N Teeth Plus 1 Encoder Style

General Description

The N teeth plus 1 encoder style describes those encoder patterns that consist of N equidistant teeth distributed about a wheel with one additional tooth located midway between two equidistant teeth. The 36 plus 1, and the 6 plus 1 are supported examples of this encoder.

Synchronization

The additional tooth is used to communicate synchronization information. Speaking generally, the additional tooth is detected if the time from the last synchronous crank encoder edge to the last detected edge is less than half the previously observed crank tooth period.

A crank synchronization error will be flagged if, upon detecting the additional tooth, the number of real teeth previously observed was incorrect. Thus the 36 plus 1 pattern would issue a synchronization error if 36 normal teeth were not observed between observing the an "additional" tooth and the next observed "additional" tooth.

A resynchronization can temporarily impact the behavior of crank-synchronous blocks. Impacts include a pulse not scheduling as expected, having it occur twice, or being malformed. The system will recover, but the initial impact is unavoidable because the correction to the crank angle position results in a step change in position that may disrupt the underlying pulse scheduling software.

Tooth and Halfmoon Cams

Tooth Cam No Go Region

The Tooth Cam and the Halfmoon Cam are both considered to be of tooth cam style. Neither edge of the halfmoon cam nor the synchronous edge of the tooth cam should fall within the additional tooth region for all engine operating conditions. Unreliable crank encoder synchronization may occur (which will lead to encoder errors) if an edge transition were to occur within this region. Best practice is to not locate a significant edge between the equidistant teeth that encompass the additional tooth.

Halfmoon Cam Width Tolerance

The width of a half moon cam should approximate 360 crank angle degrees. That is, a cam edge should be observed approximately every 360 crank angle degrees. The tolerance will vary depending upon the number of teeth in the system. For a 60 tooth crank encoder, consecutive cam edges shall be:

Utilizing a Tooth Cam configuration in place of the halfmoon will still allow a fully featured crank-synchronous model to be developed. Therefore a Tooth Cam can be used in place of a halfmoon cam if there is an issue with the width. Cam errors will be reported if a halfmoon is in use that does not conform to the width restrictions.

Halfmoon Cam Sensor Assumptions

The halfmoon cam arrangement assumes that a zero speed sensor is in place. Such a sensor is able to reliably report the presence or absence of metal with minimal to no rotation. A halfmoon should be treated as a tooth cam if such a sensor is not available.

Crank Teeth from Cam Edge to Crank Tooth Zero Setting

The crank teeth from cam edge to crank tooth zero setting is used to assist ControlCore in making an informed guess as to what the half cycle state is when it first observes missing teeth. Correctly setting this value will improve start quality. The setting is ignored when it has a value of -1. Instead ControlCore determines an appropriate value that won't be optimal. A positive value implies that the setting is to be used and communicates to ControlCore the number of crank encoder teeth that need to be observed from speed zero before the system can categorically conclude whether the cam edge would have been observed if it was going to occur prior to the observed missing teeth.

The figure illustrates a setting of 3 teeth. If at least 3 teeth had been observed before the first missing tooth is observed (from speed zero) and a cam tooth has not been observed then the system knows that the missing tooth corresponds to the half-cycle that does not have the cam tooth. The system can't make that assumption if 2 or fewer teeth had been observed before the first missing tooth.