Mobasher, B., Engstrom, J., and Anderson, H., “A Low-Cost Retrofit System for Digital Closed Loop Mechanical Testing of Materials,” Proc., third Annual Undergraduate Faculty Enhancement Symposium, Teaching the Material Science, Engineering, and Field Aspects of Concrete, Univ. of Cincinnati, July 9-12, 1995, pp.133-139. (zipped MS Word V.7 Office 97)
A Low-Cost Retrofit System for Digital Closed Loop Mechanical Testing of Materials
Barzin Mobasher, John Engstrom, Jason Key,Henry Anderson,
Department of Civil and Environmental Engineering, Arizona State University, Tempe, Arizona 85287
Engineering Research Services, Arizona State University, Tempe, Arizona 85287
To expand the structural and material testing facilities at the undergraduate level, the Mechanics of Materials Laboratory at Arizona State University has developed a low cost retrofit system for closed loop testing of materials. The project involves upgrading several screw-driven universal testing machines by means of digitally controlled geared servomotors. The motion of servomotor is monitored using an optical encoder that is connected to a computerized motion control and data acquisition software. Computer software is developed to control the response of the system. The test system is directly linked to the campus computer network, providing the undergraduate students with the digitized raw data when a mechanical test is conducted. Digital data processing techniques are also included as a part of the curriculum to add flexibility in interpretation and manipulation of experimental data. This enables an efficient and accessible presentation method to undergraduate students.
Modular components of the system are developed using object oriented programming tools. Additional software is being developed to increase the scope of the project to include materials science & engineering, and experimental stress analysis aspects. Load, displacement, and strain controlled tests are developed. The laboratory has been successfully integrated in the curriculum of undergraduate programs including, Civil, Mechanical and Aerospace, and Chemical, Bio, and Materials Eng. Departments.
One of the major challenges facing the educational institutions is the lack of funding to keep up with the ever changing field of engineering education. Instructional labs are among the facilities that suffer due to the budget shortfalls since the cost of replacing major test instruments are quite high. It is well known that the instructional laboratories are a fundamental component of education that present the students with hands on experience of the fundamental concepts. During the recent years, use of computers in the laboratories have enabled active participation of students in the experiment. State of the art computer facilities can enhance the quality of education by keeping the students’ attention focused on the fundamental concepts. A mechanical testing laboratory integrates the understanding of engineering properties of materials with a variety of instrumentation and measurement techniques.
Electromechanical test systems have been traditionally used for teaching and research both. The original systems utilized a vacuum tube technology. Despite their age several of them are still in operation, however, many of these instruments have undergone a partial or complete retrofit. The mode of operation of these systems is based on a displacement controlled test that can be used to conduct tensile, compressive, flexural, and fatigue tests. The test data are normally plotted using a strip chart recorder. The rotation of a DC motor is converted to the appropriate cross head speed using a high precision gear speed reducing system. This generates the necessary torque for turning the loading screws that move the cross head. Typical gear ratios range from 10:1 to 100:1.
As long as these machines are in a good mechanical condition, they could be converted to full scale digital machines. A partial retrofit is defined as a condition where the commands to move, speed selection, and the control of the cross-head, are still conducted with the original instrumentation. The chart recorder is however replaced with a stand-alone data acquisition system that may include appropriate signal conditioning devices for amplification and scaling of load and displacement transducers.
Full retrofit is defined as a project where the entire control and data acquisition of the system are replaced with solid state electronics. The DC motor is replaced with a stepper motor for an open loop system, and a servo-motor for a closed loop system. Stepper motors are brushless motors that contain either a permanent magnet, a variable reluctance, or hybrid types. Motor construction may vary depending on the number of windings, pole counts, and mechanical step angles. A stepper drive sequentially regulates the current into the motor phase windings in order to produce a stepwise motion. A chopped current therefore causes the motion according to the frequency specified. Due to the inherent open loop nature of these systems, the torque delivered by stepper motor is a function of the speed, and the motion profile. It is mainly dependent on the motor’s ability to overcome the inductance of the windings and push the maximum current into the phase windings as quickly as possible. In a servomotor that is based on a closed loop system, the current into the motor is continuously updated according to three loops controlling the system. These include a torque (current) loop that controls the amount of the current into the windings, the position loop, and the velocity loop. In most servosystems, there is direct relationship between the current into the winding and the torque supplied. Optical encoders are used to measure the position, and the velocity data are calculated by real time digital processing of the position data.
Figure 1 presents a schematic of the Mechanical Test System. The procedures to retrofit these machines are quite straightforward. Only the structural components of the testing system are used while the motion control and data acquisition are done through the computer. A brush servo motor replaces the DC motor in providing the necessary torque to drive the gears. The overall system consists of a brush servo motor, an optical encoder, an amplifier, and a controller card. The function of the encoder is to measure the rotation of the servomotor with a resolution of 0.09 degree. The controller and a PC-bus plug-in interface card connect the computer to the mechanical testing system. Recently developed plug-in cards can provide up to 4 axis of motion control, 7-12 bit analog inputs, error handling, and a PID filter (used to tune the closed loop parameters). Analog inputs such as load cell, displacement, and strain gage transducers are also connected to this card using the auxiliary channels for data acquisition.
Initial testing of the system can be achieved between the controller card, the amplifier, and the servo motor to assure proper response. The command response must be first tested with the controller board editor to achieve direct communication with the controller card. All the various command functions must be tested, and any mechanical and electronic errors in the system corrected before moving into the graphical programming environments.
A graphical software package such as the LabViewr for Windows, from National Instruments,Austin, Texas was used to provide the user interface. LabView provides tools for instrument control, data acquisition, data analysis. With the graphical programming language, the user wires block diagrams in a logical sequence to create what are called virtual instruments (VIs). Each VI can be programmed to run alone or be embedded into a single hierarchical VI. This modular approach to programming is extremely versatile and allows greater complexity and sophistication, without creating a large and indecipherable assemblage of wires and icons. For each specific control, acquisition, and analysis function provided by the controller card, a library of VIs can be assembled. These VIs can then be used together to create specific testing procedures.
An example of this graphical programming language is seen in Figure 2. Instead of writing code, the programmer wires together a block diagram in much the same way an electrical diagram is drawn. This diagram illustrates how a simple data point is acquired and displayed. The program is split between two windows. The first is a user-interface panel with controls and display outputs, Figure 2.a; the second is a diagram of VIs that combine to build the application. The user specifies which control and display functions are needed in the user-interface panel and then switches to the wiring diagram to connect the block elements as shown in Figure 2.b.
It is best to first configure the system and use the basic control interface provided by the vendor to ensure that all components are communicating properly. When proper communication has been established the LabViewr environment can be implemented. In order to develop the link between LabViewr and the controller card, a Windows DLL (Dynamic Link Library) file is used.
The controller card uses several user-defined variables. The commands fall into two categories, commands and responses. Commands such as start and stop are sent to the motor and an action immediately follows. The response commands are sent to the motor or encoder and a response is sent back to the controller card. These variables can be used to tell the controller card to display position, velocity, torque, and analog input. The controller card definitions can be accessed through Windows Using the DLL (Dynamic Link Library) file structure. To facilitate programming, a separate VI can be written for each command type, that is, one for simple commands and one for commands that display a response. This gives the user total control of the controller from the graphical environment. Examples of these commands include: stop, start, jog, and position. Using the command variables and the modular VIs, a simple displacement controlled tension test routine can be written in a few short minutes. The front and back panel for this test areshown in Figures 2a and 2.b. The programmer defines the input to the test such as testing rate, and in the wiring mode, wires together a sequential command structure. This speed is converted to an encoder count rate, sent to the controller card and the input on the analog input is displayed on the graph.
Development of a closed loop system is somewhat more demanding than a simple open loop test, since the definitions of the command signal, and the response signal as described earlier has to followed. Since the operation of the servomotor is based on the fundamentals of closed loop control systems, no additional circuitry or electronics is required. The user has to specify the control signal as the feedback parameter.
Figure 4 represents a typical tensile stress strain response of a cast iron specimen. Note that the raw data is collected, and the user is required to use a digital data analysis software of a spreadsheet to calculate the engineering properties of the test specimen.
When developing such in house systems, several safety precautions must also be considered. Various interlock mechanisms for operator protection and fail-safe operation may be employed. The system can be equipped with a visual indicator for the operational status, and a computer controlled stop switch. Two mechanical limiters can mechanically shut the motor off and disengage all torque when exceeded. These may be located at the upper and lower ranges of motion, and activated by the testing arm itself. The third level of safety is provided by the main power switch on the front of the test frame. Cutting power releases all torque. The fourth level of safety is a built-in limit in the encoder motor. The motor shuts off and disengages all applied torque when a certain error limit is reached. Its internal PID filter (proportional, integral, and derivative) keeps track of exactly where the motor should be and compares this with the data coming in from the encoder. Any discrepancy is stored as error. This error accumulates while the motor is in servo mode. When the error limit is exceeded the motor is automatically shut off and all torque released. This error limit can be set by the operator and a typical value corresponds to 1000 counts, which is one quarter of revolution of the servo motor. If the testing arm tries to move and is unable to do so, it accumulates error and will eventually shut-off. This eliminates concern for a runaway motor crushing up against the top or bottom of the apparatus.
The safety of the system may also be augmented by providing a door access to the testing arm. If the door is open, an open is generated in an electrical circuit that will disengage the motor. This will ensure that no hands or other body parts are inadvertently caught in the machine during a test. The safety door will also protect the user and observers from any flying debris generated by an explosive rupture.
The completed system allows the operator to load a sample and then control the entire testing procedure from the computer terminal. Tension, compression, fatigue, and bending tests have been developed and tested with relative ease. The graphical user interface is significantly easier to use than older, text based, command shells. This will allow the operator to be more actively involved with the testing.
Procedures to carry out a full scale retrofit of a screw driven mechanical test system are presented. Modular components of the system are developed using object oriented programming tools. Additional software is also developed to extend the scope of the project to materials science & engineering, and experimental stress analysis aspects. The laboratory has been successfully integrated in the curriculum of three engineering programs that include, Civil, Mechanical and Aerospace, and Chemical, Bio, and Materials Engineering Departments.
This laboratory is part of the integrated mechanical properties of materials laboratory (IMTL) of the College of Engineering and Applied Sciences. Support of National Science Foundation (Grant No. MSS9211063 , Program Director Dr. Ken Chong) is greately appreciated.