Ahstmet.--4 inixed sensitivitj-'FIX control law. and it,s roiist~ailt approxiiilatioi1 are proposed. Linear Tiine hivarialit (LTI) Multi-Input Multi-Out,put (hIIhIO) systems are considered. The Const,ant Xpproxinlation of a Mixed Sensit,ivit,y ' H , control law (CoXhISe) is based on asj-inpt,ot,ic Bode approxiinations of low frequency. Necessary and sufficieiit st,ahility coiidit~ioiis are gix-ell for bot,h coiitrol lan-s applied to the LTI SIISIO syst,eiii. Ail ilnpleiiieiit~at~ioii of t,lie CoASISe for t,lie speed colitrol of a DC inot,or laboratorj-prot,otj-pe. is present,ed. The inlpleinentation is realized in a Prograinnlable Logic Colit,roller (PLC) of low colnputatiolial resources. Also. a Linear l I I l I O uilst,ahle model of a nlecllanical syst,ein. illust,rates t,he result,^. Keyu~ords.-Robust st,ahilitj-. robust performance. strong stability. stabilizing con~pensators. duality. pseudo inverses. t,rackingWe colisider robust coiiipelisat~ors for syst,eiiis under admissible external dist,urhances and uncertainties. and freyuelicy design specificatiolis wliicli give iiiore insight for the engineer [11]. The problems of robust stabilit,y and perforlnaiice call he coiiserx-at,ix-ely coiiibiiied oil a single 'FIX ~ioriii specificat,ioii [Id]. i. e.. t,lie mixed seiisitivitj-problem. and solx-ed using classical algorit,hnls [3]. In [5] and [7] the inixed sensiti~ity problem is solx-ed using an intersection function of the 'FIX norm of the sensitivity and coiiiplelneiitary seiisitivity fulictiolis. without an augmented system. and allowing to fix t,he free paraineter of the stabilizing coinpensat,or [11][12] t,o a constant value. These allows to get low conlplexit,y coinpensators. The 'FIX cont,rol law of [7] solves t,he inixed sensit,ivit,y prohleiii through a pre-colnpelisat,or aiid its dual post-coinpensat,or. in a control-observer scheme. Suficient st,ahilitj-condit~ions are gix-en in [7] for t,lle ox-era11 sj-st,ein. The four-parameter tuning procedure of [7] iinproves the perforiiialice. Tliis ~net,liodology was test,ed ill a t,hree-tank-system laboratory prototype ill [8].Lon-order coiiipeiisators for LTI systems liax-e beell dex-eloped in [I]. because they are easier to analyze and to evaluate and for reasons of reliahilitj-. cost and less coinput~at~ional effort. This reduction of conlplexit,y allows for a inore silit,aI)le iinpleiileiitatioii. I~erause t,he fast dylialnics of t,lie colnpelisat,or (high gaiiis) are cut, off. Then. it is possible to increase t,he sampling period and to avoid sat,uration of the actuat,ors. There are at, least t,hree inetllods t,o design a low order coinpensator for a liigli order plant,. The first oiie is to approximate the plant aiid t,heii desigii t,lie coiiipeiisat~or. The effects of t,he approxilnatioii call he propagat,ed through t,lie design process. and a sat,isfact,ory approxiination of the plant requires some knowledge in adx-ance of the coinpelisat,or [4] leadiiig to all it,erative process. 111 t,he direct second one. the parameters of the low order coinpens...