Linear Actuator Help

I bought a Concentric LACT12P-12V-20 Linear Actuator with Feedback - 12" Stroke, 12V, 0.5"/s and a jrk 21v3 motor controller and I power it with a 12VDC power pack. I tried the linear actuator with battery alone and it works perfectly. I connected the jrk, the Concentric and the 12VDC and I got all the blinking lights and interface graphics using the Pololu, jrk, 21v3, 12v12, umc01a, umc02a, umc download but the linear actuator wont budge. I tried configuring the Pololu, jrk, 21v3, 12v12 according to the instructions contained within this site but the directions seemed vague and in some cases items mentioned could not be found. The instructions contained in the pop ups were particularly perplexing. Anyone got a fool-proof way to configure the program for this novice? Thanks.


Did you follow the instructions on the bottom of the linear actuator product page? Can you elaborate on what you found confusing/incomplete and what you mean by “items mentioned could not be found”?

- Ben

Ben - Thanks for the info. No I did not dl the values and upload them to my program. The “items mentioned could not be found” statement will be in another post very soon.

I dl the values. It worked. WHAT A RUSH!!! Guess who is going to be buying a lot more from Pololu? Thanks for the help. I will post what I think were errors as I have not forgot about it.

I’m sorry, but shouldn’t I be in junior college learning Pololu robotics? Anyway, yesterday my linear actuator stopped prior to full extension but did retract completely. And yesterday I was not aware that my linear actuator stopped prior to full extension. Today, 09OCT12, my linear actuator did not completely extend but did completely retract. And on today, 09OCT12, using the “Motor” tab, the “Detect Motor Direction” tile, the “Invert motor direction” checkbox and the “Apply settings to device” tile, I did get full extension with a slight wobble as it stopped at full extension. At all times my linear actuator was in the horizontal position. At some point I got an error number that I forgot and a “Not Connected” notice that I quickly reset using the “Connected to” drop-down. I also got a “PID period exceeded: YES”. Can anyone be so kind and tell me what I must do to make my linear actuator function without flaw? I truely wish I were versed in math and physics so that I could troubleshoot the problems myself.

I deleted the Jrk download to duplicate conditions so that I could regain the errors and pop-ups but Pololu has written their program too deeply in my computer registry and my Jrk Configuration Utility appears with the “jrk21v3_linear_actuator_settings” loaded. But during today’s linear actuator experience I did receive one error message pop-up that I was unable to understand: “Center the output, then click OK”. This was what I was referring to. I racked my brain and strained my eyes trying to find anything faintly resembling an “output” so that I could “center” it. I could not.

I want to learn about my linear actuator so that I can use it without it stopping short of where I want it to.


The jrk’s settings are not stored in your registry: they are stored in EEPROM on the jrk itself so that your device can operate when USB is not plugged in. There is an option in the menus to reset the jrk to its default settings, if you want to do that.

The “Output” of the system is explained in the “PID Calculation Overview” section of the jrk user’s guide. In your case, the output is the position of the linear actuator, and you can manipulate it by connecting the linear actuator motor directly to a power source.


What does PID mean?

PID is in algorithm for performing closed-loop (feedback-based) control. Specifically, the algorithm output is determined by three terms: “proportional”, “integral”, and “derivative”. Each term is a function of the error (the difference between where you are and where you want to be), and the output is always trying to drive the error to zero. The three terms are each scaled by a constant (Kp, Ki, and Kd), and the values of these constants affect how well the algorithm works. When people talk about tuning the PID, it means they are choosing the appropriate PID constants for the particular system so that the performance is optimized (e.g. quick response, little-to-no overshoot, etc).

You might find the wikipedia article on PID helpful.

- Ben

Ben thanks for the Wiki. PID exceeded most helmsmen steer ships. That’s impressive. I open the Jrk Config Utility and I still don’t know what the “Manually set target (Serial mode only)” on the “Input” tab is for. All I know is that if I move the slider and press the “Run Motor” tile the linear actuator moves. And on the “Motor” tab the “Invert motor direction” function has reversed. Initially, I left the “Invert motor direction” check box blank to extend the linear actuator and I had to place a check in the check box to retract the linear actuator. 1.) What is the “Manually set target (Serial mode only)” for and what should I do with it? 2.) Why has the “Invert motor direction” check box reversed functions? I have searched for my answers and I cannot find any. I need a start to finish instruction manual, I guess. Thank you for any help that you may provide.

The “Manually set target” box allows you to issue a Set Target command to the jrk over USB. This sets the jrk’s “Target” variable, which represents the position that the user wants the system to go to. I recommend that you read all of the jrk’s user’s guide and in particular the section entitled “PID Calculation Overview”, if you want to understand what the Target variable is and why it is important.

I don’t understand your question about the “Invert motor direction” box. That box is fully documented in the “Motor options” section of the jrk user’s guide. Please read that section, and let me know if something is still unclear.


Mr. Grayson: One month ago I bought a Parallax motor and I emailed Parallax to find out how the motor was powered. Parallax said something about pulse and a remote control unit to fly RC flying devices. Parallax then recommended I buy their cheapest kit and go from there. Well, I didn’t and instead looked around and decided to buy from Pololu. THAT is the extent of my knowledge about RC and getting a motor to do what you want it to do. All the algebra and all the terms, abbreviations and instrutions mean nothing to me as a novice. Your instruction manuals are written for engineers, not novices like me. Your answers to my questions are never at my novice level so I cannot understand anything you say. Maybe you are kidding around. I am not kidding around. Forum members never seem to answer my questions. Perhaps you will reread my questions and post a more basic answer. I too have time constraints but I will ask again question number 2. Many days ago when I dl jrk and finally got my linear actuator to move, (I mean that right there is a highly technical name which is vague and misleading. Linear actuator? How about “electric plunger rod”?) I saw the check box that said “Invert motor direction”. I took a chance and checked the check box and hit the “Run motor” tile and it worked! The plunger retracted! But now, for some unexplained reason that confuses even you, the role of that check box has reversed. I now have to check the box to get my linear actuator to extend and uncheck the box to get it to retract. It is funny that for some reason you cannot understand my most basic language and explanation but unfortunately another attempt at explanation is just with more basic language. I hope you now see what I mean. NOW, I am not going to ask Pololu questions that I can answer myself. I just want to know the basics that will enable me to operate my linear actuator and know when any problem that I encounter is the system’s fault and not my own. I chose the feedback version so I can operate my linear actuator when it is out of my visual field. Thanks for listening.

I am sorry that you are having trouble understanding our documentation. I will do my best to answer any specific questions about sentences in our documentation that you don’t understand.

I try to answer your questions.

Setting the target lets you tell the jrk how what position you want the actuator to go to. Checking the Invert motor direction box is equivalent to swapping the wires of the motor: it causes the jrk to flip the direction it drives the motor.

In general, if you don’t understand an answer, you should ask a more specific follow-up question that lets us focus on what is confusing you.

I’m not sure if you are joking; how is the term “linear actuator” vague and misleading? Do you have a problem with the word “motor”?

Ok, that makes sense. Somehow when I read your earlier post I didn’t make the connection between question 2 and the other stuff you were saying earlier in the post. Thanks for explaining it again. You should not have to check the “Invert motor direction” checkbox and click “Apply settings to device” just to get the actuator to move in a different direction. Your goal should be to set up your system so that you can tell the jrk a target using the “Manually set target” box and it will drive the actuator to the corresponding position. The numerical value of the target should correspond to the position of the actuator. If you carefully follow the instructions at the bottom of the linear actuator product page that Ben linked to, I am confident you will be able to achieve this result. If any steps of those instructions are ambiguous or unclear to you in any way, please let us know.


Oooooooooooh. Gotcha. (Error free in twenty one three!) And stay off the motor tab!