.PS
# https://tex.stackexchange.com/questions/575592/how-to-create-a-controls-system-diagram
gen_init
  linewid = linewid*0.8
  circlerad = 0.35/2
  bw = boxwid/2
  bh = boxht/2
Observer: [
  define(`thickarrow',`arrow thick 3 wid 7bp__')
  define(`thickline',`line thick 3')
  fillval = 0.6
  boxwid = 0.3
  boxht = 0.4
  circlerad = 0.1
  {"$r$" above ljust}
R: arrow
S: circle fill
  {"$-$" below ljust at S.s; "$+$" above rjust at S.w+(2bp__,0)}
{ OLP: box invis fill 0.9 wid 3.1 ht 1.4 with .nw at Here+(0.3,boxht*5/4)
  "\sf plant" at OLP.sw above ljust }
U: arrow right 0.4
  {"$u$" at last arrow+(2bp__,0) above }
Bp: box fill "$B$" 
  thickarrow right 0.4 
Sp: circle fill
  {"$\;+$" below ljust at Sp.s; "$+$" above rjust at Sp.w+(2bp__,2bp__)}
  thickarrow right 0.4
  {"`$\dot{\mathbf x}$'" at last arrow + (0,7bp__)} 
Ip: box fill "$I/s$"
  thickarrow right 1
  {"$\mathbf x$" at last arrow + (0,7bp__) }
Cp: box fill "$C$"
# print Cp.e.x - Bp.w.x + 0.2
  thickline right 0.4
Yp: thickarrow
  {"$\mathbf y$" at last arrow + (0,7bp__) }
Ap: box fill "$A$" with .n at Ip.s+(0,-boxht/2) 
  thickarrow <- from Ap.e right 0.4 then up Ip.y-Ap.y
  thickarrow from Ap.w to (Sp,Ap) then to Sp.s
# print OLP.n.y-Ap.s.y+0.1
#
K: box fill "$K$" at Ap + (0,-boxht*3/2)
  arrow from K.w to (S,K) then to S.s
  {OBS: box invis fill 0.9 wid 4.3 ht 1.8 with .nw at (S.w,K.s)-(0.1,0.1)
   "\sf observer" at OBS.sw ljust above } 
Io: box fill "$I/s$" at K + (0,-boxht*3/2)
  thickarrow <- from Io.w left 0.4
  {"`$\dot{\hat{\mathbf x}}$'" at last arrow + (0,7bp__)} 
Sxo: circle fill
  {"$\;+$" below ljust at Sxo.s; "$+$" above rjust at Sxo.w+(2bp__,2bp__)}
  thickarrow <- left 0.4
SLo: circle fill
  {"$\;+$" below ljust at SLo.s; "$+$" above rjust at SLo.w+(2bp__,2bp__)}
  thickarrow <- left 0.4
Bo: box fill "$B$"
  arrow from 1/4 between U.start and U.end up boxht \
    then left 2*boxht then down Bp.y-Bo.y+boxht then to Bo.w
Ao: box fill "$A$" with .n at Io.s+(0,-boxht/2) 
  thickline from Io.e right 0.4
  {thickarrow <-> from K.e to (Here,K.e) then to (Here,Ao) then to Ao.e
   thickarrow from Ao.w to (Sxo,Ao.w) then to Sxo.s }
Xhat: Here
Co: box fill "$C$" at (Cp,Here)
  thickarrow right 0.4
  {"$\hat{\mathbf y}$" at last arrow + (0,7bp__) }
  { thickarrow from Xhat to Co.w }
  {"$\hat{\mathbf x}$" at last arrow + (0,7bp__) }
Syo: circle fill
  {"$\;+$" above ljust at Syo.n; "$-$" below rjust at Syo.w+(2bp__,-2bp__)}
  thickarrow <- from Syo.n to (Syo,Cp)
Lo: box fill "$L$" with .n at Ao.s+(0,-boxht/2) 
# print Io.n.y+0.1 - (Lo.s.y-0.1)
  thickarrow from Syo.s down Syo.s.y-Lo.y then to Lo.e
  thickarrow from Lo.w to (SLo,Lo.w) then to SLo.s
]# with .nw at last [].sw+(0,-0.3)
# "(b) Single-input plant with feedback from a full-order observer" \
#   below ljust at Observer.sw+(0,-5pt__)
.PE