Torque measurement

Torque measurement

Torque is defined as the product of force and moment arm and is expressed in Nm. Torque is measured by measuring the strain on the shaft and

The most commonly used method is to measure the relative torsion angle between two cross sections of the rotating shaft.

10.2.1 Strain gauge torque measurement

From the material mechanics, we know that when the shaft is subjected to torque, the shaft surface has the maximum shear stress τ max.

The element is in a pure shear stress state, with maximum normal stresses σ 1 and σ 2 in the direction 45 degrees to the axis .

Its value is | σ 1 |= | σ 2 |= τ max. The corresponding deformations are ε 1 and ε 2. When the strain is measured,

During measurement , the strain gauge is pasted at a 45° angle to the axis.

If the strain ε 1 along the 45° direction is measured , the corresponding shear strain is

Where:

E — elastic modulus of the material;

μ — Poisson’s ratio of the material;

Therefore, the torque on the shaft is

Where:

Wn — Torsional modulus of the material.

For solid circular shaft

When measuring torsion, the resistance strain gauge must be in the direction of the principal strain ε ₁ and ε ₂ (at 45° and 135° angles with the axis). The arrangement and bridge assembly of the strain gauge should take into account the requirements for sensitivity, temperature compensation and offsetting interference from non-measurement factors such as tension, compression and bending.

Transmission of torque measurement signals

1. Current collector for torque measurement

Strain measurement of rotating parts such as shafts requires solving the problem of signal transmission. The strain gauges and bridge wires attached to the rotating parts rotate with the rotating parts, while the measuring and recording instruments such as strain gauges are fixed. In addition to using telemetry, a current collector is required.

The current collector consists of two parts: a collector ring (slip ring) connected to the strain gauge and rotating with the rotating part, and a brush (pull wire) connected to the external measuring instrument and pressed against the slip ring. The current collector should accurately and reliably transmit the strain signal to prevent interference and reduce measurement errors. The change in contact resistance between the collector ring and the brush is the main factor that generates interference and affects normal measurement ( general requirements for the current collector ).

There are many types and forms of current collectors, and their principles and structures are the same as those of motor current collectors. Commonly used current collectors are:

There are two types: wire pull type and brush type

2. Wireless transmission method

Wireless transmission can overcome the shortcomings of wired transmission and is increasingly used. It is divided into radio wave transceiver and photoelectric pulse transmission. From the perspective of use, both methods eliminate the intermediate contact link, wires and special current collecting devices. The radio wave transceiver measurement system requires reliable transmitting, receiving and telemetry devices, and its signals are easily interfered with; while photoelectric pulse measurement has strong anti-interference ability. It digitizes the test data and transmits it in the form of optical signals from a rotating measuring disk to a fixed receiver, and then restores it to the required signal after the decoder.

Torque Measurement (II)

Other torque measurement methods

1. Piezomagnetic torque sensor ( principle of piezomagnetic torque measurement )

The structure of the piezomagnetic torque sensor is shown in the figure below. It measures torque by using the change in the material's magnetic permeability when the shaft is twisted. It is characterized by non-contact measurement and easy use, but requires no radial runout during the rotation process, otherwise the gap between the core and the shaft will change, causing measurement errors and even damaging the measuring equipment.

2. Magnetoelectric inductive torque sensor ( Principle of magnetoelectric inductive torque sensor )

The structure of the magnetoelectric induction torque sensor is shown in the figure below. It fixes two gears on the rotating shaft and measures the torque through the relative torsion angle between their cross sections.

3. Photoelectric torque sensor ( the principle of photoelectric torque measurement )

The structure of the photoelectric torque sensor is shown in the figure below. It fixes two disc gratings on the rotating shaft and measures the torque through the relative torsion angle between the two gratings.

Photoelectric Torque Sensor

As shown in the left figure (photoelectric torque sensor), two disc gratings 3 are fixed on the rotating shaft 4. When no torque is applied, the light and dark areas of the two gratings just block each other, and the light from the light source 1 does not pass through the gratings to illuminate the photosensitive element 2, and there is no output signal. When the rotating shaft is subjected to torque, the deformation of the rotating shaft will cause the two gratings to rotate relative to each other, and part of the light passes through the gratings to illuminate the photosensitive element to generate an output signal. The greater the torque, the greater the torsion angle, the greater the light flux passing through the grating, and the greater the output signal, so that torque measurement can be achieved.

Magnetic Inductive Torque Sensor

As shown in the left figure (magnetoelectric induction torque sensor), two gears 1 and 2 are fixed on the shaft, and their materials, sizes, tooth shapes and numbers of teeth are the same. The magnetoelectric detection heads 3 and 4 composed of permanent magnets and coils are installed facing the top of the teeth. When the shaft is not subjected to torque, the output signals of the two coils are the same and the phase difference is zero. When the shaft is subjected to torque, the phase difference is not zero, and increases with the increase of the relative torsion angle between the cross sections of the two gears. Its size is proportional to the relative torsion angle and torque.

When the rotating shaft of ferromagnetic material is subjected to torque, the magnetic permeability changes. In the above figure (piezomagnetic torque sensor), coils A and B are wound respectively , where AA is placed along the axis and BB is placed perpendicular to the axis, and they are perpendicular to each other. The open ends of the two iron cores maintain a gap of 1-2mm with the surface of the rotating shaft. When alternating current is passed through the AA coil, an alternating magnetic field is formed through the rotating shaft.

When the shaft is not subjected to torque, the magnetic lines of force do not interlink with the BB coil; when the shaft is subjected to torque, the magnetic permeability of the shaft material changes, the magnetic resistance decreases along the positive stress direction, and the magnetic resistance increases along the negative stress direction, thereby changing the distribution of the magnetic lines of force, causing some of the magnetic lines of force to interlink with the BB coil and generate an induced potential in the BB coil. The induced potential increases with the increase of torque and is linear within a certain range.