Contact and Coil | Nearly In Control

TAG | motion-control

This information is for a Fanuc R30-iA, RJ3-iA or RJ3-iB controller but might work with other ones.

If you’re looking for a way to send the robot world TCP position (X, Y, Z, W, P, R) over to the PLC, it’s not actually that difficult. The robot can make the current joint and world position available in variables, and you can copy them to a group output in a background logic task. There is one caveat though: the values only update when you’re running a program. They don’t update while jogging. However, there is a work-around for this too.

First you should make sure that the feature to copy the position to variables is enabled. To get to the variables screen, use MENU, 0 (Next), 6 (System), F1 (Type), Variables.

Find this variable and set it to 1 (or True): $SCR_GRP[1].$m_pos_enb

The name of that variable is Current position from machine pulse

Now create a new robot program, and in it write the following:

GO[1:X POS]=($SCR_GRP[1].$MCH_POS_X*10)
GO[2:Y POS]=($SCR_GRP[1].$MCH_POS_Y*10)
GO[3:Z POS]=($SCR_GRP[1].$MCH_POS_Z*10)
GO[4:W ANG]=($SCR_GRP[1].$MCH_POS_W*100)
GO[5:P ANG]=($SCR_GRP[1].$MCH_POS_P*100)
GO[6:R ANG]=($SCR_GRP[1].$MCH_POS_R*100)

Note that I’ve multiplied the X, Y, and Z positions by 10, so you will have to divide by 10 in your PLC. Likewise I multiplied the W, P, and R angles by 100, so divide by 100 in the PLC.

To run this program in the background, use MENU, 6 (Setup), F1 (Type), 0 (Next), BG Logic. Configure it to run your new program as a background task.

Obviously you need to send these group outputs to the PLC. Ethernet/IP is great for this, but you can use hardwired interlocks too. You need to make sure that you have enough bits to handle the full range of motion. A 16-bit integer should work pretty well for all of these. Note that the robot will happily send negative numbers to a group output as two’s complement, so make sure you map the input to the PLC as a signed 16-bit integer (a.k.a. INT in most PLCs). For the X, Y, and Z positions, a 16-bit integer will give you from +3276.7 mm to -3276.8 mm of total range. For the W, P, and R angles you’ll get +327.67 deg to -327.68 deg. For most applications this is good (remember this is TCP, not joint angles). Please check that these are suitable though.

As I said, these numbers don’t update while you’re jogging, and won’t update until the robot starts a move in a program. One little trick is to do a move to the current position at the start of your program:


This starts sending the position without moving the robot. In my programs I typically enter a loop waiting for an input from the PLC, and inside this loop I turn a DO bit on and off. The PLC detects this as a “ready for command” heartbeat, and as long as the PLC sees this pulsing, then it knows the program is running and the position data is valid.

Another trick you can use is to detect when the robot has been jogged:


The name of this variable is Robot jogged. The description from the manual is: “When set to TRUE,the robot has been jogged since the last program motion. Execution of any user program will reset the flag.”

That’s how you get the world position of the TCP into the PLC. If you just want joint angles, you can use $SCR_GRP[1].$MCH_ANG[n] as the variable, where “n” is the joint number.

Important note: The I/O will probably change asynchronously to the program scan, so what you want to do is make a copy of the X, Y, Z, W, P, R values coming into the PLC and compare the current values to the values from the last scan. If they haven’t changed, then update your actual values, otherwise throw them away because they might not be valid. If you have a fast scanning PLC and I/O then you should still be able to keep up with the robot even during a fast move. If you have a slow scan time on your PLC, then you might only get valid stable values when the robot is stopped.

Now what if you want to know what the TCP position is relative to one of your user frames? The robot controller doesn’t seem to give you access to this, but the PLC can at least calculate the X, Y, and Z positions of the TCP in your user frame itself, given the world position and the user frame parameters.

First you need to find the accurate user frame parameters. Under the normal frames screen you can only get one decimal point of accuracy, but you need the full 3 decimal points to have your numbers in the PLC match the user frame position given in the robot. You can find these accurate positions in a variable: use MENU – 0,6 (SYSTEM) – F1 (TYPE) – Variables – $MNUFRAME[1,9] – F2 (DETAIL). The second index in square bracket is the frame number, so $MNUFRAME[1,1] is frame one and $MNUFRAME[1,2] is frame 2. Copy these numbers down exactly.

Here’s the math for calculating the TCP relative to your user frame. All variables are LREAL (which is a 64-bit floating point variable). I don’t know if you can use a regular 32-bit float or not. Result is your TCP in user frame. Point is your point in world frame (from the robot) and Frame is the accurate user frame data you copied from the $MNUFRAME[] variable.

Result.X_mm := Point.X_mm - Frame.X_mm;
Result.Y_mm := Point.Y_mm - Frame.Y_mm;
Result.Z_mm := Point.Z_mm - Frame.Z_mm;

RadiansW := DegreesToRadians(-Frame.W_deg);
CosOfAngleW := COS(RadiansW);
SinOfAngleW := SIN(RadiansW);

RadiansP := DegreesToRadians(-Frame.P_deg);
CosOfAngleP := COS(RadiansP);
SinOfAngleP := SIN(RadiansP);

RadiansR := DegreesToRadians(-Frame.R_deg);
CosOfAngleR := COS(RadiansR);
SinOfAngleR := SIN(RadiansR);

// Fanuc applies rotations WPR as W (around Z), P (around Y), R (around X)
temp := Result.X_mm;
Result.X_mm := Result.X_mm * CosOfAngleR - Result.Y_mm * SinOfAngleR;
Result.Y_mm := Result.Y_mm * CosOfAngleR + temp * SinOfAngleR;
temp := Result.Z_mm;
Result.Z_mm := Result.Z_mm * CosOfAngleP - Result.X_mm * SinOfAngleP;
Result.X_mm := Result.X_mm * CosOfAngleP + temp * SinOfAngleP;
temp := Result.Y_mm;
Result.Y_mm := Result.Y_mm * CosOfAngleW - Result.Z_mm * SinOfAngleW;
Result.Z_mm := Result.Z_mm * CosOfAngleW + temp * SinOfAngleW;

Note that DegreesToRadians() is just PI*deg/180.

Run that on your PLC and check that the values in your Result variable match the user frame TCP position reported on the teach pendant.

I haven’t gotten around to calculating the W, P, and R angles of the TCP in user frame yet. Currently I just look at W, P, and R in world frame if I need to know if I’m “pointed at” something. If you get the math to work for W, P, and R, I’d really appreciate if you could share it.

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Motion control is pretty complicated.

There’s been something really bothering me about the “integrated” motion control you find in PLCs these days (notably Allen-Bradley ControlLogix and Beckhoff TwinCAT). Don’t get me wrong, they’re certainly integrated far better than stand-alone motion controllers. Still, it just doesn’t “feel” right when you’re programming motion control from ladder logic.

When I’m programming a cylinder motion in ladder logic, I would typically use a five-rung logic block for each motion (extend/retract). One of the 5 bits is a “command” bit. This is a bit that means “do such-and-such motion now”. Importantly, if I turn that bit off, it means “stop now!” This works well for a cylinder with a valve controlling it because when I turn off power to that valve, the cylinder will stop trying to move. It would be nice if integrated motion was this simple.

It’s interesting to note that manual moves (a.k.a. “jogging”) are usually this simple. You drop a function block on a rung, give it a speed and direction, and when you execute it based on a push-button, the axis jogs in that direction, and when the push-button turns off, it stops jogging. Unfortunately none of the other features are that simple.

All other moves start motion with one function block and require you to stop it with another. The reason it works like this is because motion controllers also support blended moves. That is, I can first start a move to position (5,3) and after it’s moving there I can queue a second move to position (10,1) and it will guide the axes through a curved geometry that takes it arbitrarily close to my first point (based on parameters I give it) and then continue on to the second point without stopping. In fact you can program arbitrarily complex paths and the motion controller will perform them flawlessly. Unfortunately this means that 90% of the motion control logic out there is much more complex than it needs to be.

Aside: in object-oriented programming, such as in Java or .NET, it’s pretty normal to have to interface with a relational database such as MySQL or Microsoft SQL Server. However, when you try to mesh the two worlds of object-oriented programming and relational databases, you typically run into insidious little problems. Programmers call this the Object-relational impedance mismatch. I’m sure that if you added it up, literally billions of dollars have been spent trying to overcome these issues.

My point is that there is a similar Ladder logic-motion control impedance mismatch. The vast majority of PLC-based motion control is simple point-to-point motion. In that case, the ideal interface from ladder would be a single instance of a “go-to” function block with the following parameters:

  • Target Position (X, Y…)
  • Max Velocity
  • Acceleration
  • Deceleration
  • Acceleration Jerk
  • Deceleration Jerk

When the rung-in-condition goes true on this block, the motion control system moves to the target position with the given parameters, and when the rung-in-condition goes false, it stops. Furthermore, we should be able to change any of those parameters in real-time, and the motion controller should do its best to adjust the trajectory and dynamics to keep up. That would be all you need for most applications.

The remaining applications are cases where you need more complex geometries. Typically this is with multi-axis systems where you want to move through a series of intermediate points without stopping, or you want to follow a curved path through 2D or 3D-space. In my opinion, the ideal solution would be a combination of a path editor (where you use an editing tool to define a path, and it’s stored in an array of structures in the PLC) and a “follow path” function block with the following parameters:

  • Path
  • Path Tolerance

When the rung-in-condition is true, it moves forward along that path, and when it turns off, it stops. You could even add a BOOL parameter called Reverse which makes it go backwards along the path. The second parameter, “Path Tolerance” would limit how far off the path it can be before you get a motion error. I think this parameter is a good idea because it (a) allows you to initiate the instruction as long as your position is anywhere along that path, and (b) makes sure you’re not going to initiate some wild move as it tries to get to the first point.

A neat additional function block would be a way to calculate the nearest point on a path from a given position, so you could recover by jogging onto a path and continue on the path after a fault.

Obviously there needs to be a way for the PLC to generate or edit paths dynamically, but that’s hardly a big deal.

Anyway, these are my ideas. For now we’re stuck with this clunky way of writing motion control logic. Hopefully someone’s listening to us poor saps in the trenches! 🙂

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