Method and device for detecting plastic foreign objects with low chromaticity difference in shredded tobacco through online pulse spectral imaging

A method for detecting plastic foreign objects with low chromaticity difference in shredded tobacco through online pulse spectral imaging, the method comprising the following steps:

• step 1:
• negative pressure thin-layer loading of shredded tobacco, comprising the following processes:
• a first process, wherein shredded tobacco falls from a hopper onto a horizontal conveyor belt, and when the shredded tobacco is conveyed by the horizontal conveyor belt to pass below a thin-layer thickness adjustment roller, the shredded tobacco exceeding a set thickness is removed by the thin-layer thickness adjustment roller to a shredded tobacco elimination roller and is then taken away by the shredded tobacco elimination roller into a super-thick shredded tobacco collection hopper;
• a second process, wherein the shredded tobacco adjusted by the thin-layer thickness adjustment roller is continuously conveyed by the horizontal conveyor belt and falls from a far end of the horizontal conveyor belt onto a surface of a conveying cylinder;
• meanwhile, a negative pressure control solenoid valve III connected to air chambers below the surface of the conveying cylinder is opened, and the shredded tobacco is adsorbed on the surface of the conveying cylinder under a negative pressure produced by airflows from outside into air holes of an air hole array on an outer layer of the conveying cylinder;
• and
• a third process, wherein loose shredded tobacco on a surface of a thin layer of the shredded tobacco adsorbed by the negative pressure to the surface of the conveying cylinder is blown into a loose shredded tobacco collection hopper by airflows from nozzles in an airflow nozzle array, the airflows being tangential to the surface of the conveying cylinder, and a stable thin layer of the remaining shredded tobacco is formed on the surface of the conveying cylinder;
• step 2:
• pulse line-scanning identification of shredded tobacco, comprising the following processes:
• a first process, wherein the surface of the conveying cylinder with a width of W and a circumference of L is coded, the surface of the conveying cylinder is divided into P rectangular strips, each of the rectangular strips having a width of W and a length of L/P and being marked as A_j;
• the rectangular strip A_j is divided into Q square unit areas, each of the square unit areas having a side length of L/P and being marked as A_j_k;
• and the surface of the conveying cylinder is coded with a coding array C containing Q rows and P columns and having an initial value of 0, wherein a coding rule is that C(k, j) corresponds to the square unit area A_j_k, P and Q are positive integers, j is in a range of 1, 2, ..., P-1, P, and k is in a range of 1, 2, ..., Q-1, Q;
• wherein u air holes are arranged below each of the square unit areas A_j_k, and the u air holes correspond to one air chamber, wherein u is a positive integer;
• a position trigger and an industrial computer are used to determine a corresponding position of the rectangular strip A_j on the conveying cylinder at different time;
• a second process, wherein the conveying cylinder keeps rotating at a constant speed, a line-scanning area with a width of W is determined on the surface of the conveying cylinder, and a light-emitting diode (LED) linear array light source containing N characteristic wavelengths of plastics with low chromaticity difference is used together with a line-scanning camera to capture an image of the thin layer of shredded tobacco in the line-scanning area;
• the LED linear array light source corresponding to each of the characteristic wavelengths sequentially and cyclically emits light with a pulse width of T/N, a pulse interval of T(N-1)/N, and a pulse period of T in an imaging process;
• when the rectangular strip A_j passes through the line-scanning area, the line-scanning camera obtains in real time a scanning signal I_band_ji corresponding to the rectangular strip A_j under irradiation of an ith characteristic wavelength λ_i, i, wherein i is in a range of 1, 2, ..., N-1, N and N is a positive integer;
• the pulse period T is in a range of 1.5-3000 µs;
• and
• a third process, wherein foreign object discrimination thresholds D_i of scanning signals corresponding to the N characteristic wavelengths µ_i are used together with the scanning signal I _band_ji obtained in real time and corresponding to the rectangular strip A_j under irradiation of the ith characteristic wavelength to identify whether foreign objects exist in an area corresponding to the rectangular strip A_j;
• if an identification result is that foreign objects do not exist in the area corresponding to the rectangular strip A_j, values of elements C(1 :Q, j) in a jth column of the coding array C remain unchanged;
• if an identification result is that foreign objects exist in the area corresponding to the rectangular strip A_j, the square unit area A_j_k containing the foreign objects in the rectangular strip A_j is further located, and the corresponding C(k, j) in the coding array C is set to 1;
• and
• step 3:
• positive pressure online elimination of foreign objects, comprising the following processes:
• a first process, wherein a foreign object-containing shredded tobacco elimination trigger lineis set directly above a foreign object collection container;
• when the area corresponding to the rectangular strip A_j is directly above the foreign object collection containerand a center line of the area coincides with the foreign object-containing shredded tobacco elimination trigger line, the position triggertransmits a signal to the industrial computer, and meanwhile the industrial computerreads the elements C(1:Q, j) corresponding to the rectangular strip A_j in the coding array and sequentially reads values of all elements C(k, j) in C(1 :Q, j);
• if C(k, j) is 0, foreign objects do not exist in the square unit area A_j_k corresponding to C(k, j), the air chambercorresponding to the air holes in the square unit area A_j_k maintains an original state, and the thin layer of shredded tobacco in the square unit area remains adsorbed by the negative pressure;
• if C(k, j) is 1, foreign objects exist in the square unit area A_j_k corresponding to C(k, j), the negative pressure control solenoid valve III connected to the air chamber corresponding to the air holes in the square unit area A_j_k is closed and a first positive pressure control solenoid valve also connected to the air chamber is opened for t1 seconds and then closed, so that the air chamber corresponding to the air holes in the square unit area AJ_k is in a positive pressure P1 state for t1 seconds and then in a normal pressure P0 state, and the thin layer of shredded tobacco containing foreign objects falls from the square unit area A_j_k into the foreign object collection container;
• a second process, wherein a qualified shredded tobacco unloading trigger line is set directly above a qualified shredded tobacco collection container;
• when the area corresponding to the rectangular strip A_j is directly above the qualified shredded tobacco collection containerand a center line of the area coincides with the qualified shredded tobacco unloading trigger line, the position trigger transmits a signal to the industrial computer, and meanwhile the industrial computer reads the elements C(1 :Q, j) corresponding to the rectangular strip A_j in the coding array and sequentially reads the values of all the elements in C(1 :Q, j);
• if C(k, j) is 0, the thin layer of shredded tobacco in the square unit area A_j_k corresponding to C(k, j) is qualified, the negative pressure control solenoid valve III connected to the air chamber corresponding to the air holes in the square unit area A_j_k is closed, and the first positive pressure control solenoid valvealso connected to the air chamber is opened for t1 seconds and then closed, so that the air chamber corresponding to the air holes in the square unit area AJ_k is in the positive pressure P1 state for t1 seconds and then in the normal pressure P0 state, and the qualified thin layer of shredded tobacco falls from the square unit area A_j_k into the qualified shredded tobacco collection container;
• if C(k, j) is 1, the shredded tobacco containing foreign objects in the square unit area A_j_k corresponding to C(k, j) is eliminated in the first process of step 3, and the air chambercorresponding to the air holes in the square unit area A_j_k maintains the original state in this process;
• and
• a third process, wherein to avoid that a small amount of shredded tobacco still remains on the surface of the conveying cylinder after the first and second processes in the step 3, a residual shredded tobacco clearing trigger lineis set directly above a residual shredded tobacco collection container;
• when the area corresponding to the rectangular strip A_j is directly above the residual shredded tobacco collection containerand a center line of the area coincides with the residual shredded tobacco clearing trigger line, the position trigger transmits a signal to the industrial computer, and meanwhile the industrial computercontrols a second positive pressure control solenoid valveconnected to the air chamberscorresponding to the air holes in all the square unit areas A_j_k in the rectangular strip A_j to be opened for t1 seconds and then closed, so that the air chambers are in a positive pressure P2 state for t1 seconds and then in the normal pressure P0 state, the residual thin layer of shredded tobacco falls from the area corresponding to the rectangular strip A_j into the residual shredded tobacco collection container, and the values of all the elements C(1:Q, j) in the coding array are set to 0.