Validation

1

CNC DATA ACQUISITION: SYSTEM DEVELOPMENT AND VALIDATION

Abstract The Open architecture controller is a well known concept to promote the implementation of intelligent machine performance functions on shop floor. However there are no further investigations about the system validation, considering the data acquisition on interpolator level. This study aims at the development of a CNC data acquisition system considering the implementation of a procedure to validate the data transfer. Continuous monitoring of open CNC variables was implemented and experiments for system validation were performed on milling machining centers. The results of the implemented data transfer validation procedure show the system feasibility to continuously acquire the CNC data.

Keywords: Open CNC; data acquisition; system validation

1 INTRODUCTION The machining process performance is strictly related to the machine tool axes motion, considering mainly the trajectory accuracy and the displacement, speeds and accelerations. In the CNC (Computer Numerical Control) machines, the CNC system performs the process of dealing with technological parameters, and with the open architecture concept, it promotes the access to such information. Such concept is one of the key technologies to foster the CNC data monitoring in the shop floor and had a major attention in the 90’s, mainly due to the need to implement the flexible manufacturing systems. The customization functions by the user generated the necessity of achieving a neutral communication interface, which should be independent from the manufacturer [Pritschow 2001]. Since then, communication protocols proposals arose, as well as system configuration for the open architecture controller, that, up to the current days, did not result in a broad scale applied pattern in the industrial environment [Pritschow 2001], [Teti 2010]. In the work of Pritschow et al. [Pritschow 2001], the initiatives to obtain a common interface to the open CNC show a similar pattern definition proposal for the open architecture controller. The main features of the patterns presented are the neutrality concerning the manufacturers and the function customization by means of modules, by using the Application Programming Interface (API).

Using sensor signals of the CNC system itself, with no need for additional signal conditioning, is the main advantage of open CNC data monitoring. That enables more complex experimental investigations due to the low investment in instrumentation [Oliveira 2008] . According to Yun et al. [Yun 2007], the open architecture control system structure is generally comprised of three levels: device, control and system level. In order to perform the communication among the levels, an open and standardized interface is required. The MTConnect is an engagement example to obtain a common communication interface among equipments. MTConnect does not intend to replace the existing communication patterns, but it integrates the system by using an agent that performs the communication between the field devices and the external systems using the XML language (eXtensible Markup Language) [Teti 2010].

The broad monitoring function application that relies on the machine controller opening can be developed and used in the industry. However, considering the controller opening level, CNC monitoring systems need to be developed and optimized [Schützer 2012] . The current work presents the development of a data monitoring system at the level of the interpolator control. The data is stored in a commercial CNC internal Buffer and broadcast to a PC by the data transmission module, programmed with the LabVIEW® software. A procedure to validate the system was implemented by means of the

Proceedings of the 11th International Conference on High Speed Machining

2

Data

collected

monitored data comparison with the laser interferometer data by means of experiments performed in a commercial machine tool. The described approach adds on previous CNC monitoring systems, since a CNC data transfer validation procedure is presented.

2 SYSTEM DEVELOPMENT The pieces of equipments used at this work are the machining center Discovery 760, Romi, Brazil, the high speed machining center LPZ 500, MAP Werkzeugmaschinen GmbH, Germany, the laser interferometer 5528A from Hewllet-Packard®, USA, and the data acquisition board CP5511 SIEMENS AG, Germany. LPZ 500 is a five axes simultaneous machine, however, for the present experiments, it was operated using only three Cartesian axes X, Y and Z. The board CP5511 performs the communication between the personal computer and the CNC machine-tool by means of the OPI (Operator Panel Interface) with 1.5 Mbps speed. The next topic presents the system development.

2.1 Open CNC monitoring system

The acquisition strategy presented here was applied in an open commercial CNC, with the 250 samples/second sampling frequency. This strategy uses an open CNC internal procedure called synchronous action, which can be programmed directly in the NC program. The synchronous actions record continuously the monitored data in a buffer and the data is transmitted cyclically to a personal computer where they were stored. The open CNC data transmission system was developed with the LabVIEW® 8.5 software , National Instruments Corporation, USA.

Data

External entities: open CNC and user; Processes: data acquisition, transmission and

analysis; Data depository: buffer and data storage. The data acquisition process executes the data collection of the open CNC and stores it in a buffer. The data transmission and analysis process executes the data collection and processing, and the monitored variable values are stored in a text file. At the present work, the buffer is dynamic, i.e., the data is temporarily stored and withdrawn by the data transmission module in a cyclic manner. In order to build the buffer, CNC variables named parameters R were used. In parameters R the feed rate and axes positions data are stored. There are 100 parameters R that can be expanded up to 1,000, but such expansion was not necessary for the experiments here described. The data transmission module was developed with the Labview® 8.5 software. The Labview® programming logics is based on the data flow programming. The data transmission module developed executes the communication between the CNC and the PC. The program performs the cyclical reading of the 100 parameters R.

2.2 System validation experiments: analysis of the monitored variable resolution in the open CNC

This experiment was executed in the CNC SIEMENS 810D of the machining center Discovery 760 Romi®. The following premise was developed for the experiment execution: Premise: the system collects data with appropriate resolution for process performance analyses. In order to analyze the behavior of the resolution achieved in the online monitoring, the following feed rate were used: 600, 1,000, 6,000 and 12,000 mm/min. The experiment was executed in a three axis machining

Open CNC - Position - Speed - Acceleration

Axis data acquisition

Data acquisition

using the synchronous

action function

center. The X axis position and feed rate data was simultaneously collected by means of a trigger that freezes the control system data and places them in an indexed buffer for later recovery. The CNC variables that were monitored were $VA_IM1[X1] and $VA_VACTM [X1], which are related correspondingly to the X axis position and feed rate, respectively. Moreover, the following NC program was

CNC internal buffer - Dynamic buffer - CNC variables - Parameters R

Data transmission

Data block transmitted

continuously from the CNC to the

PC

User - User interface

Monitored data - Data storage

developed for the data collection:

G54

G1 X0 Y0 F1000

F (Programmed with 4 different levels 600, 1000, 6000, 12000 mm/min)

$AC_MARKER[1]=0

$AC_MARKER[2]=1

ID=1 DO $R[$AC_MARKER[1]]=$VA_IM1[X1] $R[$AC_MARKER[2]]= $VA_VACTM[X1] $AC_MARKER[1]=$AC_MARKER[1]+2 $AC_MARKER[2]= $AC_MARKER[2]+2

X80

M30

Fig. 1: Monitoring data flow diagram

The PC communication to the CNC was performed by the board CP5511 and by the server NCDDE (NC dynamic data exchange), Siemens AG [Siemens 1997] .Figure 1 shows the data flow diagram developed to the system.

The data flow diagram shown in Figure 1 is comprised by the following components:

Although the machining center has three axes, only the X axis was chosen for monitoring, as it is supposed that the X axis analysis can be expanded to the other axes, as the sampling rate is the same. The NC program executes a linear motion in the X axis between the coordinates 0 and 80 mm, and this length is enough for the axis to accelerate and reach the programmed feed rate. This is performed in the program by the programming line ID=1. The X axis position and


3

speed values are thus stored cyclically in the CNC buffer at every 10 ms and transmitted to a file with the monitoring system developed.

2.3 System validation experiments: step input response

In this experiment, the Y axis feed rate data of the milling machining center LPZ 500 was measured simultaneously by the laser interferometer and by the CNC monitoring system. This experiment premise was: Premise: The feed rate monitored by the developed system is very close to the feed rate measured by the laser interferometer. The distance run was linear in 350 mm, with programmed feed rate of 5,000 mm/min. In order to assemble the instrument, a temperature (air and material), humidity and pressure compensation station was used for automatic correction of the laser measurement. Figure 2 show the laser interferometer assembly in the machine LPZ 500 for measurement.

Fig. 2: Laser interferometer assembly for the experiment

3 MONITORING SYSTEM VALIDATION This analysis can be expanded to other CNC models, but it must be observed that, the faster the CNC interpolator cycle time, the lower the variable resolution for the same feed rate. In order to check if the data acquisition process is controlled, the monitored variable resolution was analyzed by means of control limit graph. Thus, in case there is special cause variation (for instance, measurement instrument out of calibration) they will be presented in the graph. The upper and lower control limits are calculated by the variation of +/-3 standard deviations in relation to the resolution data average [Kume 1993]. Figures 3 and 4 shows the resolution behavior monitoring and the corresponding feed rate value influence in the collected data resolution. As Figures 3 and 4 shows, for the feed rates 600 mm/min, 1,000 mm/min, 6,000 mm/min and 12,000 mm/min, the results showed that, even though the higher feed rate have higher variability, the values are within the control limits, showing that there are no special causes influencing the process. Such limits are given by the resolution averages and standard deviations. In the numerical results presented in the graph, it can be seen that the standard deviations of the position for each of the feed rate are low in relation to the work accuracy commonly achieved in a conventional machining center. From this result it is seen that, from a practical point of view, the data monitoring system assessed here has an acceptable repeatability, with maximum variation around 0.009 mm (value achieved by the difference between the

upper limit (2.004) and lower limit (1.995) for the speed with greater variability, which was 12,000 mm/min, (Figure 4) resolution with F 12,000 mm/min). Other cause that can influence the resolution variation is the machine-tool measurement system itself, which, in the case of Discovery 760, is a rotational encoder. For the machines with linear optical encoders, a lower variation of the data measured to be expected.

Fig. 3: Resolution control graph for 600 and 1,000 mm/min

Fig. 4: Resolution control graph for 6,000 and 12,000 mm/min

In order to assess the system capacity to acquire optical linear encoder data and such data represents the behavior that happened in the tool center point, the experiment comparing the feed rate measurement performed by the system and a laser interferometer was executed. The experiments were performed in the machine LPZ 500 with CNC Siemens 840D. Figure 5 shows both behaviors of Y axis feed rate, which programmed value was a step input of 5,000 mm/min.


4

The implemented procedure to validate the system shows the difference of the maximum peak value of the feed rate signal measured by the monitoring system and by the laser interferometer was lower than 0.5%, showing a system with the ability to monitor CNC data on interpolator level.

5 ACKNOWLEDGMENTS The authors would like to thank the financial support of the CNPq, CAPES, FINEP and DFG in the scope of the Brazilian German Collaborative Research Initiative in Manufacturing Technology (BRAGECRIM). Special thanks are also due to Dra. M.C.O. Papa and Mr. J. Mewis.

Fig. 5: Step input response of 5,000 mm/min - Laser and CNC acquired data

As shown in Figure 5, the steady state values are practically the same for both measurement systems. A small difference is noticed due to the data synchronization during the ascent ramp; but in the peak value, a variation lower than 0.5 % is seen. With the results presented in Figure 5, it is possible to state that the data acquisition system developed measures accurately the behavior of the variable of interest.

4 CONCLUSIONS The development of an alternative system to monitor the CNC manufacturing was presented at the present work.

In order to execute the system proposed a CNC data acquisition procedure was developed, and the system validation procedure was implemented, that adds on previous works, since the CNC data transfer to the system is validated.

The monitored variable resolution presented values within the control limits and the largest standard deviation was 0.002 mm. It shows that the system collects data with appropriate resolution for the process performance analysis.

Documents

Validation