Abstract:
A cleaning system and method of cleaning filters that removes the ash in the plugged regions is disclosed. The filter is subjected to vibrations, which serve to loosen trapped and packed retentate from the filter. The loosened retentate is then captured by a collection bin. The cleaning system can be integral with the intended application, such as within an automobile. In another embodiment, the cleaning system is a separate cleaning station, where the filter is removing from its intended application, cleaned, and then reinstalled.

Description:
[0001]    This application claims priority of U.S. Provisional Patent Application Ser. No. 61/352,908, filed Jun. 9, 2010, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    In many applications, there is a need to effectively and efficiently clean filters. In particular, there is a need to clean expensive filters, restoring them to a condition in which the filters can be reused. Additionally, many filters are difficult to clean using conventional means. A common method of cleaning filters uses forced air to blow off the retentate. There are a number of disadvantages to using forced air for filter cleaning. 
         [0003]    First, forced air methods and systems generate airborne particles and dust that may pose health and environmental hazards. Many forced air cleaning systems require large and expensive secondary dust collection and filtration systems. 
         [0004]    Second, forced air is not always effective at removing retentate from filters. In particular, filters with small pores, channels, pleats, or the like; or filters in which the retentate has become sintered, stuck, or otherwise adhered to itself or the filter surface may not effectively be cleaned by the use of forced air. 
         [0005]    Third, forced air filter cleaning methods are ineffective at removing retentate packed in areas of the filter that experience little-to-no airflow. 
         [0006]    Finally, ineffective filter cleaning reduces the useful life of the filter, and may cause efficiency losses or damage to the systems in which the improperly cleaned filters are used. 
         [0007]    While there are many types of filters, diesel particulate filters, in particular, are especially difficult to clean using forced air. Diesel particulate filters are used to reduce soot emissions from a wide range of sources, including internal combustion engines. Although there are many types of diesel particulate filters, the most common are ceramic honeycomb-type wall-flow filters. These filters may be cordierite, silicon carbide, mullite, aluminum titanate, or any other suitable material. The wall-flow filters generally contain thousands of small channels, the walls of which are composed of a porous material. The ends of the channels may be alternately blocked at opposite ends of the filter (checkerboard pattern). Particulate-laden exhaust that enters the open end of a filter inlet channel is forced to flow through the porous filter walls to the adjacent outlet channel. The soot particles in the exhaust are trapped in the wall pores or in a cake layer along the channel surface. 
         [0008]    In addition to ceramic wall-flow filters, a number of other diesel particulate filters exist including metal foam, sintered metal, and fiber-based (ceramic, glass, paper, and others) filters. These filters may contain pleats, channels, or a porous matrix, among other configurations. 
         [0009]    Diesel particulate filters are effective at trapping in excess of 99% of engine-out soot in some cases. The trapped soot is periodically or continuously removed from the filter by regeneration. The regeneration process oxidizes the soot, but leaves incombustible ash behind. 
         [0010]    Over time, the incombustible ash builds up in the filter, either in a layer along the filter walls, or in a plug at the back of the filter channels. Ash generally consists of metallic components, originating in lubricant additives, such as Mg, Ca, and Zn forming various sulfates, phosphates, and oxides. Trace metals in fuels (Na, K, Ca, and other elements found in biofuels for example) and engine wear metals and corrosion particles may also contribute. In cases where fuel-borne additives are used for filter regeneration, such as Ce, Fe, Pt, and others, these elements may also contribute significantly to ash accumulation in diesel particulate filters. 
         [0011]    The build-up of ash restricts exhaust flow, increases exhaust backpressure, and negatively impacts engine fuel consumption. The ash further reduces the filter&#39;s soot storage capacity. In extreme cases, filter plugging can cause excessive exhaust backpressure leading to failure of the engine, exhaust, or machine system. One means of mitigating the negative effects of ash build-up on the filter and system (engine, vehicle, etc.) performance is to clean the filter, i.e., to remove the ash. 
         [0012]    Common systems and methods employed to clean diesel particulate filters include forward and reverse blowing, through the use of forced air. Additionally, the filter may be heated (generally up to 650 C) to oxidize and remove soot as well. While filter heating is effective at removing the soot, it is relatively ineffective at removing ash at these temperatures. 
         [0013]    One reason that diesel particulate filters are difficult to clean is due to the manner in which the ash accumulates in the filter, as well as its composition. Initially, beginning with a clean filter, the ash accumulates in a thin layer along the filter walls. This thin layer or membrane may actually provide a reduction in filter pressure drop when the filter is also loaded with soot, as the ash layer may prevent soot from accumulating in the filter pores (depth filtration). Generally, the beneficial ash membrane is formed with less than 15 g/L specific ash loading, resulting in an ash layer that is on the order of 50 microns thick; however the pressure drop benefits may be realized with much thinner ash layers and lower specific ash loadings. Depending on the ash composition and properties, these values may differ, however. 
         [0014]    As more ash accumulates in the filter, the pressure drop may increase significantly, such that any initial benefit is lost, and for a given level of soot load, the pressure drop across a particulate filter containing soot and ash is greater than the pressure drop across a filter containing the same amount of soot but no ash. Specific ash levels above 25-30 g/L may result in a significant increase in filter pressure drop, loss of soot storage capacity, as well as increase in regeneration frequency. 
         [0015]    At these elevated ash levels, the majority of the ash (75% or more) may be packed toward the back of the filter, forming plugs in the filter channels. In some cases, for filters with 150,000 miles of on-road use, the ash may completely plug over 50% of the length of the DPF channels. The ash accumulated in the channel end-plugs may also pack, sinter, or fuse together or to the filter surface. As the ash accumulated in the plugs completely blocks the flow of exhaust through the ash-plugged region of the filter, it contributes significantly to filter pressure drop, generally more so than the porous ash accumulated in a thin layer along the filter walls. 
         [0016]    Considering the ash distribution in the DPF, an ideal cleaning process is one that removes all of the ash from the plugs, but leaves a small thin layer along the channel walls. Removing the ash from the end plugs significantly increases the available area for soot deposition, and reduces pressure drop. Leaving a thin layer of ash along the filter walls, preserves the beneficial effect of the ash membrane from preventing soot depth filtration, and provides some reduction in pressure drop. 
         [0017]    Conventional forced air systems accomplish exactly the opposite of the ideal cleaning method proposed above. These systems and methods remove much of the ash in the layer along the channel walls (region of the DPF subject to highest flows), but are ineffective at removing ash in the plugged regions (most of the ash) of the filter which experience no, or negligible, flows. 
         [0018]    Therefore, an improved cleaning system and method for use with filters, and particularly diesel particulate filters, is needed. 
       SUMMARY OF THE INVENTION 
       [0019]    A cleaning system and method of cleaning filters that removes the ash in the plugged regions is disclosed. The filter is subjected to vibrations, which serve to loosen trapped and packed retentate from the filter. The loosened retentate is then captured by a collection bin. The cleaning system can be integral with the intended application, such as within an automobile. In another embodiment, the cleaning system is a separate cleaning station, where the filter is removing from its intended application, cleaned, and then reinstalled. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0020]      FIG. 1  represents one embodiment of a filter cleaning system; 
           [0021]      FIG. 2  represents a cross section of several channels in the filter of  FIG. 1 ; 
           [0022]      FIG. 3  represents a perspective view of one embodiment of the filter cleaning system; 
           [0023]      FIG. 4  represents a filter cleaning station; 
           [0024]      FIG. 5  represents a filter cleaning system integrated with a machine, such as a vehicle; and 
           [0025]      FIG. 6  shows the requirements for a specific impulse of 0.1 m/s, for a mass of 70 kg. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]      FIG. 1  depicts a vibration-based filter cleaning system according to one embodiment. A diesel particulate filter  100  is made up of a housing containing the filter element (not shown). The filter element may be of any type, but is generally a ceramic honeycomb contained within a metal housing or can. Together the filter element and housing form the diesel particulate filter  100 . The diesel particulate filter  100  may be removed from its application prior to cleaning. An end cap  104  is affixed to the filter outlet, and an ash collection bin  106  is affixed to the filter inlet. The end cap  104  and ash collection bin  106  may be affixed to the filter using conventional v-band clamps or other clamps commonly used in these applications, for example. One or more vibrating elements  102  may be attached to the filter housing  100 . Additional vibrating elements  108  may be attached to either or both end cap  104  and ash collection bin  106 . Additional vibrating elements  108  are shown attached to the end cap  104 . 
         [0027]    The ash collection bin  106  may be a container of suitable composition, metal, plastic, composite material, or the like. In another embodiment the ash collection bin  106  may be a filter bag, plastic bag, or any other particle containment system affixed to the diesel particulate filter  100  inlet. 
         [0028]    The vibrating elements  102 ,  108  may be mechanically connected to one or more of the diesel particulate filter  100 , end cap  104  or ash collection bin  106 . In one example, the vibrating elements  108  may be bolted to the end cap  104  or ash collection bin  106  and the vibrating element  102  may be affixed to the diesel particulate filter  100  using a band clamp, such as a hose clamp. 
         [0029]    The vibrating elements  102 ,  108  may or may not contact the filter element directly. Filter end cap  104  and ash collection bin  106  may or may not be used. In another example, the filter end cap  104  is not used and the vibrating elements  108  may contact the filter element surface directly, such as the inlet or outlet surface. In this example, the vibrating elements  108  and  102  may be hand-held and manually pressed against the filter element surface or filter housing by the operator. The vibrating elements  108  and  102  may be held in place or moved along the diesel particulate filter  100  surfaces, including the outer housing and filter element surfaces. 
         [0030]    In one embodiment, the vibrating elements  102 ,  108  may be conventional vibrating motors, magnetostrictive elements, piezoelectric actuators, pneumatic vibrating elements, or any other device suitable for generating vibrations. Vibrating elements  102 ,  108  may be supplied with power from an external supply or conventional electrical outlet, and may also be connected to a control system (not pictured). In another embodiment, the vibrating elements  102 ,  108  may be ultrasonic or acoustic transducers. Additional couplant, such as gel, water, or other suitable material may or may not be used to improve the transfer of vibration energy from the vibrating elements  102  and  108  to the diesel particulate filter  100 . The vibrations generated by vibrating elements  102 ,  108  may be periodic or random, and continuous or pulsed. 
         [0031]    The vibrating elements  102 ,  108  may be affixed in any position or orientation relative to filter  100 . In one embodiment, the vibrating elements  102 ,  108  may be adjusted (position optimized) to target regions of the filter with the highest ash deposits, such as the end-plugs. In many cases, the majority of the ash deposits are found as end-plugs in the back of the filter channels. Positioning the vibrating elements  102 ,  108  close to the end plugs represents one means of specifically focusing the vibration energy on those regions of the filter  100  with the most accumulated ash. Prior to cleaning, the location and length of the ash deposits may be measured. Measuring the insertion length of a thin wire rod inserted down one or more diesel particulate filter  100  channels represents one means for determining the length of the ash plugs. Measurements of ash deposit location, although not required, may be used to determine the most appropriate location for the vibrating elements  102 ,  108  to target the ash deposits. 
         [0032]    The position of the vibrating elements  102 ,  108  may or may not be altered or adjusted during the cleaning process. In one embodiment, the vibrating elements  102 ,  108  may remain fixed during the cleaning process. In another embodiment, the position of the vibrating elements  102 ,  108  may be continuously or intermittently adjusted during the cleaning process. In one example, vibrations are applied to a specific region of the filter  100  until no more ash is removed. The position of the vibrating elements  102 ,  108  is then adjusted to focus the vibration energy on another region of the filter  100  still containing ash deposits. Any range of vibrating frequencies and forces may be used. One or more vibrating elements  102 ,  108  may be used. 
         [0033]    In one embodiment, radio frequency-based filter soot and ash probes  110  may also be attached to the end cap  104  and ash collection bin  106  or the filter housing  100 . The filter housing  100 , end cap  104 , and ash collection bin  106  form a microwave cavity. The RF probes  110  may be connected to a control unit and power supply  116 . The RF probes  110  may be used to provide information on the amount of ash and soot in the filter as well as to detect filter failures, such as cracking or melting. Any frequency range of operation is suitable. Higher frequencies increase measurement spatial resolution. Measurements from the RF probes  110  may be used to determine when to clean the filter, and also when to stop cleaning the filter, in one example. 
         [0034]    Additionally, in some embodiments, one or more connections  112  and  114  may be used. These connections may be part of the end cap  104 , the ash collection bin  106 , or both. The connections  112  and  114  may allow for the use of air flow, or any other suitable fluid flow to remove the retentate (ash/soot) dislodged by the vibrations. In another embodiment, a vacuum may be applied to at least one of these connections to remove the retentate. Air flow or vacuum is by no means required for system operation, but may be applied. Pressure transducers (not pictured) may also be used to monitor filter  100  pressure drop before, during, and after the cleaning process, to provide one measure of cleaning system effectiveness, and suitability for reuse of the cleaned filter. 
         [0035]    The operation of the system shown in  FIG. 1  will now be described. Vibrations and forces applied by vibrating elements  102 ,  108  cause the ash deposits to break-up. In one embodiment, low frequency vibrations, in conjunction with elevated forces, may be applied to break up the ash accumulated in the plugs. This may be followed by high frequency vibrations, but low applied forces, which may be used to cause the dislodged ash to fall out of the inlet section of filter  100  into the ash collection bin  106 . One particular method may utilize frequencies below 60 Hz to loosen and break up the ash deposits, and frequencies above 100 Hz to remove the ash from the filter  100 . In another embodiment, higher frequencies around 1 kHz may be used. The vibrations may be used to remove ash, soot, and any other contaminant material from the filter  100 . 
         [0036]    The duration of the applied vibrations may be less than minutes in one example. The applied frequencies and forces may be constant or variable. In another example, low frequency and high force vibrations are applied for 2 minutes followed by the application of high frequency and low force vibrations for another 2 minutes, and the process alternated over the entire cleaning duration, which may be 10 minutes. The duration of the applied vibrations may be any duration, and may be less than or greater than 10 minutes. 
         [0037]    During the cleaning process, the location, orientation, and number of vibrating elements  102  and  108  may be varied or adjusted. The applied forces and vibrating frequencies may also be varied. In one example, the vibrations and forces may be varied by adjusting the voltage applied to a vibrating motor, or the internal weights on the motor shaft. In another example, simply placing absorbing material between the vibrating elements and filter (such as rubber matting or foam) may also damp the vibrations. In yet another example, the pressure and flow rate may be varied if pneumatic elements are used. 
         [0038]    In one embodiment, the cleaning process may be monitored and controlled using the RF sensors  110 . In such a system, the cleaning process parameters (vibration, forces, and displacement) may be adjusted or optimized based on the measured rate of retentate removal. The filter  100  cleaning process may be stopped once filter  100  retentate is reduced to acceptable levels as determined by the RF sensors  110 . In another example, visual observations of the ash removal, mass of ash removed, or filter  100  pressure drop, may be used to determine when the cleaning process is finished, or to adjust process parameters. In yet another example, the cleaning process may be terminated when a certain predetermined time has elapsed. Any number of measurement systems and methods including x-ray, terahertz imaging, borescope inspection of the filter channels, and manual measurements of ash plug length using a wire rod may be used to determine ash levels in the filter  100  to adjust cleaning process parameters or determine when to stop filter  100  cleaning. 
         [0039]    In another embodiment, the filter  100  may be first heated above 650 degrees C., preferably in the range of 700 to 1,000 degrees C. This heating may induce a volume change in the ash plugs, (expansion or shrinkage) which may facilitate their removal using vibrations. The heating may be accomplished using the existing filter regeneration system on the engine, or via external heaters or furnaces. 
         [0040]      FIG. 2  provides additional details of the vibration cleaning process. A cross section of three channels of a diesel particulate filter  200  is shown. The figure shows the porous channel walls  202  an inlet channel  204  and accumulated ash  206  in a layer along the inlet channel walls and in a plug at the back of the channel. Two outlet channels  208  are also shown. Applied vibrations  210 ,  212 , and  214  are schematically depicted. 
         [0041]    The vibrations may be applied perpendicular to the channels as in  210  and  212  or parallel to the channels as in  214 , or at any angle or orientation to the channels and filter  200 . The orientation of the applied vibrations may or may not change during the cleaning operation. The vibrations may be induced on the channels or filter directly, or through the filter housing. The vibrations  210 ,  212 , and  214  are used to break up, dislodge, and remove the ash or retentate  206  from the channels. In one embodiment, vibrations perpendicular to the channels  210  and  212  may be used to break up and loosen the ash plugs and deposits  206 . The loosened ash deposits may be removed from the channel  204  by application of parallel vibrations  214  causing the loosened ash to move down the channel  204  and fall out of the filter. Flows of air or other gases or liquids may or may not be used to aid in ash removal. Any orientation of the vibrations  210 ,  212 , and  214  may be used in any sequence. 
         [0042]      FIG. 3  presents a particular embodiment showing a particulate filter  300  to which an ash collection bin  302  and end cap  304  have been fixed. A vibrating motor  306  is mounted to the top of the end cap  304 . A vibrating collar  308  is also mounted around the circumference of the filter  300 . The vibrating collar  308  may contain one or more vibrating elements  310  such as magnetostrictive elements or other suitable elements. The position of the vibrating collar  308  may be adjusted during the cleaning process to target areas of the filter  300  containing high concentration regions of ash or ash that is difficult to remove, such as in the end plugs. 
         [0043]    Although the vibrating elements  306  and  310  are shown mounted on the housing  300  or caps  304  externally to the filter element contained within housing  300 , the vibrating elements may be mounted directly on the filter element, such as by placing the vibrating motor  306  directly on the face of the filter, in one example. In another example, the filter element may be removed from its housing  300  prior to application of the vibrations. 
         [0044]    Application of vibrations from elements  306  and  310  cause the ash to be dislodged and fall into the ash collection bin or bag  302 . In this manner, no airborne ash dust is created. 
         [0045]      FIG. 4  depicts an ash cleaning station. The station is comprised of a support structure  400 , isolating elements  406 , and a support surface  402 . One or more vibrating elements  408  are fixed to the support surface  402 . The vibrating elements  408  may be connected to a power supply and control unit (not shown). The support surface  402  is physically connected to the support structures  400  via isolating elements  406 , which serve to reduce the transfer of vibrations from support surface  402  to support structure  400 . 
         [0046]    A filter fixture  404  may be connected to support surface  402 . The support fixture  404  allows a filter to be mounted in the horizontal or vertical direction, or any direction chosen by the operator. The support fixture  404  may or may not be used. In the event the fixture  404  is not used, the filter (not pictured) may be held in place manually by the operator, or some other means. 
         [0047]    The vibrating element  408  is used to induce vibrations in the support surface  402  which are then transferred to the filter placed on the support surface  402 . The position of the filter on the support surface may be varied during the cleaning process. The filter placed on the support structure  402  may or may not contain an end cap  304  and an ash collection bin  302  as shown in  FIG. 3 . If the filter does not contain an end cap  304  or ash collection bin  302 , an enclosure or ash collection bin or system may be mounted on support surface  402  to collect and contain the ash removed from the filter. 
         [0048]    The vibrations induced on the filter placed on the support structure  402  by vibrating element  408  are used to loosen, dislodge, and remove the ash from the filter. The filter may have additional vibrating elements fixed directly to it, as shown in  FIG. 1  and  FIG. 3 , or vibrating element  408  may be the sole source of the vibrations. One or more vibrating elements  408  may be used. The frequency, force, displacement, and other process control parameters may also be adjusted. 
         [0049]      FIG. 5  shows a vibration-based filter cleaning system applied directly to a diesel particulate filter  508  installed on machine  500 . Machine  500  may be any machine containing a particulate filter  508 . The machine  500  may be stationary or contain one or more traction devices  502  for moving the machine. Particulate filter may be connected to an engine  504  via a conduit  506 . Exhaust from the filter  508  may be directed away from the machine  500  via conduit  510 . The particulate filter  508  and conduit  506  and  510  may be oriented horizontally, vertically, or at any angle with respect to the machine  500 . 
         [0050]    One or more vibrating elements  512  may be attached to filter  508 . The vibrating elements  512  may further be connected to a power supply and controller, not shown. A connection  516  may be used to allow for an ash collection bin  518  to be connected to exhaust conduit  506 . A valve, plug or gating mechanism  514  may also be inserted into exhaust conduit  506 . The gating mechanism  514  has a first position where exhaust flows from the engine  504  to the filter  508 , and a second position where retentate flows upstream from the filter  508  to the ash collection bin  518 . 
         [0051]    The vibrating element  512  may be used to loosen, dislodge, and remove ash from filter  508  while it is still installed on machine  500 . In one example, the ash thus dislodged and removed, is blown from the filter  508  and exhaust conduit  506  by application of reverse flow directed into exhaust conduit  510  with the engine off. A source of fluid flow, such as an air compressor or blower, may be in communication with conduit  510  to blow fluid upstream through the filter  508 . The ash may be carried by this flow into collection bin  518  via a conduit  506 . The collection bin  518  may be a container, bag, or second filter element. The collection bin  518  may be removed to dispose of the collected ash. Valve, plug or gating mechanism  514  may or may not be used to prevent the ash from blowing to the engine  504 . Application of reverse flow may or may not be required to remove the ash from the system. A vacuum system (not pictured) may be connected to connection  516  in place of the collection bin  518 , in another example. In another embodiment, filter cleaning may be accomplished using vibrations alone, such as when the filter  508  is mounted in a vertical orientation. 
         [0052]    A particular example of a cleaning method will now be discussed. The vibrations applied to the filter can be characterized by an impulse on the filter. In this manner, the different “shaking” processes can be scaled and compared. Effective filter cleaning using vibrations can be obtained with relatively short shaking times, if the filter experiences a sudden loss in momentum (that is, it hits a surface). In one example, the speed just before the impact is about 5-10 cm/s without compromising the material integrity. Faster speeds result in substantial vibrations and sound. The specific impulse resulting for an almost instantaneous deceleration of the filter is then &lt;0.1 m/s (normalized by the mass of the filter). 
         [0053]    The impulses can be generated in a harmonic manner, by using either mechanical or electrical means using pneumatic vibrators, vibrating motors, magnetostrictive elements, piezoelectric elements, ultrasonic or acoustic transducers, and the like. In the case where magnetostrictive elements are used, they may be driven in a non-harmonic manner, using a square-wave-like driving current. 
         [0054]    In a particular example, the filter assembly, which includes the DPF, the can, matting and sensors, has a mass of about 70 kg. If the frequency of the vibrating element is f, then the time the force is applied is, approximately, 0.5/f. The force F required to generate a specific impulse of 0.1 m/s with a mass of 70 kg operating at 500 Hz is about 7000 N. The corresponding acceleration provided by such a system is approximately 10 g. 
         [0055]    The required power can also be calculated. The displacement of half a cycle is on the order of 50 microns. Thus the power required to drive the system is about 400 W. 
         [0056]    Lower frequencies require lower power but larger displacements. Thus, at 60 Hz, the required force is 800 N, and the power is about 42 W. The acceleration from the shaking motion is however, only about 1 g.  FIG. 6  shows the requirements for a specific impulse of 0.1 m/s, for a mass of 70 kg. The lower frequencies can be achieved using conventional motors (unbalanced motors). The higher frequencies can be achieved by using a magnetostrictive element, or other suitable devices. 
         [0057]    Magnetostrictive materials are a class of ferromagnetic materials that change their length upon the application of a magnetic field. Several materials experience magnetostriction. At room temperature, TERFENOL-D is one example. In another example, three such actuators are fixed to support surface  402  in  FIG. 4  or vibrating collar  308  in  FIG. 3 . Each actuator is about 1 cm in diameter and 3 cm long, but wider or longer actuators can be used. They are driven by a high power audio amplifier. 
         [0058]    The actuators have an elongation of about 100 microns, and require power of a few hundred Watts. The resonant frequency of the relatively short actuator is much higher than the operating frequency, and the field penetration is also fast. Operated at 500 Hz, the 3 actuators deliver about 6700 N, agreeing with the force required for shaking a mass of 70 kg at 500 Hz 
         [0059]    Using square waveforms to drive the actuator is particularly helpful when the downward motion of the filter assembly is stopped, as it is at this time that it is likely that the ash will dislodge and fall to the receptacle at the bottom. The direction of acceleration can be easily changed by driving the actuators with different phases. It is possible even to drive the support surface  402  ( FIG. 4 ) with different frequencies. 
         [0060]    The vibration-based cleaning system and methods disclosed herein may be applied independently, or in conjunction with other cleaning methods such as forced air and thermal regenerators. Measurements of filter loading state and filter integrity (cracking, melting) may or may no be applied. Radio-frequency and microwave sensors, pressure sensors, flow measurements, ultrasonic measurements, x-ray measurements, ash mass measurements, and similar systems and parameters may also be used to determine the effectiveness of the cleaning process. Detectable powders, such as fluorescent powders, in one example, may be introduced into the reverse flow through the filter to detect filter failures. Other powders may also be used. Detection of the powders on the opposite side of the filter from which they were introduced may indicate a failure of the filter. In situ measurements or estimates of filter retentate levels may also be used for real-time control of process parameters such as vibration orientation, frequency, and applied force during the cleaning process for example. The cleaning process may be stopped after a pre-determined amount of time has elapsed, or after a certain amount of retentate has been removed from the filter, or after residual retentate levels in the filter have been reduced to acceptable levels. 
         [0061]    To aid the cleaning, charged particles can be added to the flows applied to the filter in order to neutralize charges present in the ash or soot or other retentate that helps it agglomerate and/or hold on to the walls of the filter. For best neutralization, both positive and negative charges can be applied. In the case of both charge polarities, the charges can be in the same space, or they can alternate in space or time. The charges can be generated by external corona discharges, among other production means. 
         [0062]    It is intended that the examples and description provided in this disclosure be interpreted as illustrative and not in a limiting sense. The vibration-based cleaning system and methods disclosed are equally applicable to all types of diesel particulate filters, and all type of filters for that matter (pleated filters, paper filters, filter bags being some examples), regardless of material composition, configuration, or design. Further, the retentate may be soot, or ash, or any other material accumulated in filters. The retentate removed from the filter may be accumulated in plugs at the end of the filter channels, in layers on the filter surface, or in the filter pores.