Technical performance and application of digital torque sensor

1. Introduction

The most frequently involved parameter in the rotary power system: rotary torque. In order to detect the rotary torque, the traditionally used torsional angle phase difference sensor is to install two sets of gears with exactly the same number of teeth, shape and installation angle at both ends of the elastic shaft, and install a proximity (magnetic or optical) sensor on the outside of each gear. When the elastic shaft rotates, the two sets of sensors can measure two sets of pulse waves. By comparing the phase difference between the front and rear edges of the two sets of pulse waves, the torque borne by the elastic shaft can be calculated. Advantages of this method: It realizes non-contact transmission of torque signals, and the detection signal is a digital signal; Disadvantages: It is large in size and difficult to install. At low speeds, the front and rear edges of the pulse wave are slow and difficult to compare, so the low-speed performance is not ideal. (See Figure 1)

Figure 1 Schematic diagram of torsion angle phase difference torque sensor

The more mature detection method for torque testing is strain electrical measurement technology. It has the advantages of high precision, fast frequency response, good reliability and long life.

The special torque measuring strain gauge is pasted on the elastic shaft to be tested with strain glue to form a strain bridge. If the working power is provided to the strain bridge , the electrical signal of the elastic shaft being torsion can be tested. This is the basic torque sensor mode. (See Figure 2) However, in the rotary power transmission system, the most difficult problem is how to reliably transmit the bridge pressure input of the strain bridge on the rotating body and the detected strain signal output between the rotating part and the stationary part. The usual practice is to use a conductive slip ring to complete it.

Figure 2 Basic torque sensor

Since the conductive slip ring is in friction contact, it is inevitable that there will be wear and heat, which limits the speed of the rotating shaft and the service life of the conductive slip ring. And because of the unreliable contact, the signal fluctuates, resulting in large measurement errors or even unsuccessful measurements. In order to overcome the defects of the conductive slip ring, another way is to use radio telemetry: amplify the torque strain signal on the rotating shaft and convert it into a frequency signal by V/F, transmit it from the rotating shaft to the outside of the shaft by radio transmission through carrier modulation, and then use radio reception to obtain the signal of the rotating shaft being torsion (see Figure 3).

Figure 3

The energy supply on the rotating shaft is a battery fixed on the rotating shaft. This method is a telemetry torque meter. (See Figure 4) The success of the telemetry torque meter lies in overcoming the two defects of the electric slip ring, but there are also three shortcomings. First, it is easily interfered by electromagnetic waves at the site of use; second, because it is battery-powered, it can only be used for a short period of time. Third, because the structure is attached to the rotating shaft, it is easy to cause dynamic balance problems at high speeds. It is more prominent when the range and diameter of the shaft are small. The digital torque sensor absorbs the advantages of the above methods and overcomes their defects. Two sets of rotary transformers are designed on the basis of the strain sensor to achieve non-contact transmission of energy and signals. And the transmission of the torque signal has nothing to do with whether it rotates, the speed, or the direction of rotation.

Figure 4

II. Features

1. It can measure both static torque and rotational torque;
2. It can measure both static torque and dynamic torque;
3. High detection accuracy, good stability and strong anti-interference;
4. Small size, light weight, multiple installation structures, easy to install and use;
5. It can continuously measure forward and reverse torque without repeated zeroing;
6. It has no wear parts such as conductive rings and can run at high speed for a long time;
7. The sensor outputs high-level frequency signals that can be directly sent to the computer for processing;
8. The strength of the measured elastic body is large and can withstand 100% overload.

III. Measurement principle

The special torsion strain gauge is pasted on the elastic shaft to be measured with strain glue to form a strain bridge. The electrical signal of the torsion of the elastic shaft can be measured by providing power to the strain bridge. After the strain signal is amplified, it is converted into a frequency signal proportional to the torsion strain through voltage/frequency conversion. The energy input and signal output of this system are undertaken by two sets of special toroidal transformers with gaps, thus realizing the contactless energy and signal transmission function. (The dotted line is the rotating part) (see Figure 5).

Figure 5

4. Sensor principle and structure

A special torque measuring plate is pasted on a special elastic shaft to form a variable bridge, which is the basic torque sensor; fixed on the shaft are: (1) the secondary coil of the energy toroidal transformer, (2) the primary coil of the signal toroidal transformer, (3) the printed circuit board on the shaft, which contains the rectifier stable power supply, instrument amplifier circuit, V/F conversion circuit and signal output circuit. Fixed on the outer shell of the sensor are (1) the excitation circuit, (2) the primary coil (input) of the energy toroidal transformer, (3) the secondary coil (output) of the signal toroidal transformer, and (4) the signal processing circuit.

5. Working process

Provide ± 15V power to the sensor, the crystal oscillator in the excitation circuit generates a 400Hz square wave, which is then passed through the TDA2030 power amplifier to generate an AC excitation power supply, which is then transmitted from the stationary primary coil to the rotating secondary coil through the energy toroidal transformer T1. The obtained AC power is passed through the rectifier filter circuit on the shaft to obtain a ±5V DC power supply, which is used as the working power supply for the operational amplifier AD822; the high-precision voltage-regulated power supply composed of the reference power supply AD589 and the dual operational amplifier AD822 generates a ± 4.5V precision DC power supply, which is used as both the bridge power supply and the working power supply for the amplifier and V/F converter. When the elastic shaft is twisted, the strain signal of mV level detected by the strain bridge is amplified into a strong signal of 1.5v ± 1v by the instrument amplifier AD620, and then converted into a frequency signal by the V/F converter LM131. It is transmitted from the rotating primary coil to the static secondary coil through the signal toroidal transformer T2, and then filtered and shaped by the signal processing circuit on the sensor housing to obtain a frequency signal proportional to the torque borne by the elastic shaft. The signal is TTL level and can be provided to a dedicated secondary instrument or frequency meter for display or directly sent to a computer for processing. Since there is only a gap of a few tenths of a millimeter between the dynamic and static rings of the rotary transformer, and the upper part of the sensor shaft is sealed in a metal housing to form an effective shield, it has a strong anti-interference ability.

The frequency signal output by this sensor is 10kHz at zero point, 15KHz at full scale for forward rotation, and 5KHz at full scale for reverse rotation. That is, the full-scale variable is 5000 numbers/second. The speed measurement adopts the measurement method of photoelectric gear or magnetoelectric gear. 60 pulses can be generated for each rotation of the shaft. The frequency measurement method can be used for high-speed or medium-speed sampling, and the period measurement method can be used for low-speed sampling. The accuracy of this sensor can reach ± ​​0.2% to ± 0.5% (F·S). Since the sensor output is a frequency signal, it can be directly sent to the computer for data processing without AD conversion.

VI. Scope of application

1. Detect the output torque and power of rotating power equipment such as generators, motors, and internal combustion engines.
2. Detect the load torque and input power of equipment such as reducers, fans, pumps, mixers, winches, propellers, and drilling machinery. 3. Detect the torque during the working process of
various machining centers and automatic machine tools 4. The torque and efficiency transmitted by various rotating power equipment systems; 5. The speed and axial force can be detected while detecting the torque. 6. It can be used to manufacture viscometers and electric (pneumatic, hydraulic) torque wrenches. VII. Installation and use 1. Use two sets of couplings to install the sensor between the power source and the load.

Figure 6

2. It is recommended to use flexible, elastic or universal joint couplings to ensure that the concentricity is less than ∠ 0.1mm.
3. The power and load equipment must be fixed reliably to avoid vibration.
4. The base of this sensor must be fixed reliably to the base of the equipment, and the center height must be properly padded to avoid bending moment.

VIII. Multiple installation methods

After five years of mass trial production, this sensor has been widely praised by users. This sensor belongs to the through-type installation method, which generally needs to be installed through a coupling. In order to facilitate use under special requirements, this sensor has seven deformation structures:

1. If the axial direction of the transmission system is not required to be lengthened due to the installation of the sensor, an intelligent coupling can be used-it can detect torque and assume the function of a coupling. That is, a coupling with detection function.
2. When the transmission system to be tested is not allowed to be lengthened but can be disassembled, a set-type torque sensor can be used-it can be put on the shaft to be tested.
3. When the transmission system to be tested is neither allowed to be lengthened nor disassembled, a card-mounted torque sensor assembled on site can be used. The test can be carried out by clamping it on the shaft from both sides. It can be used for long-term monitoring or short-term detection. It is similar to the current clamp meter.
4. When some test systems have both torque and axial force, a torque, speed and axial force three-parameter sensor can be used; it can avoid the interference of axial force on torque test and measure the signal of axial force;
5. When testing at high speed or ultra-high speed, a bearingless torque and speed sensor can be used;
6. If the system under test is in a non-rotating state, a torque sensor transmitter can be used;
7. If the maximum value of the system under test is between 5mN·m and 1N·m, a small-range torque and speed sensor can be used.

IX. Intelligent torque and speed measuring instrument

In order to use this series of torque sensors more conveniently, an intelligent torque and speed measuring instrument and a computer virtual instrument are specially designed. The two instruments can directly display the values ​​and curves of the torque, speed and power values ​​being measured, and can also set the torque limit value, over-value alarm, and save the torque peak value and curve of this measurement, and can print out the above parameters and curves.