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DMI Accuracy Explained

For reliable measurements, DMI accuracy is of utmost importance in surveying. There are several factors that effect the accuracy of a DMI:
  1. Type of sensor used
  2. Computational and Rounding errors
  3. Tire slippage and pressure

Sensor Types

There are several types of sensors that can be used to interface with a vehicle-installed DMI. Different types of sensors have been developed in an attempt to address ease of installation and/or accuracy.

1. OBD Sensor - This sensor plugs into a vehicle's OBD port and to the DMI.  This type of sensor was recently introduced in an attempt to make installation easier and faster.  The OBD sensor relies on the OBD diagnostic information being exchanged between the vehicle and the OBD sensor.  Although relatively easy to install, this type of sensor suffers inaccuracies caused by slow movement, usually at the end and beginning of the trip.  For this reason, Microdynamics does not recommend using this sensor in any application requiring accuracies better than 50 ft. / mile.

2. Magnetic Sensor - This type of sensor senses axis rotation by sensing magnetic poles which are installed to a vehicle's axis.  Requiring an installation of a mounted magnetic sensor assembly and attaching several magnets to an axis, this installation is the most tedious and time consuming of the different sensor types.  While more accurate than the OBD sensor, the accuracy of this sensor is highly dependent on the proper spacing of the magnets and is subject to failure in the case any magnets detach from the axis.

3. VSS Wired Connection - This sensor connects directly to a vehicle's speed sensing wire such as a VSS feed to a speedometer/odometer, speed signal to the braking system, or other type of VSS wire.  Microdynamics believes this is typically the most accurate type of sensor as it does not rely on the presence of user-installed magnets or sensor assembly.  It also offers easier and faster installation than the Magnetic Sensor, as it simply requires a one-wire connection to a spliced wire in most vehicles.

 Accuracy
Installation
 OBD Sensor
Poor
Easy
 Magnetic Sensor
Good*Difficult
 VSS Wired
Excellent*
Medium
                                                                                            * In most cases

Computational and Rounding Errors


The DMI has to process the incoming pulses being received either from an external source such as a sensor module or a direct connection to an analog sensing source.  The distance computation involves the pulse count and calibration number, both of which are critical to the accuracy of the measurement.  The calibration number is previously computed by traveling a precisely marked distance, and calculating the calibration number based off the distance traveled and pulse count.  The fractional precision of the calibration number is vital to the accuracy of the DMI.  Most other DMI's such as the Jamar RAC Plus I/III and Numetrics Nitestar NS-50/NS-60 utilize a calibration number significant to only 3 decimal places.  On the other hand, the Microdynamics DOT-Z1 utilizes a calibration number significant to an extra 4th decimal place, which results in 10 times better accuracy.  This loss of accuracy can be realized in the following graph, which illustrates the accuracy in terms of feet per mile based on calibration number for both 3-digit and 4-digit fractional precision.


As an example, a vehicle with a calibration number close to 0.50 can suffer 5 feet/mile accuracy loss when using a 3-decimal place calibration number, compared to a potential accuracy loss of only 0.5 feet / mile when using the additional 4th-decimal place of precision.

Furthermore, some DMI's require the use of an additional divider, which reduces the number of meaningful pulses being sent to the DMI, further reducing the theoretical precision and accuracy.

Tire Slippage and Pressure

Another source of inaccuracy is the possibility of tire slippage, which can occur during abrupt acceleration or breaking, especially if the road conditions are wet.  This is because the vehicle's wheel doesn't roll the entire exact length of the survey, causing some pulses to be missed that would have otherwise been detected.  For this reason, Microdynamics recommends accelerating and braking slower during wet road conditions.

Additionally, it is highly recommended to recalibrate the DMI when the vehicle has gotten new tires, tire pressure adjustment, or if the ambient temperatures have changed drastically since the previous calibration attempt.  This is due to the tire-diameter change that happens during pressure or temperature changes.


Jamar RAC Plus III/GPS is registered and owned by JAMAR Technologies, Inc.  Nu-metrics Nitestar NS-60 is registered and owned by Vaisala Corporation.  All information is assumed to be correct as of July 18th, 2011. Microdynamics assumes no liability in the accuracy of this information, and we urge customers to do their own research.