Introduction to Motion Control

Direct Computer Control, commonly known as DCC, is the process where Geomet controls the motion of the CMM to perform part inspections and higher level functions such as scanning operations. Every feature in Geomet can have motion attached to it. For example a Point would contain the necessary motion to traverse to the approach vector for the point to capture, slow to a probing speed and capture the data point then retreat to a stand off position. All the information for the moves is contained in a Motion Map which is attached to the feature.

Geomet initially creates the motion map through a process of "learning" as the operator measures the part being inspected the first time. On Helmel CMMs, this process can be done either using the Joystick controls, or by turning the motors off and moving the CMM by hand. All motion is considered Point-to-Point where every start and end point consists of one of the following point types:

IP points are XYZ positions captured during the self-teach process that are required to provide safe passage for the probe to clear obstacles. IP points are also are used to establish the vector approach for measurement data points. see figure 1.

figure 1, Motion Path

In the example above, we show a motion map moving from IP1, which corresponds to the retract position of the last feature, to the next Measurement Point (MP) of the current feature. The operator would capture the current position (IP1) by pressing the <IP> button on the joystick. The CMM would then be positioned to IP2 and again the <IP> would be pressed. This continues until position IP4 has been reached. The move after IP4 captures the data point MP.

The operator chose the path IP1 -> IP2 -> IP3 -> IP4 to allow safe passage for the probe  around the part. The motion defined as IP4 to the MP is considered the approach path. Geomet will calculate the Stand Off (SO) point based on the stand off distance defined in the DCC parameter settings. If the distance from IP4 to MP is greater than the stand off distance, the SO will be inserted as shown in figure 1. If the distance from IP4 to MP is less than the distance define as the stand off distance, then IP 4 becomes the SO.

There are two Stand Off points created, one before the MP and one after. Both of these SO point share the same XYZ location. The first SO is where Geomet will switch from Traverse Speed to Probing Speed. The second SO which follows the MP is the retract position and the completion of a normal feature motion map.

This point brackets the Measurement Point. The preceding SO will act as the location the motion of the CMM will slow to the Probe Speed. The SO that follows the Measurement Point is the location the probe will retract towards after the MP point has been captured.

The MP point is the point of contact where the probe contacts the part.

As the name suggests, the Auto-IP is calculated at the moment it is used during motion. This point is used when Geomet has an Clearance Coordinate System established. The Auto-IP is define not by an XYZ location, but by an axis of movement. If the Auto-IP is defined as using the Z axis, then the motion will calculate a direct path from the current location into the XY plane of the current Clearance Coordinate System. The actual motion will be parallel to the Z axis of the Clearance CS.

The teaching of the motion path can be accomplished when motors are on and the operator uses the joysticks, or when the motors are switched off and the operator moves the CMM by hand. Manual operation is often the preferred method of teaching due to the speed and ease of movement when the operator moves the CMM by hand. However manual or joystick motion can lead to erroneous IPs when moving the CMM at a high speed.

For example, the CMM interface is responsible to control all communications between Geomet and the PMAC motion controller. The interface polls the PMAC motion controller at a variable rate such as 5 times per second and transfers position and status data between Geomet and the PMAC motion controller with each cycle. The buttons on the joystick are interpreted by CMM interface and transferred to the motion controller at the interface polling rate. Therefore, when the IP button is pressed while moving 10 inches per second, the actual capturing of data can happen at worse case, 0.2 seconds later, or 2 inches after you pressed the IP button! The procedure of capturing an IP starts with the IP button press. Then the CMM interface acknowledges the button press and sets a flag to inform the motion controller to capture an IP and return the XYZ position. This requires one cycle. Therefore when teaching a motion path that requires positional accuracy, slow down or stop at the IP positions.

The PMAC motion controller maintains the current position of the CMM at rates higher than 8MHz per second. When a probe trigger occurs, PMAC latches that position in memory and waits for the CMM interface to cycle around before transmitting the positional data to Geomet.

Motion paths often require adjustments due to changes in the part or the need to adjust motion to eliminate accidental collisions due to a missing IP or other conditions. These adjustment tools are:

Motion Edit Tool	Allows full adjustment of the motion map by allowing access to adjust a all or a single motion point. Appending, Inserting and Moving of motion points can be done.
CMM Homing	On DCC style CMMs, a initialiation of the motion control system is required to establish a point of reference for the XYZ scales. This process is known as CMM Homing.
One-Step Edit	This tool will provide access to adjust motion attributes (speeds and distance) or move the entire motion map a desired relative distance.
DCC IP/MP XYZ Targeting	Accuratly position the probe under DCC control to capture MPs or set IPs.
DCC Settings	Provides access to the speed and distance parameters used in the building of a motion path.
Motion Path Tests	Building a motion path requires certain conventions to be followed. Should a common error, such as missing IP or SO points, or distance too small, the Geomet Motion Paths Tests will report common errors.
Point Targeting	In certain conditions it may be important to hit the exact position called for in the motion path. An example would be the capturing of a MP on a knife edge.
Clearance Coordinate Systems	Clearance Coordinate Systems (CS) are defined as an offset to an existing PCS and is used to provide safe transition planes for CMM movement.
Offline IP	The Offline IP generator allows the creation of Interim Points at the beginning of a motion map. These points can be created as a XYZ defined point or as a Auto-IP which utilizes Clearance CS for safe transition.

Point	Creates 1D, 2D and 3D Point Features in offline and interactive modes.
Line	Creates 2D Line Features in offline and interactive modes.
Circle	Creates 2D Circle Features in offline and interactive modes.
Cone	Creates a Cone Feature in offline and interactive modes.
Cylinder	Creates a Cylinder Feature in offline and interactive modes.
Plane	Creates a Plane Feature in offline and interactive modes.
Bolt Hole	Creates two or more 2D Circle Features surrounding a common center point in offline and interactive modes.

Geomet offers a test to ensure the motion map is consistent with the rules of motion as define by Geomet. The test definitions can be found here.