An incremental cascade PID closed-loop control method and a hardness tester test force precise control method

Abstract

An incremental cascade PID closed-loop control method, the outer loop is the pressure loop, the inner loop is the pressure change rate loop, according to the formula Δu k =Ae k ‑Be k‑1 +Ce k‑2 Obtain the control increment Δu between the outer loop and the inner loop k ; In the formula, A, B, C are coefficients, e k 、e k‑1 、e k‑2 The control deviations are respectively the expected pressure values ​​and actual pressure values ​​at the kth, k-1st, and k-2nd times of the constant sampling period T. Compared with the existing hardness measurement control technology, the hardness tester detection method based on the incremental cascade PID closed-loop control method can control the change of parameters and the change rate of the feedback physical quantity, which can not only improve the accuracy of hardness measurement, but also prevent overshoot from damaging the workpiece being measured, and at the same time reduce the measurement error caused by indentation rebound.

Description

Technical Field

[0001] The invention belongs to a hardness tester control technology, and in particular relates to a hardness tester test force precise control method. Background Art

[0002] In the process of daily product quality inspection, the inspection of material hardness is the most important link to ensure the quality of finished products. At present, the metal materials used in industry range from very soft materials to very hard materials. Vickers hardness tester is widely used in hardness inspection due to its wide measurement range. The hardness inspection method mentioned in GB / T 4340.1-2009 "Metallic Material Vickers Hardness Test Part 1: Test Method" is to press a regular tetrahedral diamond indenter with a specified angle on the top two opposite faces into the sample surface with a certain test force, keep it for a specified time, remove the test force, and measure the diagonal length of the indentation on the sample surface.

[0003] Through the investigation of Vickers hardness testers on the market, we found that most of them are manually pressed and unloaded, which is very cumbersome to operate and cannot control the time and speed of applying the test force. At the same time, we found through experiments that during the test force maintenance stage, traditional operations cannot maintain the test pressure at the bottom of the indentation, causing the indentation to rebound to a certain extent. What may happen is that the rebound amount of the indentation exceeds the normal rebound amount, resulting in the residual depth of the indentation being less than its actual value, affecting the measurement accuracy of the Vickers hardness. Summary of the invention

[0004] In order to make up for the above-mentioned deficiencies in the prior art, the present invention proposes a method for accurately controlling the test force of a hardness tester, which can control the changes in parameters and feedback physical quantities, thereby not only improving the accuracy of hardness measurement, but also preventing overshoot from damaging the workpiece being measured and reducing the measurement error caused by indentation rebound.

[0005] An incremental cascade PID closed-loop control method, the outer loop is the pressure loop, the inner loop is the pressure change rate loop, when the expected force value is F, the control deviation formed by the expected pressure value and the actual pressure value at time t is shown in formula (1):

[0006] e(t)=r(t)-y(t) (1)

[0007] Where r(t) is the expected pressure value, g; y(t) is the actual pressure value, g; e(t) is the control deviation input to the PID controller, g;

[0008] Assume that the control law of the simulated PID controller is the output of the PID controller and the input signal u(t) of the controlled object:

[0009]

[0010] In the formula, K pis the proportionality coefficient; T I is the integration time constant; T D is the differential time constant; u 0 To control the constant;

[0011] Then, the output values ​​of the PID controller at the kth and k-1th sampling moments are:

[0012]

[0013] Substitute formula 3 to 4 into formula 5 to calculate the increment Δu of the control amount. (k) :

[0014]

[0015] It can be seen from formula (5) that if the control system adopts a constant sampling period T, once the coefficients A, B, and C are determined, the control increment can be calculated by simply detecting the deviation of the three measured values ​​before and after.

[0016] when:

[0017] a) When the pressure value of the pressure head on the workpiece reaches 20%F, the system enters the cascade loading PID mode;

[0018] b) When the pressure value of the pressure head on the workpiece reaches 90%F, it enters the cascade buffer PID mode;

[0019] c) When the pressure value of the pressure head on the workpiece reaches 98%F, it enters the cascade anti-overshoot PID mode:

[0020] d) When the pressure value of the pressure head on the workpiece reaches F, the cascade full-load PID mode is entered.

[0021] The present invention also provides a method for accurately controlling the test force of a hardness tester, comprising the following steps:

[0022] Step S1) the system is started, a desired test force F is given, and the motor drives the pressure head to press down the workpiece to be tested;

[0023] Step S2) when the pressure head contacts the workpiece to be tested, the test force begins to increase, and the cascade loading PID mode, cascade buffering PID mode, cascade overshoot prevention PID mode, and cascade full load PID mode are entered respectively;

[0024] Step S3) when the system is in the cascade full-load PID mode, the desired test force F is maintained for a period of time until the workpiece no longer undergoes plastic deformation;

[0025] Step S4) unloads, the motor rotates upward, and the pressure head returns to the initial position.

[0026] The hardness tester test force precision control method provided by the present invention adopts incremental cascade PID closed-loop control and anti-integral windup algorithm, sets the outer loop as the pressure loop, and the inner loop as the pressure change rate loop, that is, the outer loop controls the increase of the test force, and the inner loop controls the motor speed at the same time. The final actuator is the motor, and the pressure head descending speed is controlled within the required range. The motor output generates speed and pressure. We give the expected test force in the outer loop, and the output of the outer loop PID is the expected value of the inner loop PID. The output of the inner loop PID will generate the corresponding speed size, and finally the motor generates the corresponding loading test force. During the process of pressing the indenter, the several states to which the workpiece to be tested is subjected are artificially divided into four PID modes: cascade loading, cascade buffering, cascade overshoot prevention, and cascade full load. Compared with the currently known hardness measurement control technology, not only the change of control parameters but also the change rate of the control feedback physical quantity can be improved. This can not only improve the accuracy of hardness measurement, but also prevent overshoot from damaging the workpiece to be tested, and at the same time reduce the measurement error caused by indentation rebound (elastic recovery is a property of metal, which is related to the type of metal, but not to the load that produces the indentation. That is to say, no matter what the load is and how big the indentation is, elastic recovery must exist. This is also one of the shortcomings of Vickers hardness measurement, and the present invention makes up for this.). The problem that the force cannot be maintained in manual measurement, which easily causes the vibration of the indenter, is solved. At the same time, a precise timer is used to control the action and holding time of the test force, which meets the national standard test. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is the flow chart of the motor control test force;

[0028] Figure 2 It is a principle block diagram of the test force closed-loop control system of the present invention;

[0029] Figure 3 The topological diagram of the incremental cascade PID control system provided by the present invention;

[0030] Figure 4 The test force curves of the method provided by the present invention when the test forces are 1KG, 2KG and 3KG;

[0031] Figure 5 The test force curves of the method provided by the present invention when the test forces are 50KG, 55KG, 65KG and 70KG;

[0032] Figure 6 The present invention provides a test force curve when the test force is 80KG, 100KG, 120KG and 150KG. DETAILED DESCRIPTION

[0033] The present invention discloses a method for accurately controlling the test force of a hardness tester, and the flow chart of controlling the test force is as follows: Figure 1 As shown, the following steps are included:

[0034] (1) The motor, motor pulse controller and serial port in the Vickers hardness tester control system are reset, and the displacement sensor detects whether the pressure head returns to the initial position, completing the initialization of the control system.

[0035] (2) Start the system, input the expected test force F (different expected test forces can be set according to the thickness of the workpiece to be measured), and the motor drives the pressure head to press down on the workpiece to be measured.

[0036] In this step, the pressure sensor installed on the pressure head reads the pressure of the workpiece under test, and the upper and lower limits of the motor are controlled by the optical coupling switch (to protect the safe operation of the motor); in addition, the motor and the pressure head are connected by a screw rod, and the rotational motion of the motor is converted into the linear motion of the pressure head through the screw rod. In this embodiment, during the press-fitting process, as the degree of pressure changes, the pressure also changes accordingly. The pressure and the degree of pressure are in a one-to-one correspondence. The larger the pressure value, the greater the corresponding degree of pressure.

[0037] (3) When the pressure head contacts the workpiece to be tested for the first time in step (2), the test force applied by the motor to the workpiece to be tested by the pressure head gradually increases from 0 to the expected test force F; in the present invention, the inventors have innovatively proposed an incremental cascade PID closed-loop control mode, in which the outer loop is a pressure loop, the inner loop is a pressure change rate loop, and the final actuator is a motor, thereby driving the pressure head to press down to generate pressure. That is, when the system is working, the outer loop measures the current load test force value of the workpiece to be tested through the pressure sensor as feedback of the outer loop PID control system, the expected test force F is given in the outer loop, the test force of the pressure sensor is compared with the expected value, and the result is sent to the PID module for calculation, and the test force is quickly increased through the pressure PID controller. The output of the outer loop PID is the expected value of the inner loop PID. At the same time, the inner loop obtains the load test force change rate through the pressure change rate controller, and sends it to the inner loop pressure change rate PID controller, and compares it with the mapping table of the motor speed direction. The inner loop PID output will generate a corresponding speed, thereby controlling the motor speed, and finally the pressure head presses down to generate a corresponding load test force. The pressure change rate can reflect the trend of pressure change. If the pressure change is too large, it is easy to cause overshoot and damage the device. The present invention monitors the pressure change rate and intervenes in the pressure change in advance, making the measurement safer and more efficient.

[0038] The incremental cascade PID control algorithm flow chart provided by the present invention is as follows: Figure 2 As shown in the figure (ε is the error threshold), the pressure PD controller is the outer loop, and the pressure change rate PID controller is the inner loop (such as Figure 3As shown). The pressure PD controller set in the outer loop has its output as the input of the inner loop. The load test force detected by the pressure sensor is differentiated as the feedback of the inner loop. The inner loop sets the pressure change rate PID controller, and its output is used to control the pressure head pressing speed, thereby controlling the test force. Then,

[0039] 1) The deviation of the outer ring pressure value is:

[0040] e 1 (k) = R 1 (k)-Y 1 (k) (6)

[0041] In the formula, e 1 (k) is the pressure deviation of the outer ring at the sampling time; R 1 (k) is the expected pressure value of the outer ring at the sampling time; Y 1 (k) is the actual pressure value of the outer ring at the sampling time.

[0042] 2) The output control quantity of the outer loop pressure PD controller (inner loop input value) is:

[0043] U 1 (k) = U 1 (k-1)+ΔU 1 (k) (7)

[0044] In the formula, ΔU 1 (k) is the control increment acting on the pressure PD controller.

[0045] 3) The inner ring pressure change rate deviation is:

[0046] e 2 (k) = U 1 (k)-Y 2 (k) (8)

[0047] In the formula, e 2 (k) is the pressure change rate deviation of the inner ring at the sampling time; Y 2 (k) is the rate of change of inner ring pressure at the sampling moment.

[0048] 4) The output control quantity of the inner loop pressure change rate PID controller is:

[0049] U 2 (k) = U 2 (k-1)+ΔU 2 (k) (9)

[0051] In the formula, ΔU 2 (k) is the control increment acting on the pressure change rate PID controller.

[0052] During the downward pressure process, the system gradually enters the following different stages of PID mode:

[0053] a) When the pressure value of the workpiece under test reaches 20%F, the system enters the cascade loading PID mode:

[0054] In the cascade loading PID mode, the outer loop obtains the current loading test force through the pressure sensor, and quickly increases the test force through the pressure PID controller. According to national standards, the loading operation is usually completed between 2s and 8s, and in this embodiment, it is completed within 2s to 4s. The inner loop obtains the pressure change rate through the pressure change rate controller, and compares it with the mapping table of the motor speed direction, thereby controlling the motor speed and the pressure head continues to press down.

[0055] The mapping relationship between the pressure change rate and the motor speed direction is that a pressure value is read from the pressure sensor and then converted into a pressure change rate through differential operation. The conversion between the motor speed n and the pressure change rate can be realized through formula (10):

[0056]

[0057] Where n is the motor speed (r / min); F is the test force (g); and K is the unit conversion coefficient (r·s / g·min).

[0058] According to formula (11), the motor pressure head pressing speed v (mm / s) is obtained:

[0059]

[0060] Where, v is the pressing speed of the pressure head, mm / s; S is the lead of the screw, mm / r.

[0061] b) When the pressure value of the pressure head on the workpiece reaches 90%F, it enters the cascade buffer PID mode:

[0062] The cascade buffer PID mode, i.e., the pressure change rate controller calculates the change rate of the head pressure, which is the difference between the two differences before and after. In other words, the differential term gives a corresponding adjustment action in advance according to the rate of change of the pressure difference, which can buffer the oscillation to a certain extent.

[0063] According to the national standard test method, for small force Vickers hardness test and micro Vickers hardness test, the force application process cannot exceed 10s and the pressure head drop speed should not exceed 0.2mm / s. The above-mentioned cascade buffer PID mode obtains the pressure change rate through the pressure change rate controller, which can buffer the oscillation of the loading test force to a certain extent, which is conducive to a more stable approach to the expected test force.

[0064] c) When the pressure value of the pressure head on the workpiece reaches 98%F, it enters the cascade anti-overshoot PID mode:

[0065] There are large-scale errors in the process of cascade loading, so the integrator accumulates a large value. When the loading test force is close to the expected test force, the integral is saturated, which may cause a large overshoot, and it is always in an overshoot state for a long period of time, which is not conducive to the measurement of the workpiece. Therefore, we introduce an anti-integral windup algorithm in the cascade anti-overshoot PID mode, that is, the mapping relationship between the inner loop pressure change rate and the motor speed is used to obtain the corresponding motor speed according to formula (12), and the motor speed is nonlinearly reduced to reduce the overshoot and protect the workpiece.

[0066] This mapping relationship forms a limited-increment PID mode by limiting the step size of the control quantity increment:

[0067]

[0068] Where Step is the step size, which is used to limit the increase of the control amount to keep it within the allowable value.

[0069] The principle is that the differential component in the PID algorithm is used to introduce a correction signal into the system to provide appropriate braking for the greatly changed control quantity, thereby suppressing the change of deviation.

[0070] d) When the pressure head pressure reaches the expected value, enter the cascade full load PID mode:

[0071] Maintain the expected test force for a period of time until the workpiece no longer undergoes plastic deformation. The purpose of this is that when the test pressure is maintained during the Vickers hardness test, the indentation may rebound or deepen (deepening: the indentation may deepen due to the vibration of the indenter or external interference, and the measurement result is inaccurate. Therefore, it is necessary to increase the test force to keep it constant, that is, the cascade full load PID, in order to keep the indenter pressure near the expected value and reduce the measurement error.), so the test force needs to be maintained for 10s to 15s. For special material samples, the test force holding time can be extended until the sample no longer undergoes plastic deformation, but it should be noted in the hardness test results and the error should be within 2s. At this time, a cascade full load PID is required to maintain the pressure of the indenter near the expected value during the entire test force holding period, so that the hardness tester should be able to resist the impact and vibration and reduce the measurement error. When the surface pressure of the workpiece to be tested reaches the expected value F, the force value is maintained to the set time.

[0072] At the same time, e(t) is fed back to the microcontroller for calculation, and the formula is as follows:

[0073]

[0074] When δ% = ±2% (or δ% = ±5%), that is, the actual test force is (1±2%) times the expected test force, it can be considered that the workpiece no longer undergoes plastic deformation. According to national standards, the test force needs to be maintained for 10s to 15s.

[0075] (4) Unloading: The motor controls the pressure head to leave the workpiece to be tested and reset to the initial position.

[0076] Experimental verification

[0077] The test process is: make the indenter contact the surface of the workpiece to be tested, apply the test force perpendicular to the test surface until the test force is applied to the specified value. The time from the start of force application to the completion of the full test force application is between 2s and 8s. For small-force Vickers hardness tests and micro-Vickers hardness tests, the force application process does not exceed 10s and the indenter descent speed is not greater than 0.2mm / s. For micro-Vickers hardness tests, the indenter descent speed should be between 15μm / s and 70μm / s.

[0078] A fully automatic hardness indentation automatic measuring device was used to apply test forces of 1KG, 2KG, 3KG, 50KG, 55KG, 65KG, 70KG, 80KG, 100KG, 120KG, and 150KG to the test piece. The test force was maintained for 10s to 15s. The test force curve was as shown in the figure. Figure 4-6 shown.

[0079] Depend on Figure 4-6 It can be seen that the overall test force curve follows the process of loading, buffering, overshoot prevention, and full load. The time from the start of force application to the completion of the full test force meets the requirements of the national standard. At the same time, due to the effect of the full load PID, the final loading test force value is almost close to the expected test force value, reducing the damage caused by the overshoot phenomenon. The final test force value of the curve drops because the measurement is completed, the motor rotates upward, and resets to the initial position.

[0080] Therefore, it is proved through experiments that the force sensor is used to control the force in the present invention, and the test force has high precision. The electronic closed loop can automatically control the precision of the force, feed back and consume the external interference force, and reduce the measurement error caused by the indentation rebound. At the same time, the loading force range is very large, and the loading force can be from 1kgf to 3kgf, or from 80kgf to 150kgf. It effectively improves the hardness measurement accuracy, and solves the problem that the force cannot be maintained in manual measurement, which easily causes the vibration of the striker. It complies with the national standard test method and has practical application significance.

[0081] According to the national standard test method, when measuring hardness, the indenter contacts the surface of the workpiece to be tested, and the test force is applied perpendicular to the test surface. There should be no impact and vibration during the force application process, that is, to ensure that the workpiece cannot be penetrated, and the operation complies with the national standard. According to the above national standard test method, when measuring hardness, the indenter contacts the surface of the sample, and the test force is applied perpendicular to the test surface. There should be no impact and vibration during the force application process until the test force is applied to the specified value. The time from the start of force application to the completion of the application of all test forces should be between 2s and 8s, and the test force holding time is 10s to 15s. For small-force Vickers hardness tests and micro-Vickers hardness tests, the force application process cannot exceed 10s and the indenter descent speed should not be greater than 0.2mm / s. For micro-Vickers hardness tests, the indenter descent speed should be between 15μm / s and 70μm / s.

[0082] The technical means disclosed in the scheme of the present invention are not limited to the technical means disclosed in the above-mentioned implementation mode, but also include the technical scheme composed of the above-mentioned technical features.

Claims

1. An incremental cascade PID closed-loop control method, characterized in that: The outer loop is the pressure loop, and the inner loop is the pressure change rate loop. The control increment Δu between the outer loop and the inner loop is obtained according to formula (5): (k) : Δu (k) =Ae k -Be k-1 +This k-2 (5) In the formula, A, B, C are coefficients, e k 、e k-1 、e k-2 are the control deviations formed by the expected pressure value and the actual pressure value at the kth, k-1th, and k-2th times of the constant sampling period T respectively; When the outer loop force value reaches 20%F, 90%F, 98%F, and F respectively, the incremental cascade PID closed-loop control system enters the cascade loading PID mode, cascade buffering PID mode, cascade anti-overshoot PID mode, and cascade full-load PID mode respectively; The cascade loading PID mode is that the outer loop obtains the current loading test force, and the inner loop calculates the pressure change rate through the pressure change rate controller by differentiation. The motor speed is controlled by comparing it with the mapping table of the motor speed direction, and the pressure head continues to press down; The mapping relationship between the pressure change rate and the motor speed direction is that a pressure value is read from the pressure sensor, and then the pressure change rate is obtained through differential operation, and the conversion between the motor speed n and the pressure change rate is realized through formula (10): Where n is the motor speed, r / min; F is the test force, g; K is the unit conversion coefficient, r·s / g·min; According to formula (11), the motor pressure head pressing speed v, mm / s: Where, v is the pressing speed of the pressure head, mm / s; S is the lead of the screw, mm / r.

2. The incremental cascade PID closed-loop control method according to claim 1, characterized in that: The cascade buffer PID mode obtains the pressure change rate through the pressure change rate controller, controls the force application process to not exceed 10s and the pressure head drop speed should not be greater than 0.2mm / s.

3. The incremental cascade PID closed-loop control method according to claim 1, characterized in that: The cascade anti-overshoot PID mode obtains the corresponding motor speed according to formula (12): In the formula, Step is the step length, which is used to limit the increase of the control amount to keep it at the allowable value; u (k) and u (k-1) are the output values ​​of the PID controller at the kth and k-1th sampling moments respectively.

4. The incremental cascade PID closed-loop control method according to claim 1, characterized in that: The cascade full-load PID mode maintains the surface pressure of the workpiece to be measured at the expected value F for more than 10 seconds.

5. A method for accurately controlling the test force of a hardness tester using the incremental cascade PID closed-loop control method according to any one of claims 1 to 4, characterized in that: The following steps are involved: Step S1) the system is started, a desired test force F is given, and the motor drives the pressure head to press down the workpiece to be tested; Step S2) when the pressure head contacts the workpiece to be tested, the test force begins to increase, and the cascade loading PID mode, cascade buffering PID mode, cascade overshoot prevention PID mode, and cascade full load PID mode are entered respectively; Step S3) when the system is in the cascade full-load PID mode, the desired test force F is maintained for a period of time until the workpiece no longer undergoes plastic deformation; Step S4) unloads, the motor rotates upward, and the pressure head returns to the initial position.

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