[align=left] 1 Introduction Electromagnetic relays (referring to DC-excited electromagnetic relays in this article) exhibit a double-closing phenomenon during the closing process due to the characteristics of their electromagnetic mechanism and mechanical structure. When the relay first closes, although the moving and stationary contacts are closed, the closure is not tight, making them susceptible to external interference and prone to malfunction. Furthermore, the contact resistance of the relay contacts during the first closing is much greater than that during the second closing, significantly weakening the relay's load-carrying capacity. Therefore, the second closing voltage is a crucial electrical indicator in relay manufacturing and application. [b]2 Secondary Closing Phenomenon[/b] When the electromagnetic relay coil is energized, as the excitation coil current increases, the first incomplete closure of the moving and stationary contacts occurs (as shown in Figure 1a). At this point, the spring tension, electromagnetic attraction, and the spring force of the moving contact reach equilibrium. As the excitation current continues to increase to a certain value, the electromagnetic force causes the spring connecting the moving contact to elastically deform, and the armature continues to move towards the electromagnetic coil core, achieving maximum tight contact. This results in a more reliable contact between the moving and stationary contacts, known as the secondary closing. [img=431,191]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/51-3.jpg[/img] When the excitation voltage of a relay is changed, the voltage across the excitation coil when the relay contacts initially close (incomplete closure) is called the primary pull-in voltage, and the voltage when the relay secondly pulls in is called the secondary pull-in voltage. Previously, the measurement of the secondary pull-in voltage was limited to military products. However, with increasing requirements for relays, many civilian applications also require the calibration of the secondary pull-in voltage of electromagnetic relays. Therefore, some relay manufacturers use the secondary pull-in voltage as one of the performance indicators for factory inspection in their internal standards. The tenth station in the 115F type relay automatic testing line, jointly developed by the Electrical Research Institute of Hebei University of Technology and Xiamen Hongfa Electroacoustic Co., Ltd., is the secondary pull-in voltage testing station. (See Figure 2.) [img=363,188]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/51-1.jpg[/img] The PLC-controlled robotic arm places the relay to be tested onto the testing production line and controls the conveyor belt to run. Once in position, it stops and sends a testing signal to the computer. The computer controls the testing equipment to detect and record the data, determines whether it is qualified, and sends a qualified signal to the PLC. Finally, the PLC sorts the qualified and unqualified products. [b]3 Dynamic Process Analysis of Excitation Current When the Relay is Engaged[/b] The electromagnetic mechanism of a DC excitation relay is mainly a DC electromagnet. When an excitation current is passed through the coil, a dense magnetic flux is generated in the electromagnet circuit. This magnetic flux acts on the armature, causing the armature to be displaced by electromagnetic attraction. Figure 3 shows a simplified model of a DC electromagnet. According to Ohm's law for magnetic circuits: Φ—magnetic flux, Rm—magnetic reluctance, l—magnetic circuit length, μ—permeability, S—cross-sectional area of ​​the magnetic circuit, F=NI—ampere-turns. Figure 3 shows that the magnetic circuit of an electromagnet can be divided into three segments: the electromagnet core, the armature, and the air gap. [img=191,31]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/52-1.jpg[/img] During the relay closing process, only the length of the air gap magnetic circuit changes, i.e., Rmx and Rmt are constants, while Rmq changes, where μq is the air gap permeability coefficient. Therefore, as the excitation voltage increases, the dynamic changes of the relay excitation current are as follows: (1) When the electromagnetic force of the electromagnet increases to a level sufficient to overcome the tension of the release spring, the armature moves rapidly toward the electromagnet core. At this time, the air gap l decreases rapidly, causing Rm to decrease, thereby increasing the magnetic flux Φ, causing the excitation current i to drop suddenly. As shown in segment 4ab. (2) When the moving contact contacts the stationary contact, it hinders the movement of the armature, and the air gap cannot continue to decrease. [img=25,35]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/52-5.jpg[/img], the excitation current increases again. As shown in segment 4bc. (3) When the excitation current continues to increase, and the electromagnetic force is sufficient to overcome the sum of the release spring tension and the spring force of the spring plate connecting the moving contact, the spring plate bends, and the armature continues to move toward the electromagnet, causing the excitation current to drop suddenly again. As shown in segment 4cd. (4) When the armature and the core are in close contact, the air gap length reaches its minimum value, [img=25,35]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/52-5.jpg[/img] decreases, the excitation current increases, and finally the excitation circuit is balanced, and the equation becomes U=iR. See section de of Figure 4. According to the above analysis, the two activations of the relay correspond to two sudden drops in the excitation current. By detecting the two drops in the excitation current, the secondary activation voltage of the relay can be detected. [img=220,179]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/52-6.jpg[/img][img=258,179]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/52-7.jpg[/img] 4. Principle of the Relay Secondary Engagement Voltage Measurement System This system consists of three parts: a computer, an adjustable power supply, and a signal processing circuit. The voltage output of the adjustable power supply can be controlled by the computer. The adjustable power supply supplies power to the relay excitation coil, converting the current signal into a voltage signal through a current detection circuit. The voltage signal is processed by the signal processing circuit. Signal processing includes: signal amplification, followed by differentiation to produce two pulses at the point of current decrease. The voltage comparison circuit normalizes the pulses into digital pulses, which are then sent to the trigger-holding circuit to hold the pulse signal. The adjustable power supply is controlled by a computer to linearly apply voltage to the excitation coil at a certain rate. At the same time, the time when the pulse appears is monitored and the voltage when the second pulse appears is recorded. This voltage value is the secondary pull-in voltage. The system block diagram is shown in Figure 5. [img=366,141]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/52-8.jpg[/img]  [b]5 Signal Processing Hardware Circuit Design  [/b] The signal processing circuit in this system is the key part of the system. It consists of five parts: current detection circuit, inverting amplifier circuit, differentiating circuit, voltage comparison circuit, and trigger holding circuit.  (1) The current detection circuit is mainly used to detect the current in the relay excitation circuit. It can be implemented by current detection resistor or current transformer. If a current detection resistor is used, the current detection resistor in series should not have too much influence on the circuit current. Therefore, the current detection resistor should be as small as possible. (2) Amplification Circuit: The excitation current of a relay is generally not large (maximum tens of mA), so the voltage signal output by the current sensing circuit is very small and needs to be amplified by one stage. This system uses the ICL7650 chip to form an amplifier circuit with an amplification factor of 14. (3) Differentiating Circuit: The differentiating circuit is composed of the OP37 chip, which differentiates the output signal of the amplifier circuit and produces a pulse when the excitation current decreases. (4) Voltage Comparison Circuit: The voltage comparison circuit is composed of the LM339 chip, which normalizes the pulse generated by the differentiating circuit into a pulse with an amplitude of 5V. (5) Trigger-Holding Circuit: This part is composed of an inverter 7404, a JK flip-flop 74LS112, and a dual D flip-flop 7474, where the D flip-flop is connected in the form of a shift register. The function of this circuit is to provide the computer with the rising edge level change when the excitation current decreases. The computer judges the relay's activation action based on the level change signal. The signal processing process is shown in Figure 6. The work needs to be processed, so the hardware timer interrupt processing method is adopted. The software processing process is shown in Figure 7. [img=264,419]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/53-1.jpg[/img][img=212,434]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/53-2.jpg[/img] 6. Software Design The computer is programmed in C language. For the secondary pull-in voltage detection station, the main task is to control the adjustable power supply to apply voltage to the test coil at a certain speed and repeatedly read the level (UD1, UD2) changes of the trigger and holding circuit. Since the computer has other station work to process, a hardware timer interrupt method is adopted. The software processing process is shown in Figure 7. 7. Conclusion This system can accurately and easily measure the secondary pull-in voltage of a relay (DC excitation). Because it is computer-controlled, it can be easily interfaced with other automated equipment and is suitable for industrial online measurement. This system has been successfully applied in Xiamen Hongfa Company. [b]References[/b] ...