Measuring Torque with PMX
New application possibilities for innovative torque measurement in test benches – with PMX
Measuring torque, rotational speed, angle of rotation and the quantities derived from these variables is assuming an ever greater role in the design of new test benches for use in industrial environments. In addition to higher requirements for accuracy and speed, further determining criteria for the selection are options for automation and efficient operation. How can all of this be achieved?
This article considers the most important success factors:
- What requirements are placed on innovative test benches?
- How should torque sensors be designed?
- What can be done to further enhance the performance of torque measurements?
- What performance features should the data acquisition and automation system have?
- Which automation task concept should be selected – fieldbus based measuring technology?
- What are the features of the service concept?
1. What requirements are placed on innovative test benches?
Main target of politics and economics is to develop standards for the next phase of more fuel efficient vehicles, particularly directed towards medium to heavy duty fleets. This will help with bolstering energy security & cut carbon pollution, thereby saving money and supporting the manufacturing innovation. As such, engine manufacturers and drivetrain engineers have a challenge - to enhance the performance of current and future engines with fuel efficiencies so as to be compliant with the standards, as well as lead over their competition & satisfy customer expectations.
Today's essential success factors include lightning-fast response to market needs with new products, which are nevertheless mature. Manufacturers must respond to this with shorter development times and test methods that are both efficient and flexible. This in turn requires separation organizationally and in terms of time when preparing and conducting test tasks. Efficiency improvement by a factor of 10 can be achieved here. Energy efficiency is an important issue in the automotive and aviation industries. Focus is shifting increasingly to engine development, rolling resistance and energy conversion efficiency.
First, it must be possible to implement the test structures quickly. This can be achieved with intelligent sensors and measuring amplifier systems that communicate with each other and exchange configuration data, for example by using TEDS sensor data detection. Essential preconditions for this are high quality of measured values and accuracy. HBM's torque flanges of series T10, T12 and T40 meet both these preconditions – high accuracy combined with high dynamics and rotational speed.
In addition, the measuring amplifier and control system must be capable of further processing the measurement data in real time, so that the test bench can then be regulated. It is also essential to make measurement data available at high resolution for analysis and to save it. To achieve real efficiency gains, all these functions must be combined in a single device. The PMX® measuring amplifier system was developed by HBM based on these requirements. It allows for use in testing and proving grounds and can also be used as a measurement and automation system in the production area. This is made possible by flexibly fitting it with measurement and output channels. Depending on the level of automation, analog or Ethernet-based fieldbus interfaces can be used in real time.
It is exactly this flexibility of hardware combined with the possibility of data recording at the highest data rate and resolution that brings users a further efficiency gain by a factor of up to 30.
2. How should torque sensors be designed?
Modern torque transducers of series T10, T12 and T40 from HBM must work with digitized data and at high sampling rates to be able to meet the high requirements of the function tests. Available output signals include not only torque but also rotational speed and angle of rotation. These quantities are important to be able to calculate quantities derived from them such as power and energy conversion efficiency in the downstream PMX® automation system from HBM. The measurement signals are converted into frequency signals to ensure noise-free transmission. This is indispensable for the harsh ambient conditions frequently encountered, as even larger engines or frequency inverters with their electromagnetic fields must not adversely affect measurement quality. The most important metrological properties of torque sensors include:
- Accuracy class
- Sensitivity tolerance
- Temperature stability
- Linearity deviation and hysteresis
Special emphasis was placed on the quality and compliance of operational data in the development of HBM torque sensors.
However, the user should also note the application areas and load limits:
- Rotational speed limits
- Permissible oscillation bandwidths
- Lateral limit and longitudinal forces
- Maximum temperatures
3. What can be done to further enhance the performance of torque measurements?
Measurement signals from torque sensors are acquired with the PMX plug-in module for frequency measurements, the PX460. It works with an accuracy of 0.01% and up to four torque sensors of series T10, T12 or T40 can be operated. Mixed operation is also possible. To further optimize the measurement data, the PMX® measuring amplifier system includes a whole series of internal computing channels specially designed for operation and use of torque transducers. They work exactly like measurement channels in real time at a calculation rate of 50 microseconds.
This includes for example a 21-point linearization of the characteristic curve of the transducer for the torque sensor. The result is an improvement in the raw signal of the sensor in PMX® – beyond the accuracy specified in the data sheet. The improved measurement signal can then be further processed, which increases the measurement quality of the test bench.
Other ways of scaling is the use of polynomials and straight pitches. Especially with the use of polynom-scaling, an increase by a factor of 10 can be achieved since these represent the sensor characteristic again significantly more accurate. The coefficients of the polynomial of the sensor is determined already are in production and subsequent calibration of the sensors during the calibration. Since only 3rd order polynomials are needed here, later parameterization of the PMX measurement channels is very simple and avoids incorrect entries.
To rise the accessible accuracy of torque sensors, calibration equipment can be used to capture the behavior of the sensor under various load cases. These load case include on one hand the dynamic right- and left rotation, on the other hand beside the 100% measuring range also a high accurate measurement in partial ranges are needed. This is necessary e.g. to capture the residual breaking torque. For this purpose, these different applications are measured during the calibration of the sensor and determines the corresponding characteristic curves according to DIN51309 or VDI / VDE 2646 and held firmly in the calibration protocol. These characteristics can then be stored in PMX® and then be used in the test depending on the application. PMX® recognizes from the current measured parameters which application is present and then automatically share-fourth the predefined sensor characteristics.
Another important function is parallel, independent processing of raw measurement values, for example filtering. This makes it possible to adapt the signals for regulation and automation of the test bench. It is this combination of analog outputs and/or PMX® real-time Ethernet fieldbuses that makes it possible to implement efficient test bench automation.
Special filter for testing combustion engines: Due to the work cycle with compression and expansion in the individual cylinders and the corresponding fluctuations in combustion, the torque generated by an engine exhibits highly dynamic behavior. In many measurement systems it appears as "noise" (or rapid changes). This can be eliminated by using a CASMA filter (a filter that works angle-synchronously).
The diagram above shows the result of implementing this type of CASMA filter. It can be clearly seen that the CASMA filter achieves excellent stabilization of torque measurements in correlation to the engine speed, which also changes over the course of time. The greater the width of this filter, the better the results.
Additional functions include determining peak values or mean values of measurement signals to determine and document test limits. These control values can be monitored in turn with limit values or tolerance bands in real time, making it possible to control the test bench.
If the raw values of the torque measurement with torque and speed are available, they can be used to calculate and output the application of torque in real time using mathematical computing channels. Connectible timing elements can be used to correct additional runtime differences in the measurement signals. These differences occur in high-performance load cases on the transducer side and can have negative effects on measurement results.
Test signals: PMX provides users with the convenient option of being able to simulate signals and system states and to test functional capability already during startup without having to place the test bench completely in operation. This can be done on the sensor side by activating the "shunt signal." Then the torque transducer emits 50% of its nominal (rated) signal and the function can already be tested in a "dry run." PMX also has internal signal generators that can be used to simulate test sequences statically and dynamically.
4. What performance features should the data acquisition and automation system have - fieldbus based measuring technology?
The range of measurement signals to acquire is extensive and runs from simple signals, acquired at a low frequency (a slowly changing temperature value, for example) to complex measurement data that has to be simultaneously measured at a high measurement frequency, for example torque signals with angle of rotation signals and rotational speeds that must be acquired synchronously. Decisive factors here include not only the sturdy and accurate sensors, but also robust and accurate measurement acquisition. The two should be in the same accuracy class and should be at least 0.1%, or better still 0.01%. The sampling rate of the signals is just as important as measurement accuracy. It should be high enough so that fast or small partial changes can still be reliably resolved and displayed. To be able to cover acquisition of peak values, computing speed and regulating quality, all measuring and computing channels must be sampled in parallel at a rate of at least 20 kHz, which is equivalent to a measuring and calculating grid of 50 microseconds. In the area of torque measurements, PMX offers the PX460 plug-in board which runs at 38.4 kHz to take full advantage of the bandwidth of measurement signals of torque sensors.
In addition to the process data, the user also has access to extensive diagnostic information in the standard configuration. Due to the design of its real-time hardware, PMX supports bus cycle times as low as ≤ 10 kHz with its real-time Ethernet interfaces and also minimizes the latency of message transfer.
Depending on the automation application, the following real-time Ethernet interfaces are available for selection:
- PROFINET (IRT protocol)
In addition to generating measurement and control signals, these fieldbuses can also be operated in parallel in PMX. PMX then works as a "slave" together with a control master in the test bench. In this way the system is able to provide the dynamics needed to implement simulation of highly dynamic drive and load cycles.
5. Which automation task concept should be selected?
A basic distinction is made between PC-based systems and embedded systems. This applies to acquisition of measurement data, control/ regulation and also visualization. If high real time (deterministics) is required for regulation, embedded systems are used. The amount of data involved is quite small, but also very time-critical.
Regulation in hard real time cannot be implemented on PC-based systems. Because resources are distributed uniformly over all components, control tasks must "wait" in some cases before they can be executed. The cycle times are therefore 50 ms or more, which is by no means adequate for fast and reliable test bench regulation.
Embedded systems demonstrate their full strength in this case, as they reserve their resources entirely for regulation tasks through the internal CPU. The same is true for soft PLC solutions integrated into measurement systems. PMX can be equipped with a CODESYS soft PLC for this purpose, which makes it possible to control the entire test sequence.
Visualization systems are more and more in demand today with the new, web-based technologies. They have the unbeatable advantages of needing only a modern web browser and being available on all modern terminal devices. That includes PCs, tablets and also smart phones. The mobility of these devices, and thus their availability, is appreciated increasingly not only by users, but also maintenance personnel. Another great advantage is that no additional software has to be installed on the terminal device. There is always a browser present.
Two distinctions must be noted again with data storage for test benches. If only the end results of the test will be logged or stored, this can also be done with embedded systems. PC systems have a clear advantage if larger amounts of data and raw data have to be stored, however, due to their bulk storage options such as hard disks.
In this case the DAQ software can record the data on a PC in parallel to measurement and control operation. The bandwidth ranges from ready-to-use standard software prepared in advance for these types of tasks to special solutions which accomplish these tasks by means of software drivers and APIs (Application Programming Interface).
Documentation of test results can also be implemented in various ways. PC systems can easily save results individually and if necessary send them to a pdf printer. However, the trend continues in the direction of database-oriented documentation with the results and measurement data transferred to large databases where they are archived. Then the required reports can be generated through queries and statements describing status, capacity utilization or the quality of components can be generated.
6. What are the features of the service concept?
Service requirements may be categorized as either "on-site service" or "remote service." Measurement and control systems must actively support personnel "on site" during startup and maintenance. This means it must be possible query the status of measured values and the device and to obtain the desired information directly as an LED display on the device or by using a menu in the web browser. Log files also make it easy to record and query all errors and details of device operation. This is especially helpful when searching for sporadic errors or effects. Another possibility is to make "monitoring signals" available. These are voltage signals to which the measurement signals or the computing channels as well can be interconnected for monitoring purposes. Then a simple measurement can be conducted on site by any service engineer. The log files are stored power failsafe in the device and can also be downloaded and archived via web browser for documentation purposes.
Modern and powerful torque sensors such as series T10, T12, T40 from HBM, combined with embedded systems with open communication interfaces such as the PMX® amplifier platform from HBM, are suitable for use in high-quality measurement and regulation tasks.
The general trend observed here is that conventional measurement technology systems on the one hand and automation solutions on the other are moving ever closer together. As well as controlling the measurement sequences themselves, this type of modern system can also control machines and implement modern, forward-looking test benches.