Speedometer Calibrataion

**goku** · October 7th, 2003 12:15 AM

meron bang shop na nagcacalibrate ng speedometer? is there such thing? What if you feel na di na tama yung reading ng speedometer mo or you change your tire size and you want to match the reading of the speedometer of your new tire size? :confused:

**Karding** · October 7th, 2003 02:30 AM

merong nag conduct ng ganyan sponsored by AAA sa US, ewan ko lang sa pinas.

anyway, calibration stuff:

Automated Calibration System Requirements

An automated speedometer calibration system contains a calibrated source to generate input signals for the speedometer, a sensing system to precisely determine the deflection of the needle, and a control system to adjust parameters that control the torque that causes the needle to deflect. The most sophisticated component in the application is the sensing system that reads needle deflection.

The most widely used sensing system for this application is a camera, an image acquisition board, and image processing software that searches the image for the needle. The system provides an output in terms of the angle of deflection from the start point or directly as speed in miles per hour (mph) or kilometers per hour (kmph).

Soliton’s customer wanted a machine vision system that would work with the more than 100 types of speedometers it presently manufactures plus future models. The speedometers vary in shape, types of graduations, needle designs, dial background designs, and nonlinear dial graduations. As a result, mapping between the deflection angle and the corresponding speed is not linear.

The mechanical speedometers vibrate in the fixture when subjected to significant electromagnetic forces during calibration. Consequently, the second major requirement was to create image-processing software that tolerated rotations and offsets in the meter image due to relative movements between the meter and the camera.

In some of the assembly lines, the manufacturer frequently runs small batch jobs where different speedometer models are produced in the same day. This means that the time to switch the system from one type of meter setup to another must be negligible. The calibration stations also needed to be networked so calibration data can be transferred to a central database for online statistical process control (SPC) analysis.

System Solution

Off-the-shelf stand-alone machine vision systems reviewed by the manufacturer did not meet these requirements. Soliton determined that a computer-based system was the right solution to provide the required features and flexibility.

The most challenging part was developing the image-analysis algorithm that could read any gauge and correct for rotations and offsets. Soliton needed a development platform that allowed quick prototyping and access to a large image-processing library.

The company chose LabVIEW™ and the IMAQ™ Vision Image Processing Toolkit from National Instruments to build the system. After extensive designing and testing, Soliton developed GaugeVIEW, which met all the customer’s requirements.

GaugeVIEW

After studying numerous speedometer types and analyzing the images using various algorithms, Soliton came up with the solution involving a five-step configuration process. Once configured, GaugeVIEW could read the speedometer and give the output in terms of speed (mph or kmph), even if the meters tested subsequently were not imaged under the identical conditions present during configuration.

GaugeVIEW can read the gauge as long as the image contains the complete gauge and is true, meaning the camera axis should be perpendicular to the plane of the dial surface. It correctly processes the image even if the meter is imaged upside down.

The algorithm tolerates variations in lighting, and the trade-off between insensitivity to lighting and accuracy can be selected during the configuration process. Other trade-offs between the speed of processing and accuracy also can be decided during the configuration.

A combination of pattern matching and reference pattern identification is used to determine the position and orientation of the meter in the image accurately. The needle pivot point (center) and the start position of the needle are identified on the image. Then some details about the needle are provided to the software, such as size and color (dark or light with respect to the background).

The needle-detection algorithm searches for the needle and gives its angle of deflection. The final step maps the degrees of deflection to speed, where the user simply clicks on the screen with a mouse to identify where 10 mph, 20 mph, 40 mph, etc. lie.

GaugeVIEW records the angle against the speed and automatically generates a polynomial to map the deflection angle to the speed. There is complete flexibility to specify the speed units (or any engineering units), and the user has control over the order of the polynomial, if he/she chooses to modify the default values.

The complete configuration process is presented via user-friendly screens, and a new gauge can be fully configured in about five minutes. Depending upon the demands of the application, the user can modify any of the default settings, such as scan resolution which will determine the trade-off between the speed of processing and accuracy.

On a Pentium III processor running at 700 MHz with 128 MB of RAM, Windows 2000, and LabVIEW 6i, a typical speedometer image is processed in 70 ms with a scan resolution of 1/3 degree. On typical speedometers, this translates to an accuracy better than 1/3 mph in the reading.

Calibration Control System

For a mechanical speedometer, the speedometer cable is driven by a DC servomotor whose speed can be fixed with an accuracy of better than ±1 rpm. A demagnetizer present in the fixture brings the permanent magnet in the meter to the required magnetization level from its initial overmagnetized state.

Soliton developed a fuzzy logic-based calibration system to control the demagnetizer. First, the DC servomotor is set to a specified rpm, and then a closed-loop control system, developed in LabVIEW, takes over. The controller reads the needle position through GaugeVIEW and applies current pulses to the demagnetizer until the needle deflection is brought to the right amount for the given input rpm.

In the case of an electronic speedometer, the mechanical input is replaced by a frequency input, which is generated using a counter/timer card in the PC. The input frequency is adjusted until the needle deflection is brought to the right level. Then a signal is given to the processor on the speedometer, which uses this information to calculate the internal calibration factors that it stores in its internal nonvolatile memory.

**OTEP** · October 7th, 2003 10:25 AM

I don't think there are calibration shops for speedos here. Even 4x4's running bigger than stock tires don't have calibrated speedos so the readings are way off.