Five test steps, one solution: FASTpress for the final inspection of highly complex steering systems
Modern steering systems in vehicles have highly complex functions – and therefore have significant influence on the driving and safety feeling.
The requirements for the assembly and testing of steering systems are correspondingly high – these systems consist of numerous electrical and mechanical components that have to be connected, assembled and finally tested. Such tests for individual components remain important – but what about the testing of the fully assembled complete steering system? The industrial process controller MP85A FASTpress offers an efficient and secure complete solution for the final inspection of this complex and safety-relevant vehicle assembly – and also guarantees 100% documentation of the measurement results.
The inspection of a steering system consists of various test steps in which different measurement parameters are analyzed. It is therefore even more important to have an integrated complete system for process monitoring that collates all data and evaluation parameters. Precisely this integrated approach is followed by the process controller MP85A FASTpress in combination with the corresponding FASTpress Suite software.
Leading system constructors use MP85A FASTpress successfully in the testing of steering systems. Process monitoring starts as soon as the steering system is transferred to the test station after assembly. The various components in the FASTpress family then control the following test steps:
- Feeling characteristic curve
- Valve curve
- Displacement force
- Torque
- Play measurement
Even though these test tasks are different – they all have the following in common: In each test step, the object is to evaluate curve forms and positions, and to maintain precisely defined tolerance limits. A further task in process monitoring is the acquisition and storage of statistic data which are separately determined for each test step: For example: min/max values, mean values, target/actual deviations and tolerance window data.
The process controllers for the various test steps are connected with each other via Ethernet. All data can therefore be stored in a common target drive. Each tested steering system has – via the component carrier – its own identification number. The process data are therefore directly combined with the ID number so that all relevant characteristics for each steering system can be easily and simply made available.
These are the individual test steps:
Feeling characteristic curve and valve curve: Is the feel of the ride correct?
Safety and drive feeling of a vehicle are influenced significantly by the steering system. The steering must always react in the expected manner for the driver. In addition, the manner in which a steering functions expresses specific driving characteristics of a vehicle.
In all these perceived steering characteristics of a vehicle, the feeling characteristic curve plays a decisive role. By changes in the feeling characteristic curve, the behavior of the steering can be influenced in the factory. Possible settings, for example, can be "Comfort" or "Sporty". The reset torque in the steering wheel changes according to the driving characteristic.
To check the feeling characteristic curve, the connecting rod is fixed and equipped with a force transducer to measure the tensile and compressive forces. A power shaft then applies a torque to the steering column. The evaluation algorithms in MP85A FASTpress and the corresponding parameter sets enable the simulation and individual setting of the reaction behavior of the steering for various speeds.
The valve curve must also be checked for setting the feeling characteristic curve. This means that The pressure of the hydraulic system is monitored on the basis of the applied torque so that the curve must follow a predefined form.
In this operating mode, the sequence of evaluation windows is significant as the curve consists of a rising and falling branch. Thanks to the special window technology, the FASTpress monitoring criteria differentiates between these sequences. FASTpress only activates the falling branch for monitoring once the curve has reached its maximum.
Further steering system tests
Measuring the displacement force: A numerically controlled machine axis (NC axis) applies the displacement force to the connecting rod for this test step. The displacement itself is also recorded by the absolute value encoder of the NC axis. The test here is passed when the curve trace does not exceed specific min/max limits in both tensile and compressive directions and the maximum deviation from the mean value is sufficiently small.
Torque testing: In the torque test, a rotary motion is applied via the steering column. The friction moment is recorded and monitoring according to the same criteria used for the displacement force for peak and mean values.
Play measurement: The setting of the play between steering column and connecting rod must be separately tested and set. This is essential for the stiffness of the transmission elements pinion and gear racks. The NC axes also applies a rotary motion to the steering column for this test. The position of the thrust piece on the gear rack is measured with a separate displacement probe and set symmetrically to the mean/comfort position in the setpoint.
Evaluation and data backup: An integrated system for secure process monitoring
The secure acquisition of measurement parameters is just one aspect in process monitoring. Just as essential for the security and efficiency of test procedures are the statistical tools of the checking software.
The FASTpress software suite has the perfect requirements for this – even and specifically for the testing of steering systems: Firstly, the FASTpress software suite collects statistical facts from the data in a pilot series. These data then flow as evaluation limits for the mass production series later on.
This upstream process data analysis with statistical evaluation using the "EASYteach" software module facilitates the identification of relevant evaluation criteria – and does this in just a few steps:
- First of all the required number of processes are recorded with the monitoring software PME Assistant
- The data series obtained here are read into the EASYteach module of the PME Assistant and are available graphically both as curve family and mean value curve
- After the curves are processed (e.g. edited, deleted), they can be aligned relative to each other. It is also possible to print or save evaluation windows and outline parameters in a report
- Finally, the evaluation windows or tolerance ranges can be loaded back into the MP85A FASTpress process controller.
This procedure provides an integrated system for process monitoring: From measurement of the release sample to continuous process monitoring during ongoing production to determination of reproducible evaluation parameters.
Simple individualization possible
In the application case described here, the central visualization and control of all process parameters is implemented via a Siemens MP277 Panel directly at the test cell. In addition to the visualization of the entire system, the Siemens Panel also displays the process monitoring data acquired by the MP85A FASTpress.
Depending on the application, the FASTpress software also enables the development of customized views with the .net class library. Individually tailored display windows and a clearly structured process depiction are therefore available to the users. These individual views can be programmed directly by HBM – or by each user themselves (e.g. in C#, VB.net and all other .net-compatible programming languages).
Special displays are not even required for these individual presentations. Thanks to the open structure of the FASTpress suite, the process monitoring data can also be displayed using the human-machine interfaces (HMI) – in this case, the Siemens MP277 – in general use or standard industrial PCs (IPCs).
FASTpress: Integrated for more security
The industrial process controller MP85A FASTpress and the matching FASTpress software suite form the best basis for integrated process monitoring – even then when the requirements regarding monitoring, data backup and integration are extremely high. This applies in particular to safety-relevant components such as steering systems.
The perfect interplay of hardware and software allows, for instance, system builders to simply plan the overall system, including control and monitoring elements (as here for the steering tests). Users of the process monitoring system profit from the high integration level and the simple, transparent operation of the control system.
