Contact and Coil | Nearly In Control

Archive for September 2015

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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Automation and the Guaranteed Minimum Income

In recent years I’ve been interested in the effects of automation on our economy and our society. Throughout history every advance in technology has brought more wealth, health, and opportunity to pretty much everyone. With every revolution people changed jobs but their lives got significantly better. When farms mechanized, workers moved into the city and got factory jobs, and Henry Ford’s assembly lines made use of this labor to great effect.

Early factories needed labor in great quantities and as industrial processes became more efficient at utilizing labor, the value of human labor rose and the demand kept increasing. So did pay. Factory workers up through the 70’s could afford a nice house to raise a family, a big car, and even a boat or nice vacations. Since the 70’s however, the purchasing power of a factory worker or even a bank teller has been pretty flat. These are two professions that have seen the most advances in automation in the last 30 years, due to industrial robots and automated tellers. If automation makes workers more productive, why aren’t we seeing that translate into purchasing power?

There are two types of technological improvements at work here. A farmer with a tractor is very productive compared to one with a horse and plow. The displaced farm workers who went to the city were given the tools of the industrial revolution: steam engines, motors, pumps, hydraulics, and so forth. These technologies amplified the value of human labor. That’s the first kind of technological improvement. The second kind is the automated teller or the welding robot. The older technology adds value even to the lowest skilled employees, but the new technology is reducing their value and the new jobs require significantly higher skill levels. There’s something about this new revolution that’s just… different. The demand for low skill labor is drying up.

The increasing divide between the “haves” and the “have-nots” has been documented extensively. Some divide is good and promotes the economy and productivity. Too much separation is a recipe for significant problems.

I’m not the only one worrying about this issue, and as I’ve followed it over the last few years I’ve been surprised by the amount of interest in a Guaranteed Minimum Income or some such plan. Basically it involves getting rid of every low-income assistance plan such as social security, welfare, minimum wage laws, etc., and creating a single universal monthly benefit that everyone is entitled to. Some people are talking about a number as high as $24,000 per year per adult. Considering that the 2015 federal poverty level in the US is just below $12,000 for a single adult, you can see that $24,000 per adult isn’t just a trifling amount.

For comparison, a little Googling tells me that the US GDP per capita is around $55,000. Think about that for a second. You’re talking about guaranteeing almost 45% of the productivity output of the country to be distributed evenly across all adults. One presumes you would also provide some extra money per child in a household, but to be fair the “per capita” figure includes kids too. It’s possible. Sure seems a bit crazy though.

Is it practical? Won’t some people choose not to work? Will productivity go down? It turns out that we’ve done some experimenting with this type of program in Canada called MINCOME. The results were generally positive. There was a small drop in hours worked by certain people, mostly new mothers and teenagers. These costs were offset in other areas: “in the period that Mincome was administered, hospital visits dropped 8.5 percent, with fewer incidents of work-related injuries, and fewer emergency room visits from car accidents and domestic abuse.” More teenagers graduated. There was less mental illness.

I’m fiscally conservative, but I’m mostly pragmatic. It’s only my years of exposure to automation, technology and working in factories that makes me ask these questions. Not only do I believe that people should contribute, I believe that people need to contribute for their own happiness and well-being. That’s why I don’t think paying people to sit at home is the ultimate solution.

The elephant in the room is this: as technology improves, a greater proportion of the population will simply be unemployable. There, I said it. I know it’s a disturbing thought. Our society is structured around the opposite of that idea. Men are particularly under pressure to work. The majority of the status afforded to men in our society comes from their earning potential. The social pressure would still be there to work, even as a supplement to a guaranteed minimum income, so we still need to find something for those people to do. Perhaps if we expand the accepted role of men in society then we can fill that need with volunteer work. Maybe.

What’s the right answer? I don’t know. For lack of a better term, the “American Dream” was accessible to anyone if you were willing to work hard and reinvest that effort into yourself. Not everyone did that, but many people created significant fortunes for themselves after starting in the stockroom and working their way up. That security gave people a willingness to take risks and be entrepreneurial. Proponents of the idea say that a minimum income would bring back that innovative edge. Entrepreneurs could try new ideas repeatedly until they found one that worked, and not worry about their family starving. With your basic necessities met, you could start to realize your potential..

I do know that as we continue down this road of increasing automation, we can’t be leaving a greater and greater proportion of the populace without the basic resources they need to survive. Do we expect them to grow their own food? On what land? Do we expect them to do a job that I could program a robot to do, if the robot’s average cost is only $10,000/year? Do you have some valuable job we can retrain them to do? One that pays enough to support a family?

Look, I don’t like the alternatives either, but it’s better than an armed revolt.

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