Dream Team: Torque Sensor Integrated In Coupling

 

by Dr. Wilfried Krimmel*

*Dr. W. Krimmel has been active in the area of the torque measuring technique for over 20 years.
 In leading position at Lorenz Messtechnik GmbH he is responsible for the in house calibration laboratory.
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Torque Sensor DR-2554  -  Click for Torque Sensors Product Overview
Picture 1: Setup of Torque Sensor DR-2554

The natural frequency of the mechanical construction of a torque measuring device appertains to the important dynamic features of the testing equipment layout. The new torque sensor DR-2554 succeeded in increasing this natural frequency beyond factor 3, whereby dynamic measurements with even higher accuracy become possible. This was achieved by a consistent advancement of the torque sensors and a closely cooperation with a coupling manufacturer. In addition, great flexibility arose in adaptation possibilities at concurrent short design.

In torque measurement technology, dynamic measurements are carried out very frequently. Here it is to be differentiated whether the excitation of the torque occurs periodically, or whether it is a single procedure. Typical periodic torque courses can be watched at piston engines, presses, tooth shocks in gears, AC-induction motors etc. Torque courses at start-up and brake procedures represent non-periodic excitations.
In case of periodic courses, the torque sensor with the parts mounted, shows the characteristics of a mechanical oscillator with an increase of the amplitude in the natural frequency. In consequence of this, the excitation frequency should not be the same as the natural frequency of the layout because due to the low damping of the testing device, intense increases of the torque must be reckoned. Thus, torques can arise at an excitation amplitude close to the rated torque which will damage the parts in the shaft assembly. However, a fast pass-through of the natural frequency is absolutely possible, because the system does not have the time to allow a dangerous rise of the amplitude. Further it should be noticed that by means of a Fourier series, each periodic excitation can be fragmented in discrete sinusoidal frequency portions. Hereby the frequencies of the sinus oscillations are a multiple of the basic frequency.
The jump function, for instance, represents non-periodic signals. A jump wise change in the torque causes an oscillation with the natural frequency of the mechanical setup. The amplitude of the oscillation is depending on the damping and can accept maximally the twofold amount of the jump amplitude.
Thus, adulterations of the measuring signal due to the natural frequencies of the mechanical setup are to be expected because the testing setup acts like a mechanical low-pass filter. Below the natural frequency, the torque amplitudes are measured very well. In the resonance range, intense increases have to be expected and at larger torque amplitudes the damage of parts is to be reckoned. Above the resonance range, smaller measured values are obtained. In practice therefore, a preferably high natural frequency of the sensor is desired.


Determination of the Natural Frequency

Picture 2: Left Side: Model for 2 Mechanical Mass Oscillators, Right Side: Typical Setup of a Torque Sensor  - Click for Torque Sensors Product Overview
Left Side: Model for 2 Mechanical Mass Oscillators,
 Right Side: Typical Setup of a Torque Sensor
In the simplest case, as represented in picture 2, a torque sensor can be considered as a mechanical oscillator and a torsion spring with a mass attached on each end.
The measuring side and the drive side (opposite side of the measuring side) are to be differentiated. The mass moment of inertia J of a body represents its resistance to rotational accelerations. The torsion body which mainly contributes to the elastic and resilient effect is located between the two inertial masses. The spring constant c is the decisive measure for the torsion body. For the approximate computation of the natural frequency, the masses are assigned to both sides of the torsion element.

Thus, the torsion natural frequency f of the layout is computed according:

Equation of Torsion Natural Frequency



The equation shows: The higher the spring constant, the higher becomes the natural frequency of the testing layout. Inversely: The smaller the moment of inertia , the higher becomes the natural frequency. A good torque sensor therefore has a preferably stiff mechanical construction and low masses. Since the torque sensor is installed into the shaft assembly via torsionally stiff couplings, it is suggestive to consider the applied couplings with regard to the natural frequency.


Torque Sensor DR-2554 with Integrated Double-Jointed Coupling


Natural Frequencies for Sensors with and without Coupling
Picture 3: Natural Frequencies for Sensors with and without Coupling
At first, the torque sensor newly designed by Lorenz Messtechnik GmbH was calculated without couplings and clamping-parts. Thereupon the recommended combination of clamping hub at the drive side and double-jointed coupling at the measuring side was calculated. On picture 3 the results are represented graphically.
For comparison, a calculation was carried out in combination of a conventional setup, consisting of a torque sensor between two single-jointed couplings. The most important data are summarized in following table 1.

Sensor Type Sensor without Coupling Sensor with Coupling
Conventional Sensor 1 kHz...6 kHz 300 Hz...1 kHz
DR-2554 6 kHz...20 kHz 2,3 kHz...3,3 kHz
Table 1: Comparison of the Natural Frequencies of a
Conventional Sensor and DR-2554


The comparison shows that the DR-2554 has a more than threefold higher natural frequency than a conventional torque sensor. This applies for the sensor as well as for the combination of torque sensor with couplings. Thus, for dynamic measurements the new sensor shifts the necessary high natural frequency to considerably higher values than conventionally constructed torque sensors.


Setup of the Torque Sensor

The sensor consists of a stationary part, the stator, and a rotating part, the rotor; see picture 1.

The stator electronics, the coils necessary for the signal transmission and the connector for the electrical connection of the sensor are located in the stator. Here, you can also find the optional speed sensor. The free-floating rotor without bearings consists of the measuring body on which the Wheatstone-bridge-circuit with strain gauges is applied. The rotating electronics and the rotating part of the rotating transformer is attached at the measuring body. A tension ring is attached on the left side of the measuring body and the double-jointed coupling is attached on the right side (see picture 1).
By the free-floating bearing-free design of the testing equipment layout no additional bearing friction of the torque sensor has to be expected. Every friction, as well as the friction of a ball bearing, means an adulteration of the torque and an additional warming of the sensor. Since the sensor does not contain any wearing parts, it is almost maintenance-free. By decreasing the number of part components, the sensor has very small mounting dimensions. The measuring element has a high measuring accuracy as well as a very large bore fit which can be used in special cases for the laying of cables and/or hoses.
The integrated electronics has a microprocessor with an appendant memory in the stator as well as in the rotor. The measured value acquisition occurs on the rotor by means of strain gauges, the measuring signal is immediately amplified and digitalized. This digital signal then attains to a processor which prepares it in form of a serial word with checksum for the transmission to the stator. In the stator, the data signal is converted to a DC voltage signal of 0 V ± 5 V or a serial RS 485 interface in a processor. The torque sensor is supplied by DC voltage of 12 V to 28 V. By the use of processors, serial numbers, calibration values, measuring ranges, calibration dates etc. can be stored on the shaft as well as in the stator. This data can be selected on request. The supply of the sensor occurs by means of a power supply unit controlled by the processor, which can switch on a calibration control for the function and calibration check of the sensor. A very high reliability in operation of the measuring equipment is achieved by digitalization of the measurement signal directly at the point of origin and the storing and readout of sensor data.


Combination Possibilities for Couplings


Kombinationsmöglichkeiten für Kupplung und Drehmomentsensor - - Click for Torque Sensors / Couplings Product Overview
Picture 4: Combination Possibilities for Couplings
By the combination of sensor DR-2554 and Roba-DS couplings, manufactured by Mayr Antriebstechnik, (picture 4) a module accrues, which corresponds to the current state of the art in regard to power density and market requirements. Next to various keyway hubs, tension rings and clamping hubs, a flange connection and 6 different shaft-hub-connections are available.
In order to balance axial and angle offsets as well as corresponding lateral offsets, the couplings in principle are offered in a twin-cardinal design.

Requirements such as:
  • Compact Design
  • Flexible Overall-Length
  • High Speed at Short and Long Design
  • Small Imbalances
  • Cree page Current Isolation
    can be implemented, depending on the design of the chosen connection components.

    This bearing-free torque sensor - coupling - combination succeeded in combining high dynamics with great flexibility. By this, in addition, a low-cost sensor with short design was originated.

    Literature

    Holzweißig, F.; Dresig, H.: Lehrbuch der Maschinendynamik
    (4. neubearbeitete
    Auflage). Fachbuchverlag Leipzig-Köln, 1994