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Switching Between Cams

In principle, you can switch between different cams at any time. However, you should consider some points:

  • In the cam editor, the position of the slave is defined uniquely as the function value of the cam function. This function is defined in the master value range and can be expressed as follows:

    SlavePosition = CAM( MasterPosition )

  • Because the current position of the master drive usually deviates from the master value range, you need to scale the master position in the definition range of the cam function in order to represent a valid argument:

    SlavePosition = CAM( MasterScale*MasterPosition + MasterOffset )

  • In a similar way, you need to scale the function value (the slave position) if the start of the cam in the mode Absolute would lead to a jump:

    SlavePosition = SlaveScale*CAM( MasterPosition ) + SlaveOffset

  • You may have to apply both scaling values, which results in the following:

    Slaveposition = SlaveScale*CAM( MasterScale*Masterposition + MasterOffset ) + SlaveOffset

  • The appropriate values for scaling and offset parameters can vary from period to period.

  • Switching between cams can be done in three ways:

    1. Start the second cam via a second instance of MC_CamIn with BufferMode = MC_BUFFER_MODE.Buffered, StartMode = relative, MasterAbsolute = FALSE, and SlaveAbsolute = FALSE.

      Important

      As of CODESYS SoftMotion version 4.17.0.0

    2. Start of the new cam by assigning MC_CamIn.CamTableID to the new cam (no restart of the MC_CamIn function block required).

      This variant is recommended if the following points apply to your use case:

      • All of the following cams should be executed relatively with MasterAbsolute = FALSE or SlaveAbsolute = FALSE.

      • The new cam should start exactly at the end of the previous one.

      • No other parameters are changed (example: SlaveOffset).

        Important

        • The jump, which is explained in the following example for periodic cams and SlaveAbsolute = FALSE, does not apply to this variant because the next cam is placed exactly at the end positions of the previous cam.

        • The new cam is placed exactly at the end of the previous cam only if the switch to the new cam is done in the same cycle when the previous cam reports EndOfProfile = TRUE.

    3. Start of the new cam by restarting the MC_CamIn function block.

      This variant is recommended if parameters have to be changed for the correct function of the new cam (for example, SlaveOffset from the following example). The restrictions of the following example apply.

Example 9. Example

In the following example, it switches from CAM1 to CAM2:

CAM1 consists of a 5th order polynomial followed by two line segments.

_sm_img_cam_switching_1.png

CAM2 consists of two line segments followed by one 5th order polynomial.

_sm_img_cam_switching_2.png

When switching between both cams, you should consider the following:

  • To prevent jumps, the values of velocity and acceleration at the end point of the first cam should agree with the values at the starting point of the second cam. In the example, this condition is fulfilled because the same velocity (=1) and acceleration (=0) is assigned to the end point of CAM1 and the starting point of CAM2.

  • You can start the second cam in Relative mode when you have defined the start position of the slave as 0. However, the first cam has to be running in non-periodic mode. Otherwise, if CAM1 were periodic, then the Relative setting would result in a jump.

_sm_img_cam_switching_3.png

The magnification shows the transition from CAM1 to CAM2. The blue lines mark the evaluations of the cam functions at the master positions x1 and x2.

Now, we will look at the unfavorable case of periodic:

MasterAbsolute := TRUE; SlaveAbsolute := FALSE;

CAM(x1, CAM1, PERIODIC:=TRUE);

The call starts an evaluation of the cam at the master position x1, which is less than the end position of the master of CAM1. Then CAM1 is evaluated by default and yields point 1 as the position for the slave.

CAM(x2, CAM1, PERIODIC:=TRUE);

For the following call of the module, the master position x2 is outside of the master value range of CAM1, whose limit is marked by the green dashed line and agrees with the horizontal axis of the point 3p. Therefore, the EndOfProfile is set. Because CAM1 was started in periodic mode, its restart occurs at the end of the value range, which finally yields the point 2p as the result of the module call.

CAM(EXECUTE:=FALSE);

Switch to the new cam

CAM(x2, CAM2, PERIODIC:=TRUE);

Second evaluation at master position x2. This time, the new CAM2 is evaluated. After CAM2 is started in Relative mode, the current slave position (2p) is added as offset to the image of the cam function of CAM2. This moves the starting point of its graph to the point 3p and its evaluation at the master position x2 yields the point 4p, and therefore an unfavorable jump.

Select the non-periodic mode in order to prevent jumps:

MasterAbsolute := TRUE; SlaveAbsolute := FALSE;

CAM(x1, CAM1, PERIODIC:=FALSE);

The call starts an evaluation of the cam at the master position x1, which is less than the end position of the master of CAM1. Then CAM1 is evaluated by default and yields point 1 as the position for the slave.

CAM(x2, CAM1, PERIODIC:=FALSE);

For the following call of the module, the master position x2 is outside of the master value range of CAM1, whose limit is marked by the green dashed line and agrees with the horizontal axis of the point 3n. Therefore, the EndOfProfile is set.  Because CAM1 was started in non-periodic mode, slave position (2n) assigned to master position x2 is identical to the position of the slave upon reaching the end of the value range of CAM1 (3n).

CAM(EXECUTE:=FALSE);

Switch to new cam.

CAM(x2, CAM2, PERIODIC:=FALSE);

Second evaluation at master position x2. This time, the new CAM2 is evaluated. After CAM2 is started in Relative mode, the current slave position (2n) is added as offset to the image of the cam function of CAM2. This moves the starting point of its graph to the point 3n and its evaluation at the master position x2 yields the point 4n, which is on the specific line through the points 1 and 3n.

To start the cam in Absolute mode, you need to make sure that the slave is in an appropriate start position. If the value range of the master agrees with the period of the slave, then switching between cams does not have any complications, regardless of whether the cams are periodic or not.

In the example above, you can start CAM2 in Absolute mode when the periods of the master and slave agree with the master value range of CAM2 (each is 360°).

If not, for example when the period of the slave is 270° (indicated by the light blue line), then the Absolute option is not permitted without taking additional actions.

_sm_img_cam_switching_4.png

In this case, the slave is at 90° when switching from CAM1 to CAM2. Starting CAM2 in Absolute mode causes a jump to 0° (indicated by a gray line).

However, the jump can be prevented by setting the slave offset to the appropriate value of 90°.



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