuel

Exhaust

Brakes and brake fluid

Fuel and Fluid Flow

Fuel efficiency (miles per gallon or kilometers per liter) is an important performance measurement both for customer satisfaction and for compliance with regulatory mandates. Fuel efficiency is

calculated from fuel flow and vehicle speed.

The air/fuel ratio is another value of interest that is calculated from fuel flow and air flow rates.

Vehicle Controls

Vehicle and engine control values can include

Throttle position

Brake position

Gearshift position

Discrete Digital Signals

Discrete digital signals are on/off status indicators or control connections.

Noise and Vibration

Noise and vibration are frequency-domain parameters that are important in evaluating noise, vibration, and harshness (NVH) design factors.

Emissions

Low emissions levels are important for achieving compliance with environmental regulations. Typical emissions measurements include:


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Oxygen

Carbon dioxide

Carbon monoxide

Nitrogen oxides

Methane and other hydrocarbons

Third-party gas analyzers are used in test applications that require emissions measurements.

Appendix B Relating Torque to Horsepower

Torque is the twisting force produced by an engine (typically measured in units of pound-feet).

The torque magnitude is defined (and often measured) as the moment arm length multiplied by a perpendicular force applied to that moment arm. The moment arm (also called torque arm) is the

distance between the center of a rotating axis (such as an axle or drive shaft) and the point where the force is applied (e.g. the surface of the tire where it touches the road), measured in feet.

TORQUE = FORCE x MOMENT ARM (lb ft)

Note: The metric unit for torque is newton-meters (N m)

Note: Engine torque is not the same as drum (wheel) torque, because the gearing through the drive train has the effect of changing the moment arm. As we will see below, power is proportional

rpm and torque. Because power is nominally constant throughout the drive train, the ratio of wheel torque to engine torque is directly proportional to the ratio of engine rpm to wheel rpm.

The word "horsepower" was coined by James Watt, who compared the work of his invention, the steam engine, to that of a horse. Watt wanted to know how many horses his steam engine would

replace. He found that a robust horse could lift a 150-pound weight 220 feet in the air (using a pulley system) in 60 seconds.

Note: It is important to distinguish between pounds of force and pounds of mass. A 1 pound mass experiences a 1 pound vertical force (weight) in a 1 g gravitational field. In the case of the horse

lifting the weight, the lifting force opposes the gravitational force. Obviously, if the horse were pulling the weight along the ground on a cart, the force involved in moving the weight would be

significantly different. So, for the purpose of this discussion the term pound refers to force, not mass or weight.

Work is force applied over a specific distance. In Watts example, the work is lifting the 150 lb weight a distance of 220 feet.

WORK = FORCE x DISTANCE (lb ft)

Power is the rate at which the work is performed. In Watts example, you can lift the 150 pound weight 220 feet in 60 seconds if you use a 1 hp engine, 30 seconds using 2 hp, etc.

POWER = WORK/TIME (lb ft/s)

= FORCE x DISTANCE/TIME

Plugging in Watts figures for horsepower,

1 HORSEPOWER = 150 lb x 220 ft / 60 s = 550 (lb ft/s)

Note: The metric unit for power is watts (1 W = 1 N m/s).

1 hp = 746 W

Horsepower is related to torque as follows:

POWER (lb-ft/s) = FORCE (lb) x DISTANCE (ft) / TIME (s)

For a wheel,

DISTANCE = CIRCUMFERENCE x REVOLUTIONS

= 2 P x R (radius or moment arm in ft) x REVOLUTIONS

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POWER = FORCE x 2 P x MOMENT ARM x REVOLUTIONS / TIME

= TORQUE x 2 P x REVOLUTIONS / TIME

= TORQUE x 2 P x rpm (revolutions/minute)/60(s/minute)

= TORQUE x rpm x 2 P /60

HORSEPOWER = POWER / 550 (lb ft/s)

= TORQUE x rpm x 2 P /(60 x 550)

HORSEPOWER = TORQUE x rpm / 5252

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