Control And feedback Power source Sensor Measurand Primary
Control And feedback Power source Sensor Measurand Primary Sensing element Calibration signal Variable Conversion element Signal processing Output display Data storage Data transmission Perceptible output Radiation, electric current, or other applied energy Figure 1. 1 Generalized instrumentation system The sensor converts energy or information from the measurand to another form (usually electric). This signal is the processed and displayed so that humans can perceive the information. Elements and connections shown by dashed lines are optional for some applications. © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
Electrodes vecg Z 2 Zbody Z 1 +Vcc 60 -Hz ac magnetic field + Differential amplifier - Displacement currents vo -Vcc Figure 1. 2 Simplified electrocardiographic recording system Two possible interfering inputs are stray magnetic fields and capacitively coupled noise. Orientation of patient cables and changes in electrode-skin impedance are two possible modifying inputs. Z 1 and Z 2 represent the electrode-skin interface impedances. © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
(xd – Hfy)Gd = y (1. 1) xd. Gd = y(1 + Hf. Gd) (1. 2) (1. 3) (1. 4) (1. 5) (1. 6) (1. 7) (1. 8) © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
Characteristic with zero and sensitivity drift Total error due to drift y (Output) D x'd + Sensitivity drift D y' - Dy Intercept b Slope m = Dxd Dy Dxd + Zero drift - Sensitivity drift y = mxd + b xd (Input) (a) xd (Input) (b) Figure 1. 3 (a) Static-sensitivity curve that relates desired input xd to output y. Static sensitivity may be constant for only a limited range of inputs. (b) Static sensitivity: zero drift and sensitivity drift. Dotted lines indicate that zero drift and sensitivity drift can be negative. [Part (b) modified from Measurement Systems: Application and Design, by E. O. Doebelin. Copyright 1990 by Mc. Graw-Hill, Inc. Used with permission of Mc. Graw-Hill Book Co. ] © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
(1. 9) (1. 10) (1. 11) © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
x 1 Linear system y 1 (x 1 + y 2) and x 2 Figure 1. 4 (a) Basic definition of linearity for a system or element. The same linear system or element is shown four times for different inputs. (b) A graphical illustration of independent nonlinearity equals A% of the reading, or B% of full scale, whichever is greater (that is, whichever permits the larger error). [Part (b) modified from Measurement Systems: Application and Design, by E. O. Doebelin. Copyright 1990 by Mc. Graw-Hill, Inc. Used with permission of Mc. Graw-Hill Book Co. ] Linear system (y 1 + y 2) Linear system and y 2 Kx 1 Ky 1 Linear system (a) Least-squares straight line y (Output) B% of full scale A% of reading Overall tolerance band xd (Input) (b) Point at which A% of reading = B% of full scale © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
(1. 12) (1. 13) (1. 14) (1. 15) (1. 16) (1. 17) © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
Figure 1. 5 (a) A linear potentiometer, an example of a zero-order system. (b) Linear static characteristic for this system. (c) Step response is proportional to input. (d) Sinusoidal frequency response is constant with zero phase shift. © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
(1. 18) (1. 19) © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
Output y(t) R + + C x(t) Figure 1. 6 (a) A low-pass RC filter, an example of a firstorder instrument. (b) Static sensitivity for constant inputs. (c) Step response for larger time constants ( L) and small time constants ( S). (d) Sinusoidal frequency response for large and small time constants. Slope = K = 1 y(t) - - Input x(t) (a) (b) Log scale x(t) 1 Y (jw) X (jw) 1. 0 0. 707 y(t) S L t (c) w. L Log scale w (d) f y(t) 0° 1 0. 63 - 45° S w. S L L S -90° t © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998. Log scale w
(1. 20) (1. 21) (1. 22) (1. 23) (1. 24) © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
Figure 1. 7 (a) Force-measuring spring scale, an example of a second-order instrument. (b) Static sensitivity. (c) Step response for overdamped case = 2, critically damped case = 1, underdamped case = 0. 5. (d) Sinusoidal steady-state frequency response, = 2, = 1, = 0. 5. [Part (a) modified from Measurement Systems: Application and Design, by E. O. Doebelin. Copyright 1990 by Mc. Graw-Hill, Inc. Used with permission of Mc. Graw-Hill Book Co. ] Output displacement 0 y(t) Input Force x(t) Output y(t) Slope K = (a) 1 Ks Input x(t) (b) Y (jw) Log scale X (jw) K 2 x(t) 1 (d) y(t) yn 2 1 Ks -90° 1 2 f Log scale w wn 0° yn + 1 0. 5 1 wn t (c) Resonance Log scale w 0. 5 1 0. 5 t -180° © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
(1. 25) (1. 26) where (1. 27) (1. 28) © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
(1. 29) (1. 30) (1. 31) (1. 32) Overdamped, (1. 33) Critically damped, (1. 34) Underdamped, (1. 35) © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
and (1. 36) (1. 37) (1. 38) (1. 39) (1. 40) © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
Figure 1. 8 Design process for medical instruments Choice and design of instruments are affected by signal factors, and also by environmental, medical, and economic factors. (Revised from Transducers for Biomedical Measurements: Application and Design, by R. S. C. Cobbold. Copyright 1974, John Wiley and Sons, Inc. Used by permission of John Wiley and Sons, Inc. ) © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.
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