What is the role of the conductive slip ring on the torque sensor?

- Oct 10, 2019-

What is the role of the conductive slip ring on the torque sensor?

Torque sensor:

The torque sensor is an instrument that precisely measures various torques, speeds and mechanical powers. Involved in water conservancy and hydropower, railway transportation, intelligent construction, production automation, aerospace, military, petrochemical, oil wells, electric power, ships, machine tools, pipelines and many other industries. The torque sensor uses a strain bridge to measure the force of the elastic body, and the signal transmission is transmitted by a conductive slip ring.


A more mature torque test method is strain electrical measurement technology. It has the advantages of high precision, fast frequency response, good reliability and long service life. A strain gauge of a special torque gauge is attached to form a strain bridge on the elastic shaft to be tested. If the strain bridge provides operating power, the electrical signal of the elastic shaft can be tested. This is the basic torque sensor mode. However, the most difficult problem in a rotary power transmission system is how the bridge pressure input of the strain bridge on the rotating body and the detected strain signal output are reliably transmitted between the rotating portion and the fixed portion, usually using a conductive slip ring. Since the conductive slip ring is a frictional contact, the basic torque sensor will have some wear and heat, which limits the rotational speed of the rotating shaft and the service life of the conductive slip ring. And the signal fluctuation caused by unreliable contact, resulting in large measurement error, and even the measurement is unsuccessful.


Selection of slip ring torque sensor and non-contact miniature torque sensor

The first rotary strain gauge torque sensor employs a slip ring to effect electrical connection from the housing to the rotating shaft. Since the slip ring only carries the millivolt signal from the strain gauge, the material of the slip ring and brush must be carefully selected. The usual procedure is to use coin silver as the slip ring and silver graphite as the brush gear.


The conductive ring rotates with the sensor and uses a series of spring brushes to contact the ring and transmit electrical signals. The slip ring is relatively straight and has only minor drawbacks because the brush and the lesser extent of the ring do wear and therefore have a limited life. Therefore, they are not suitable for long-term testing, very high speeds, and are not suitable for applications that access sensors for maintenance.


At low speeds, the electrical connection between the ring and the brush is relatively noise-free, but at higher speeds, electrical noise eventually degrades its performance. The maximum speed (rpm) of the slip ring is determined by the surface speed at the interface between the brush and the ring. Therefore, for larger or higher torque capacity sensors, the maximum operating speed will be lower because the larger slip ring therefore has a higher surface speed at a given rpm. For medium capacity torque sensors, the typical maximum speed will be in the range of 2000 rpm. For very low volume measurements, the brush ring interface may be the source of "drag torque", which may be a problem with the drive torque to overcome.


Non-contact miniature torque sensor (inductive loop)

For higher speed (rpm) torque measurement applications, a resolver system can be used. The resolver system includes two coils, one static coil connected to the transducer housing and one rotary coil connected to the transducer shaft. This provides the distinct advantage of no contact between the rotor and the stator and includes power transfer and signal transmission from the rotary strain gage bridge circuit. The resolver type torque sensor provides a typical error of ±0.2% at speeds up to 18,000 rpm.


Non-contact miniature torque sensor

The versatility of the resolver system is sufficient for special torque sensors and sensors with limited space. However, since this design uses bearings, the maximum speed is greater than the slip ring design, but is limited by the maximum specified performance of the bearing. The system is also susceptible to noise and errors caused by the alignment of the primary to secondary windings of the transformer. Due to the special requirements of the resolver, special signal conditioning is required to generate signals acceptable to most data acquisition systems.