Torque meter in variable speed drive systems
Unlike electrical quantities the measurement of torque is not an easy task. Hence in most cases the torque is not measured directly but calculated (estimated) from electrical signals. But what if you need high absolute accuracy of torque control? In such case a torque meter can be used. In this post we share few things to consider when installing a torque meter within your shaft string.
Torque estimation by motor model
VFD standard control loops allow speed or torque control. In torque control mode the drive shall follow an external torque reference. For closed loop speed control there is an inner torque control loop. Therefore, actual torque value is needed in both control schemes. A common practice used in majority of applications is to calculate the actual torque by the motor model inside the VFD control system. As well-known, the electromagnetic torque can be determined as a product of current and flux components.
Accuracy of calculated torque
Now you may ask how accurate such torque calculation is. Well, it depends on the algorithm and there are few tricks to increase the accuracy. However, the absolute value of the actual torque is typically not the most important thing and small error can be tolerated.
Yet, there are some cases were the absolute value of torque matters. For very high precision of torque control an external torque meter may become necessary.
Realization of a torque meter
There are several principles how to realize a torque meter. Widely used approach is a torque measurement coupling. This coupling contains multiple strain gauges in a specific arrangement. Strain gauges measure the torque based on their deformation and corresponding change in resistance. Thus there is an analog output from each sensor. Moreover, the torque meter also includes sophisticated electronic circuits to convert, filter and amplify the signal into the final digital form that is easy to transmit, use inside a control system or further post-process.
Performance of a torque meter
What performance can you expect from a torque meter? First thing is accuracy. There are various products on the market with different accuracy classes. A really good class is 0.05% – probably sufficient for almost any application. Another quantity is the repeatability. One would expect a value of ± 0.02% or so. You should also check the frequency range. Especially when using VFD fed motors with inherent torque fluctuations make sure that you can really see them in the measurements (if they are relevant). The analog-digital signal conversion is very critical. It requires sufficient sampling rate, proper digital filtering and noise suppression. Modern torque meters are often delivered with advanced data acquisition platform (DAQ – see below) that gives the user the flexibility to configure many parameters individually for specific test campaign.
Challenges with a torque meter
As you may suspect there are also few challenges associated with using a torque meter. Let’s address few of them.
i. Purchase price
Quality torque meter is a fairly expensive device and may not be easily affordable. Besides the sensor (torque transducer or torque flange) you also need precise electronics for signal processing and data acquisition software to analyze and visualize the measured data.
ii. Overload
The torque meter generally does not have large overload capacity. Typical maximum dynamic torque is 1.6-times the nominal torque. If the threshold is exceeded the torque meter may be damaged. In case of a fault in the system the transient torque will almost certainly exceed the capacity of torque meter.
iii. Safety coupling
If you value your torque meter and want to protect it against excessive overloads then a safety coupling (overload coupling) may be something to consider. Their aim is to ensure quick, reliable and positive separation between the shaft elements. There are two main principal solutions with different response time so make sure that you select the right one.
iv. Torsional performance
If you use the torque meter to verify the torsional behavior you must be aware of one thing. The torque measurement coupling is an additional component in the shaft string. Therefore, it necessarily changes the torsional behavior. Whether it is critical or not depends on the purpose and goal of our torque measurement.
Mechanical installation
From mechanical perspective the torque meter is an additional coupling in the system. Therefore, it shall also tolerate certain misalignment (such as radial, axial and angle misalignment) that will always be present. In any case follow the instructions provided by the manufacturer. You shall also check the maximum thrust load according to specification versus expected maximum value in operation.
Data acquisition system
Besides a precise torque meter a good data acquisition (DAQ) system is very important as well. Mature DAQ systems allow easy working with various types of signals, such as analog, digital, fiber optic or CAN signals within one single system.
DAQ software enables easy configuration of each measuring channel. You can configure the desired sample rate and parameterize various digital filters. Databases are useful to make sensor settings easier and faster.
Common feature is also accessibility via web browser for maximum convenience.
Calibration
In order to achieve highest precision or to perform certified measurements (e.g. test benches) the torque meter shall have a valid calibration. It may be necessary to send it to a laboratory for re-calibration after certain period of time or certain amount of usage.
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Selected manufacturers of torque meters
Torquemeters Limited, https://torquemeters.com/
Hottinger Brüel & Kjaer GmbH (HBK), https://www.hbm.com/en/
