Title 40
SECTION 1066.235
1066.235 Speed verification procedure.
§ 1066.235 Speed verification procedure.(a) Overview. This section describes how to verify the accuracy of the dynamometer speed determination. When performing this verification, you must also ensure the dynamometer speed at any devices used to display or record vehicle speed (such as a driver's aid) is representative of the speed input from the dynamometer speed determination.
(b) Scope and frequency. Perform this verification upon initial installation, within 370 days before testing, and after major maintenance.
(c) Procedure. Use one of the following procedures to verify the accuracy and resolution of the dynamometer speed simulation:
(1) Pulse method. Connect a universal frequency counter to the output of the dynamometer's speed-sensing device in parallel with the signal to the dynamometer controller. The universal frequency counter must be calibrated according to the counter manufacturer's instructions and be capable of measuring with enough accuracy to perform the procedure as specified in this paragraph (c)(1). Make sure the instrumentation does not affect the signal to the dynamometer control circuits. Determine the speed error as follows:
(i) Set the dynamometer to speed-control mode. Set the dynamometer speed to a value of approximately 4.5 m/s (10 mi/hr); record the output of the frequency counter after 10 seconds. Determine the roll speed, vact, using the following equation:
Where:
f = frequency of the dynamometer speed sensing device,
accurate to at least four significant figures. droll =
nominal roll diameter, accurate to the nearest 1.0 mm, consistent
with § 1066.225(d). n = the number of pulses per revolution
from the dynamometer roll speed sensor. Example: f = 2.9231
Hz = 2.9231 s−1 droll = 904.40 mm = 0.90440 m 
vact =
8.3053 m/s
(ii) Repeat the steps in paragraph (c)(1)(i) of this section for the maximum speed expected during testing and at least two additional evenly spaced speed points between the starting speed and the maximum speed point.
(iii) Compare the calculated roll speed, vact, to each corresponding speed set point, vref, to determine values for speed error at each set point, verror, using the following equation:
Example:
vact = 8.3053 m/s vref = 8.3000 m/s verror =
8.3053 − 8.3000 = 0.0053 m/s
(2) Frequency method. Install a piece of tape in the shape of an arrowhead on the surface of the dynamometer roll near the outer edge. Put a reference mark on the deck plate in line with the tape. Install a stroboscope or photo tachometer on the deck plate and direct the flash toward the tape on the roll. The stroboscope or photo tachometer must be calibrated according to the instrument manufacturer's instructions and be capable of measuring with enough accuracy to perform the procedure as specified in this paragraph (c)(2). Determine the speed error as follows:
(i) Set the dynamometer to speed-control mode. Set the dynamometer speed to a speed value of approximately 4.5 m/s (10 mi/hr). Tune the stroboscope or photo tachometer until the signal matches the dynamometer roll speed. Record the frequency. Determine the roll speed, yact, using Eq. 1066.235-1, using the stroboscope or photo tachometer's frequency for f.
(ii) Repeat the steps in paragraph (c)(2)(i) of this section for the maximum speed expected during testing and at least two additional evenly spaced speed points between the starting speed and the maximum speed point.
(iii) Compare the calculated roll speed, vact, to each corresponding speed set point, vref, to determine values for speed error at each set point, yerror, using Eq. 1066.235-2.
(d) Performance evaluation. The speed error determined in paragraph (c) of this section may not exceed ±0.02 m/s at any speed set point.
[79 FR 23823, Apr. 28, 2014, as amended at 80 FR 9120, Feb. 19, 2015; 81 FR 74199, Oct. 25, 2016]