Campbell Scientific CR10 CR10 Measurement and Control - Page 173

CR.1 O vx.\ HI OR tr\ V5 i\ I1., !-, - Cvv = SECTION 13. CRlO MEASUREMENTS SHITLD R5 .{' fn FTGURE 13.3-6. Resistive Half Bridge Connected to Single-Ended CR10 Input Ro, the source resistance, is not constant because Bp varies from 0 to 10 kohms over the 0 to 360 degree wind direction range. The source resistance is given by: Ro = Rc+(Ru(R.-R6+R)/(R"+R)) = R6+(Rp(20k-Rb)/20k) [13.3-12] Note that at 360 degrees, Ro is at a maximum of 6k (R6=10k) and at 0 degrees, Ro is 1k (Ru=O). lt follows that settling errors are less at lower direction values. The value of R6 for any direction D (degrees) is given by: R5(kohms) = (10kXDy360 [13.3-13] Equation 13.3-6 can be rewriften to yield the settling error of a rising signal directly in units of degrees. Error (degrees) = ps-U(Ro(Ct+CwL)) t13.3-141 Equation 13.3-12, -13 and -14 can be combined to estimate the error directly in degrees at various directions and lead lengths ffable 13.3- 3). Constants used in the calculations are given below: Ct = 3.3nfd Cw = 41 pfdlft., Belden #8771 wire t = 450ps TABLE 13.3-3. Settling Error, in Degrees, for 024A Wind Direction Sensor vs. Lead Length Wind Direction L=1000 ft. l-=500 ft. 360" 270 1 80' 900 66" 15" 45" 9o 21" 3o 4" 00 The values in Table 13.3-3 show that significant error occurs at large direction values for leads in excess of 500 feet. lnstruction 4, Excite, Delay, and Measure, should be used to eliminate errors in these types of situations. Using a 10 ms delay, settling errors are eliminated up to lengths that exceed the drive capability of the excitation channel (- 2000 ft.). 13.3.3 TRANSIENTS INDUCED BY SWITCHED EXCITATION Figure 13.3-6 shows a typical half bridge resistive sensor, such as Campbell Scientific's Model 107 Temperature Probe, connected to the CR10. The lead wire is a single-shielded pair, used for conducting the excitation (V*) and signal (Vr) voltages. When V* is switched on, a transient is capacitively induced in V., the signal voltage. lf the peak transient level, V*, is less than the true signal, V"o, the transient has no etfect on the measurement. lf V"o is greater than Vso, it must settle to the correct signal voltage to avoid errors. 13-7