The structure and function of the independent servo controller are introduced, and a new method of the application of the independent servo controller in fatigue test is studied. Independent servo controller is a universal independent digital servo controller, which can operate independently without PC.
Each rack can control up to four control channels. Most of the operation functions of the controller can be realized by a rotary/press button on the front panel and a 640*480 resolution color LCD screen. The controller is a digital control unit. This unit realizes a high fidelity closed-loop control system. The combination of controller actuator can be considered as a “smart actuator”. When using this system, it is no longer necessary to amplify the command signal to ensure the response characteristics of command output.
The line between the controller and the real-time front end is connected by real-time optical fiber communication, which enables the controller to be placed close to the actuator. Analog cables used in the system are usually expensive and susceptible to noise interference. Now they can be shortened as much as possible, which makes their signal-to-noise ratio higher. As a local controller for controlling hydraulic or electric cylinder, the controller has control circuit, safety system, test circuit, self-test function and DC-DC conversion. In normal operation, the force sensor installed on the actuator cylinder measures the actual force acting on the test piece. The signal is fed back to the controller loop, in which the measured signal is compared with the command force signal from the computer. The control loop controls the actuator so that the actual measuring force is equal to the commanding force with high precision under the condition of a command with large frequency and bandwidth. The security system uses both hardware and software, which are embedded in the controller, and carries out uninterrupted security checks in all the optional circuits. The security system checks whether the difference between the command signal and the measuring force is smaller than the user’s preset limit; whether the measuring force exceeds the user’s preset limit; whether the difference between force A and force B exceeds the user’s preset limit; whether it exceeds other safety limits; whether the internal power supply is within tolerance; whether the communication between the controller and the real-time front end is interrupted; and whether the software iteration of the controller is No normal. Depending on the safety system exceeding the error limit, the hydraulic or electric system will shut down and the controller will start the self-test procedure. After self-checking, the state returns to the real-time front end, and the system enters the waiting mode. In the force and displacement control mode, the controller will switch to the waiting state according to the command or security reasons.
With this action, the real-time command will be removed immediately in order to control the hydraulic system to unload the oil pressure. Subsequently, the controller will continuously control the current of the servo valve for 1.2 seconds in order to mitigate any instantaneous impact caused by hydraulic switching. After 1.2 seconds, the controller will send a jitter signal lasting 30 seconds to the servo valve. This will help remove all retained energy from the actuator and apply to hydraulic systems that lack pressure relief equipment. It also prevents the actuator from being locked in 30 seconds. The magnitude of the jitter signal is equal to the current limit of the servo valve set at 20% and the frequency is 40 Hz. Firstly, the channel range, working range, current value of servo valve and safety limit are set. The absolute safety limit is defined as the percentage of full range. The maximum of force A safety protection of 8% and the minimum of force A safety protection of -8% are usually appropriate and a safe starting value. When pressurized, the system can start to adjust: the correct symbol for validation and location (usually completed in the calibration process); a small value for proportional gain (e.g. 0.2); a small value for integral gain (e.g. 0.2); the system pressurizes and monitors its response (emergency stop should be within reach); if the system pressurizes, it does not exceed safety due to excessive pressure. If all restrictions are turned off, then this gain can be considered the best. If the system shuts down due to the failure protection limit of initiating force A, verify that the polarity of the servo valve is correct. Servo valve polarity can be reversed by changing the gain of the servo valve loop to – 1. After changing the polarity of the servo valve, the system should be in control. If the system is still shut down due to excessive force, the gain is too low. Gain optimization. Adjust the command force to the average value (e.g. 0); generate a small amplitude of 5% of the whole range, low frequency (e.g. 0.3Hz) square wave command; start manual adjustment; adjust P, thermostatic element when there is excitation, change P to the original 1/4 or 1/2; adjust I, when there is excitation, change P to the original 1/4 or 1/2; minimize the damping gain, if there is low frequency instability or overshoot, slowly increase the damping gain; If force bias occurs in constant force control, servo valve bias parameters must be fine tuned. The waveform is changed to sine wave, the frequency is increased, and the PID is fine-tuned. Increase the amplitude and fine-tune the PID to reach the ideal state. Open amplitude matching and phase matching. Sensor automatic identification function, which allows the controller to automatically identify the sensors connected to it, and automatically copy important information such as serial number and calibration data to the test file. The controller is scalable and can be extended to four test channels with one rack. If you want to use more channels, you can also connect multiple racks together.
The controller has high reliability. The hardware of the controller uses mature products produced by the product line of the controller. In fact, the number of controller units used in the controller has exceeded 1000 in the world.
The controller also provides more safety performance to protect the test piece. Select the force control mode and enter the labeling guidance of the control channel set below. Parameter setting, such as sensor range, unit of force, sensor sensitivity coefficient, etc. Give the sensor a pressure to see if the polarity of the sensor is correct. If not, change the polarity to negative and calibrate again. The zero offset is measured, corrected and calibrated.
In the fatigue test under force-controlled mode, the displacement feedback signal has random value in the information bar, and then it is considered that although the displacement sensor is not connected, there is displacement amplification in the independent controller, so the displacement feedback signal is zero after the gain value of the displacement sensor is changed to zero in manual calibration, which solves the problem of interference of the displacement feedback signal. There are two ways to set the test frequency. At first, in order to increase the test frequency quickly, the frequency value is input directly in the setting bar of the test frequency. But it is found that the whole test system does not adapt to the value of PID quickly. After that, the manual knob is used to adjust slowly, and the effect is remarkable. The application of the independent servo controller provides a reliable basis for fatigue test.