in one aspect , embodiments disclosed herein relate to an activation system and related methods used in a drilling apparatus to selectively open and close expandable cutter blocks of the drilling apparatus . in particular , embodiments disclosed herein relate to methods and apparatus capable of selectively actuating primary and backup cutter blocks during downhole borehole enlarging operations . referring to fig1 a , a cross - section view of a drilling apparatus 100 is shown in accordance with embodiments of the present disclosure . drilling apparatus 100 includes a drillstring 110 having a drill bit 120 disposed on an end thereof , which drills a pilot hole 111 . drilling apparatus 100 further includes primary cutter blocks 130 and backup cutter blocks 132 , which are configured to drill an enlarged borehole 112 along the same path as the pilot hole 111 . the cutter blocks are well known in the art and typically include cutting elements and a stabilizer pad ( not shown ). the primary and backup cutter blocks are configured to travel along grooves ( not shown ) formed in the body of the drilling apparatus 100 when expanded or collapsed . the backup cutter blocks 132 may be constructed to be heavier duty than the primary cutter blocks 130 to withstand cutting in harder formations , and thereby handle more weight . in particular , the backup cutter blocks may be more robust by adding extra rows of cutters , using larger size cutters , using more robust cutting materials , varying the cutter shape , or other methods known to those skilled in the art . further , the backup cutter blocks may be devoid of cutters for “ backreaming ,” or underreaming while pulling the drilling apparatus out of the wellbore . as such , the backup cutter blocks 132 may be smaller than the primary cutter blocks 130 . embodiments of the present disclosure relate to an activation system within drilling apparatus 100 that allows primary cutter blocks 130 and backup cutter blocks 132 to be selectively activated downhole depending upon the characteristics of the formation being drilled . referring now to fig1 b , a schematic view of an upper portion of an activation system 200 in the drilling apparatus is shown in accordance with embodiments of the present disclosure . activation system 200 includes a pump 210 that is coupled to a motor 209 in the drilling apparatus . pump 210 uses fluid stored in a reservoir 211 to operate a main sleeve 204 in upward and direction directions . a toggle switch 214 may be used to route fluid to a sleeve activation - down port 222 and a sleeve activation - up port 223 . as used herein , the toggle switch may be defined as a valve to control the direction of fluid from the pump either to the sleeve activation - down port 222 or the sleeve activation - up port 223 . toggle switch 214 may be electronically connected to pump 210 and configured to toggle each time pump 210 is stopped . further , the various components ( e . g ., pump 210 , motor 209 , fluid reservoir 211 , and toggle switch 214 ) may be disposed in a wall of the main body of the drilling apparatus at a location substantially proximate main sleeve 204 . those skilled in the art will understand any number of electric pumps may be used . for example , in select embodiments , a pump supplied by bieri swiss hydraulics may be used . further , in select embodiments , a dc motor supplied by micromo electronics may be used ; however , those skilled in the art will understand any number of electric motors may be suitable . depending on the direction in which the fluid is routed , main sleeve 204 having an activation port 208 may be operated , the position of which determines which cutter blocks ( primary or backup ) are expanded . the following figures illustrate the sequence of operation of actuating the primary and backup cutter blocks while downhole . referring now to fig2 a and 2b , cross - section views of of activation system 200 of the drilling apparatus 100 in a “ run - in hole ” condition are shown in accordance with embodiments of the present disclosure . as previously shown in fig1 b , activation system 200 includes activation ports 222 and 223 which receive fluid from the reservoir 211 through pump 210 ( fig1 b ) to operate main sleeve 204 . sleeve activation - down port 222 provides fluid to move main sleeve 204 in a downward direction , while sleeve activation - up port 223 provides fluid to move main sleeve 204 in an upward direction . activation system 200 also includes primary cutter block activation port 230 ( to expand primary cutter block ) and backup cutter block activation port 232 ( to expand backup cutter block ). main sleeve 204 includes a main sleeve activation port 208 configured to align with primary cutter activation port 230 and a ball - drop sleeve activation port 207 in a ball - drop sleeve 203 . ball - drop sleeve 203 is disposed radially inward of main sleeve 204 . as shown , the drilling apparatus may be configured in a run - in hole condition in which the main sleeve activation port 208 and ball - drop sleeve activation port 207 are not aligned , thus the primary cutter block activation port 230 is closed . further , the backup cutter block activation port 232 is covered by the main sleeve 204 , and thus , is also closed . therefore , both primary and backup cutter blocks are in a retracted position as the drilling apparatus is inserted into the borehole and run downhole , as shown in fig2 b . referring to fig3 a and 3b , cross - section views of activation system 200 of the drilling apparatus are shown in which the activation system 200 is configured to expand the primary cutter blocks 130 ( fig3 b ) in accordance with embodiments of the present disclosure . initially , shear pins ( not shown ), which fixes the ball - drop sleeve 203 and the main sleeve 204 relative to each other , are sheared due to an increased hydraulic pressure after a ball drop to move the ball - drop sleeve 203 downward and align ball - drop activation port 207 with main sleeve activation port 208 . ball - drop devices are well known in the art and will not be described in detail . once the ball - drop activation port 207 and the main sleeve activation port 208 are aligned , fluid is able to flow into the primary cutter block activation port 230 and expand primary cutter blocks 130 , as shown in fig3 b . fig3 c shows an operation sequence of the activation system 200 shown in fig3 a and 3b in accordance with embodiments of the present disclosure . when the pilot hole is drilled and the borehole is to be enlarged , the primary cutter blocks are activated by a ball - drop device 323 . the main sleeve is initially in an “ up ” position in the bore 326 , therefore as described above , when after the ball drop device shears the shear pins , activation ports in the ball drop sleeve and the main sleeve are aligned . this allows the primary cutter blocks to expand , while the backup cutter blocks remain collapsed . the motor and pump remain off initially 324 , 328 . the drilling operation continues with the primary cutter block activation port open 330 and the primary cutter blocks expanded 332 to enlarge the borehole . while enlarging the borehole , weight sensors ( not shown ) of the activation system monitor the weight applied on the primary cutter blocks . to reduce premature wear on the primary cutter blocks , the backup cutter blocks may be expanded and the primary cutter blocks collapsed when the drilling apparatus enters a harder formation . the weight sensors may detect an increased weight applied on the primary cutter blocks and send a signal to operate the activation system by turning on the pump and moving the main sleeve . normal underreaming operations , for the purposes of embodiments disclosed herein , are typically run with weight on the primary cutter blocks that is about 20 - 25 % of the weight on bit (“ wob ”). in embodiments disclosed herein , the weight sensor ( e . g ., a load cell ) may detect a weight applied to the primary cutter blocks of at least about 35 % wob before the system is activated and the backup cutter blocks are expanded . one of ordinary skill in the art will appreciate that other preset limits may be used in accordance with embodiments of the present disclosure . operation of the activation after the preset weight limit is reached proceeds as follows . referring now to fig4 a and 4b , cross - section views of activation system 200 of the drilling apparatus are shown in which the activation system 200 is configured to expand backup cutter blocks 132 and retract primary cutter blocks 130 in accordance with embodiments of the present disclosure . main sleeve 204 is moved in a downward direction by providing fluid through sleeve activation - down port 222 and evacuating fluid from sleeve activation - up port 223 . moving the main sleeve 204 downward covers and seals the primary cutter block activation port 230 , which collapses the primary cutter blocks ( 130 in fig4 b ), and exposes the backup cutter block activation port 232 to allow fluid to enter and expand the backup cutter blocks 132 . fig4 c shows an operation sequence of the activation system 200 shown in fig4 a and 4b in accordance with embodiments of the present disclosure . when a weight sensor in the primary cutter blocks detects an increased weight above a preset limit 322 ( which indicates the cutter blocks are in a harder formation ), the coupled motor and pump may be turned on 324 to activate the main sleeve . the main sleeve is moved downward 326 to expand the backup cutter blocks ( by exposing the backup cutter block activation port 232 of fig4 a ). the coupled motor and pump may be turned off 328 and the drilling operation may continue with the backup cutter block activation port open ( primary cutter block activation port closed ) 330 and the backup cutter blocks expanded while the primary cutter blocks are collapsed 332 . while operating with the backup cutter blocks expanded , a second weight sensor disposed on or near the backup cutter blocks may monitor the weight applied to the backup cutter blocks . upon sensing that the weight on the backup cutter blocks has decreased below the prescribed limit ( e . g ., the weight on the backup cutters is less than 35 % wob ), the activation system may be activated to collapse the backup cutter blocks 132 and expand the primary cutter blocks and continue the operation . referring to fig5 a and 5b , cross - section views of activation system 200 of the drilling apparatus are shown in which the activation system 200 is configured to expand the primary cutter blocks and collapse the backup cutter blocks in accordance with embodiments of the present disclosure . main sleeve 204 is moved in an upward direction by providing fluid through sleeve activation - up port 223 and evacuating fluid from sleeve activation - down port 222 . moving the main sleeve 204 upward covers and seals the backup cutter block activation port 232 , which collapses the backup cutter blocks 132 ( fig5 b ), and aligns the ball - drop and main sleeve activation ports 207 , 208 , respectively , with the primary cutter block activation port 230 , which expands the primary cutter blocks 130 ( fig5 b ). fig5 c shows an operation sequence of the activation system shown in fig5 a and 5b in accordance with embodiments of the present disclosure . when the weight sensor disposed on the backup cutter blocks detects a decreased weight below the preset weight limit 322 , a signal may be sent to operate the coupled motor and pump 324 and move the main sleeve upward 326 to collapse the backup cutter blocks and expand the primary cutter blocks . after the main sleeve is moved fully upward , the coupled motor and pump are shut off 328 and the drilling operation continues with the primary cutter block activation port open ( backup cutter block activation port closed ) 330 and the primary cutter blocks expanded while the backup cutter blocks are collapsed 332 . advantageously , embodiments of the present disclosure may provide a drilling apparatus capable of adapting to real - time information provided by the weight sensors as to the hardness of the formation . as previously described , prior art cutter blocks require that formation intervals with a particularly increased hardness be identified before beginning the drilling operation . this can be characterized as an “ open loop ” system because the initial formation data is the only information available once drilling starts . open loop systems are hindered by a change in plans ( i . e ., drilling in a direction not previously foreseen ) and are unable to adapt on the fly . these limitations add considerable cost to the operation both in prep - work prior to drilling and in contingency costs if the drilling operation deviates from the original plan . in contrast , the embodiments disclosed herein provide a closed loop system that continuously receives feedback on the current properties of the formation ( i . e ., the formation hardness ) as drilling progresses . this eliminates the need for a preliminary “ mapping ” operation to find intervals with increased hardness . additionally , a deviation in the drilling operation is easily accommodated because of the closed loop system &# 39 ; s ability to provide feedback on the most current formation conditions . costs may be drastically reduced and conveyed to the customer . further , embodiments disclosed herein provide a drilling apparatus capable of maintaining a more constant rate of penetration (“ rop ”) regardless of the hardness of the material through which the drilling apparatus is passing . with only one set of cutter blocks , often the rop must be decreased in harder material to reduce premature wear of the cutters . in contrast , embodiments disclosed herein provide heavy duty backup cutter blocks to be used in harder formations , thereby allowing the rop to be maintained . this leads to faster and more efficient drilling and underreaming operations , saving the customer valuable money and rig time . moreover , embodiments of the present disclosure allow the bit and the cutter blocks to be used comparatively for the same length of time . it may be detrimental to have one component ( i . e ., the bit ) outlast the other component ( i . e ., the cutter blocks ) because then the drilling apparatus must be “ tripped out ” of the wellbore more often to replace parts . however , embodiments disclosed herein substantially reduce the chance that the bit will outlive the cutter blocks , thereby reducing tripping costs . finally , embodiments disclosed herein provide redundancy to the drilling apparatus , which is beneficial to reduce costs ( i . e ., if the primary cutter blocks fail , the backup cutter blocks may be used to complete the job thereby reducing the amount of tripping ). while the present disclosure has been described with respect to a limited number of embodiments , those skilled in the art , having benefit of this disclosure , will appreciate that other embodiments may be devised which do not depart from the scope of the disclosure as described herein . accordingly , the scope of the disclosure should be limited only by the attached claims .