NX Style Encoder

General Definition

Topology

An NX encoder pattern has N teeth distributed equidistantly about a rotating target. The 4X and 3X encoders are examples of NX encoders. MotoHawk™ nominally associates N with a 360degCA revolution. A 3X encoder pattern is illustrated.

 

Interpolation Impacts

The NX encoder patterns typically has few reference points. Consider 3X which has 120 crank angle degrees between teeth. Interpolation is used to schedule events like ignition at crank angles that do not correspond exactly with the described teeth. Because there is so few reference points (teeth), interpolated angles that are not scheduled near a reference tooth are likely to be quite inaccurate, especially when the system is exposed to strong acceleration or deceleration. The event 1 described by figure below will be less accurately positioned in the angle domain than event 2 if the period described by 'X' is significantly different than that described by 'Y'

 

Camless Operation (TDC Synchronization)

Camless operation on an NX style encoder utilizes a TDC based synchronization technique. Each tooth in an NX encoder is assumed to describe a cylinder's firing TDC. Thus a 3X pattern running camless is assumed to be in use on a 6 cylinder 4-stroke engine.

The encoder system is unable to associate specific tooth with a particular engine TDC because there are no distinguishing identifiers encoded within the pattern. Thus the MotoHawk™ encoder system must make the assumption that the first observed encoder edge will correspond to firing TDC#1. This assumption is unlikely to be correct, but an ignition system that employs a distributor and uses banked fuel injection does not need to be any more accurate than knowing position relative to a firing TDC.

Cam Synchronization

What is Cam Synchronization

The NX encoder patterns like 3X and 4X are not able to truly operate a sequential engine management system because the pattern only provides relative instead of absolute crank angle positional information. Moving to sequential control means that the TDC synchronization scheme is no longer appropriate because it makes an assumption that the system is unable to now tolerate. If the same crank encoder system is to be employed then a separate mechanism must be used to provide the synchronization. One such mechanism is to use a cam sensor in addition to the crank position sensor. This is referred to as cam synchronization.

Synchronization with a Tooth Cam

The cam has an absolute relationship with the crank position teeth, and is also periodic like crank. Thus, if a specific portion of the cam pattern can be identified, then this can be related back to crank position.

Typically the cam pattern is a single tooth so the point where this tooth is recognized represents absolute crank position. The figure below illustrates for a 6-cylinder 3X encoder system that has a simple tooth on the cam. The cam sensor recognizes a cam tooth at 'A' and thus can inform the crank position system that the next observed crank tooth actually represents encoder TDC #1. The selection of what point 'A' actually represents is arbitrary since it represents an encoder relationship, which translates into engine position via the Encoder to Engine TDC offset attribute.

 

Implementation assigns the first observed synchronous edge after a cam synchronization point on the as encoder TDC#1. The physical relationship to engine TDC#1 is described by the encoder offset. The scheme is applicable for any number of teeth subject to meeting the cam tolerance constraints.

Synchronization with a Halfmoon Cam

Halfmoon synchronization is similar to Tooth Cam Synchronization except it provides two synchronization points because the pattern has two edge events that will always map to different crank teeth. Thus a cam synchronization event can be issued to the crank encoder system on each observed cam edge. Cam event 'A', which is the rising edge in the figure, is associated to encoder TDC#1, whilst the falling edge of cam event 'B' is associated with encoder TDC#4.

 

Synchronization with Hyundai 4X Cam

The 4X Hyundai style cam is similar to Tooth Cam Synchronization except the "tooth" refers to the wide tooth in the pattern. Set the Cam Sync Polarity to the polarity of the cam edge that is equildistant. In the figure it is the falling edge.

 

Synchronization with 1X Plus 1 Cam

The 1X Plus 1 cam is similar to Tooth Cam Synchronization except the "tooth" refers to the tooth that precedes the additional tooth that is annotated in the figure. The figure also illustrates operation for two different crank polarities. The Cam Sync Polarity will define what crank tooth is associated with TDC#1.

 

The "+1 pulse edge" annotation on the cam must be located within 135° of the cam's "Sync Edge" annotation. Higher tooth count crank encoders will allow the 135° limit to approach 150°. The "Sync Edge" of the figure must always be associated with the same crank tooth when considering cam location tolerance for the 1X Plus 1 Cam.

NX Cam Location Tolerance

The location of the cam sensor and its target relative to the crank sensor and its target must normally meet the electrical relationship depicted by the figure for all operating engine speeds. Note that the setup and release times of these sensors will need to be considered because it is the electrical relationship that is important. The cam synchronization edge or edges must always fall between the same two crank tooth detection edges. Each cam edge will be associated with a specific crank tooth and so this relationship must not change while running. Thus the shaded region of the figure must never overlap a synchronous crank tooth edge. Additionally the shaded region shall cease at least 100us before the synchronous crank edge would be detected.

Cam Aligned With Crank Attribute

Check the Crank Aligned With Cam attribute if the synchronous edge of the crank encoder falls within the restricted region that is too close to the synchronous edge of the cam (as detailed in NX Cam Location Tolerance) and MotoHawk™ may be able to compensate for it if the target module is an ECM112 variant.