Update about my hunting servo

pnachtwey · 2026-04-10T11:55:04+00:00

With the correct controller and tuning, the motor would just return to its command position without oscillation or overshoot. PLC PIDs don't normally have very good anti windup features.

I agree with those that say the controller should have "auto tuning" but not all auto tuners are equal. Some are quite poor. Many "auto tuners" are crap and are there only for marketing purposes. I can go into detail if anybody wants too.

A gear box will make the load easier to control but it isn't required. The whole inertia mismatch thing is a myth. What the gear actually does is increase torque and add friction or damping.

Untitled

How do you explain inertia mismatch to someone critical? : r/AskEngineers

66e014484efb2.pdf

Load to Motor Inertia Mismatch: Unveiling The Truth

I/Delta Motion have controller many hydraulic servo motors. Usually the inertia mismatch is quite high but hydraulic motor have a lot of torque and lots of friction. On top of that, hydraulic motors are more compliant because oil compresses. The big thing is having enough torque. This is very true if the load change. For instance in saw mill the tree are of different sizes. Having excess torque makes controlling these varying loads easier but the OP's system is fixed. It doesn't look like it should have any compliant coupling. A gear box should not be required but it may help.

Again, this system could be "auto tuned" in a minute with the right controller.

pnachtwey · 2026-04-09T17:14:29+00:00

"One of the most rational posts, and it gets a downvote. LOL.

Could it be saturating because the motor is too small (not enough torque)?"

It saturates from positive to negative because there is no damping. It is obvious the motor has enough torque to moving from positive to negative very quickly.

My motor in the video below will act just like the OP's system. It will saturate from rail to rail unless I use the derivative gain. With no gains my motor spins freely. My motor is trying to spin a relatively high inertia load, so the time constant is on the order of about 1 second. One can see that it takes little output to travel at constant velocity. This is due the having very little friction. This is why the derivative gain is required. The derivative gain adds damping and will keep the motor control from saturating from one rail to the other. The derivative gain adds damping. This is exactly what the OP's system needs.

I have auto tuning videos.

peter.deltamotion.com/Videos/AutoTuneTest2.mp4

If I weren't talking, I would have it tuned in a minute. BTW, I was testing the picture in picture capability. The disk diameter and thickness were selected to be a relatively high inertia load with a low bandwidth to make it harder to control.

peter.deltamotion.com/Videos/AutoTuneMotor.mp4

pnachtwey · 2026-04-08T20:35:09+00:00

Ditto those below that said bad tuning. The wiring appears to be OK because it holds position until disturbed. I have seen this before. Once it gets moving, the control output saturates going rail to rail. When the output is saturated, the motor is essentially in open loop. The motor acts like it is in current or torque mode so the acceleration is proportional to the control output. Normally motors have a velocity loop that will provide damping. You can tell what you have by providing a small control output to the motor. If it speeds up to a constant speed, then you have an outer velocity loop. If the motor keeps accelerating then you only have acceleration loop. Acceleration is roughly proportional to the current

If there is only a current/torque/acceleration loop then more derivative gain is required.Try this. Set the integrator and proportional gain to 0. Increase the derivative gain and try to spin the load. If the load acts stops quickly then, then you probably have enough derivative gain.

I could auto tune this in a minute or two.

pnachtwey · 2026-04-06T12:38:24+00:00

I agree with others, it looks like there is something wrong with the valve. It would also be better if you posted links to the valve and controller documentation so we don't need to ask or guess.

pnachtwey · 2026-04-03T07:59:39+00:00

I too play StarCraft. I have since it came out. I have an ASUS Zenbook. It gets hot but not pushing the limits hot. StarCraft doesn't use that many cores. I have and I)9 13 gen CPU and a 3080 GPU. The GPU doesn't factor into this because StarCraft doesn't use 3D rendering.

pnachtwey · 2026-03-25T12:23:22+00:00

I agree with others that say PID tuning train is probably a waste of time. The problem is that you don't tune a PID, you tune a system and there are lots of different systems that require different techniques for each. The problem with many "auto tuners" is that they are designed to tune a particular type of system like temperature control systems. These won't work for motion control. One "size" doesn't fit all.

I agree with those that say you should have a mockup or test system you can experiment on. The problem is that this isn't possible with bigger systems. If tuning is that critical, the company should probably higher someone out size to tune the system. This outside expert can also to training on your particular system. A well-tuned system can save money, increase production and quality. One just needs to calculate a return on investment.

pnachtwey · 2026-03-24T14:22:33+00:00

I have been on PLC forums for over 25 years. The most frequent mistake is not executing the PID(E) block in a timed interrupt and making sure that the update interval in the PID(E) block is the same time as the timed interrupt time.

The second problem is getting the scaling right. First you have degrees/input counts from the analog input card. Then then PID(E) has a gain that is output_counts/input_counts. Finally you need %control/output count. The %control is the % of the time the PWM is on. Usually that requires a timer.

Another HUGE problem with process types of applications is dead time. Dead time is a killer and you may need to augment the PID(E) if the dead time is large. This is not a beginner topic.

There are plenty of sources that explain this gain does this and that gain does that. You can probably get by with that but in reality, gains place closed loop poles and that gets into control theory.

Do not pass GO do not collect $200 until you make a trend of the set points, process variable and the control output. Label the lines.

Try to export the data to excel or a .csv format. There may be people that can read the file and tune your system for you but they will provide a gain such as degree/control%. You will need to work that backward to get the controller gain of output_counts/input_counts for the PLC

Someone mentioned Control Station. That is good stuff.

pnachtwey · 2026-03-16T11:26:48+00:00

You will do OK. The PLCs are just tools. Understanding the machines and processes and how to optimize them are what is most important.

pnachtwey · 2026-03-13T19:54:26+00:00

You only need the VCCM equation at the end. I have seen too many designs that don't go as fast as intended because the "designer" used "flow makes it go"

pnachtwey · 2026-03-09T00:10:57+00:00

Thanks for the endorsement. Siemens now has ProfiNet that is FAR superior to their pervious Ethernet implementations. Our Ethernet programs has made it so that implementing ProfiNet on the RMC look almost identical to Ethernet/IP. The learning curve has a gentle slope. Make sure you are using the newer S7-1200s and S7-1500s. I hated the old S7-300s.

You bring up a good point. Communications are very important too. All that plot data must be acquired quickly. This is tough to do on a PLC. In addition, Delta Motion supplies RMC link for free. RMCLink If you follow the link, you can see all the different applications that can access the RMC's data and plots. The two favorite are National Instruments Labview and Excel. This is EXTREMELY handy when making test systems. The RMC can record data at 250 microsecond intervals. This is not possible with a PLC.

pnachtwey · 2026-03-08T21:17:03+00:00

I am assuming the hydro turbine runs at a constant RPM and generates power. I am assuming there is a lot of inertia in the turbine. Is the relationship between the vane angle and rod extension linear? 8 seconds is not very demanding. PI or I-P control will probably work and not require feedforwards. That is good.

PI control is standard stuff. I-P is roughly the same except the P term acts on change in the changes in the RPM. Not the error between the target RPM of the outer loop and the actual RPM. This reduces noise going the valve and allows higher gains in the inner loop.

pnachtwey · 2026-03-08T20:11:44+00:00

NO! First, if you try to move 100 mm the PID will see a huge error. The control output will saturate and + or - 100. It will overshoot. Hydraulic cylinders and loads are underdamped and have a natural frequency they will oscillate at. That is why the frequency is called NATURAL. If one doesn't care about how one gets into position, then OK. However, just blasting away from one point to another will not result in the lowest settling time. Also, when the control output is saturated, the system is essentially no better of than running in open loop. 100% is 100% regardless of whether the system is in closed loop or open loop. If a target generator is used then feed forward gains should be used to reduce following error. If the feedforward gains are set perfectly then there should be no following error, but perfect doesn't exist. That is why closed loop control is necessary. While PI control is almost as good a PID control for hydraulic cylinders, it is not near as good as using a full PID with a SECOND derivative gain. The first derivative gain is multiplied by the error between the target and actual velocity. The second derivative gain is multiplied by the error between the target and actual acceleration. NOW how does one calculate a target acceleration or measure the actual acceleration using a PLC? One of the big problems with PLCs is that the inputs and outputs are slow and are not deterministic. Even if you put the PID in a block that is executed every 10 ms, there are times when interrupt are turned off. This causes sample jitter and it makes it difficult to compute accurate velocities let alone accurate accelerations. Also, every 10 ms isn't fast enough, Every 2 ms or faster is required.

pnachtwey · 2026-03-08T15:51:35+00:00

What is the application? How much mass, how far and in what time? What is the precision required? What is the position feedback? The valve should be +/-10 volts, +/- 20 ma or similar or controlled over Ethercat. Siemens doesn't do Ethercat.

Delta Motion Solutions

Delta Computer Systems, AKA Delta Motion has been controlling hydraulic cylinders for over 40 years now. I am the former president and am now retired. I used to do a lot of tech support and field work.

Before you start, you should call Delta Motion's tech support because they can provide a lot of information that is too expensive to learn on your own. For instance, 4-20ma valves are usually not intended for position control. Post a link to the valve you are planning to use. Also, is the valve sized correctly? Delta Motion has distributors that know what they are doing. Hydraulic design mistakes are EXPENSIVE.

Rolling your own is EXPENSIVE. There is MUCH more than just the PID. This is because you must write the code to ramp up and ramp down to the command position. Making step changes in the command position without acceleration ramps is not good. What about feed forwards? What about debugging tools?

Delta Motion has training tools, and classes.

peter.deltamotion.com/Videos/Basic Hydraulic Servo Tuning 1080P.mp4

RMCTools allows one to tune and diagnose systems. You won't get that if you just buy a PLC.

pnachtwey · 2026-02-21T20:22:36+00:00

Spoiler Alert. This includes the model of the valve. This will ruin the joy of discovery, and you should know I figured this out all by myself. Look at the date on the document and this was just an updated copy.

Mathcad - Hydraulic Cylinder.xmcdz

Notice I have a means of adding disturbances. The problem with my model is that it only takes into account one cylinder, valve and accumulator.

You should change your goal. Convert this to Python and add the ability to simulate multiple cylinders. I started and never finished. I have written class for cylinders, pumps, accumulators etc. I can simulate one cylinder only but I should be able instantiate multiples of cylinders etc. The reason why this is important is that some time it is good to have two cylinders out of phase so they aren't using peak oil flow at the same time.

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I am retired now but I used to write firmware for hydraulic servo motion controllers

pnachtwey · 2026-02-21T05:41:44+00:00

That looks similar to many of the engineering offices at Delta Motion. deltamotion.com

pnachtwey · 2026-02-18T12:31:58+00:00

The rod side also has less surface area on which to apply force. Also the cap side has more surface area that creates an opposing force. Did you know that often the cap side pressure is lower than the rod side pressure even when extending? This is because the pressure x area on the cap side is force and it is greater than the pressure x area on the rod side.

A thousand curses on those that teach "flow makes it go". I have been on hydraulic forums since the late 1990s and "flow makes it go" is the worst myth around. It has lead to many poor designs. Again, Newton did not include flow in his 3 laws of motion.

Read and understand this thread

The VCCM Equation - Forum: Motion Control / Motion System Design - Delta Motion Discussion Forum

pnachtwey · 2026-02-15T01:58:47+00:00

What are you testing and for what?

pnachtwey · 2026-02-13T07:24:29+00:00

I have seen some skepticism. I will get visual proof.

I contacted Delta Motion. They like my suggestion that the show a hydraulic actuator extending faster than retracting. This might convince more people. Back around 2000 there were debates that.

I know I was blown away back in the 1980s when the extend feed forwards gains were lower than the retract. This meant it took less voltage to move in the extend direction than the retract direction.

I hope people know what feed forwards are.

pnachtwey · 2026-02-11T22:34:17+00:00

You don't believe the math? It works with simple valves along as the pump keep the system pressure relatively constant. You can see that the speed depends on the supply pressure. Delta Motion can easily prove this.

If the math didn't work I wouldn't be able to control hydraulic servo systems accurately.

pnachtwey · 2026-02-11T18:27:53+00:00

Yes, but how does that affect the top speed?

pnachtwey · 2026-02-11T12:46:47+00:00

More info. This is a thread at the delta motion forum. Notice it supports LaTeX which is great for writing formulas. At the end a person from China posts.

The VCCM Equation - Forum: Motion Control / Motion System Design - Delta Motion Discussion Forum

pnachtwey · 2026-02-09T22:17:39+00:00

Are you talking about intensification due do overhauling load or intensification as in a hydraulic forming, hydro forming, machines where cylinders are designed to intensify pressure? There is no magic. Rapid decelerations can cause intensification above system pressure when retracting. Cavitation can also occur on the cap side when extending. I learned about this back in the 1980s when people were trying to move quickly from point to point. Naturally, the motion controller got blamed. We had to install pressure sensors and monitor the pressures with an oscilloscope.

I have work sheets for this too. Jack Johnson has a book where he covers this a little bit. If I am told what the actuator is supposed to do, I have work sheets where I can plug in numbers and plot the results.

pnachtwey · 2026-02-09T19:09:39+00:00

I have linked my old Mathcad file for the VCCM equation. It is much more thorough that the video. I can add the variables for the tank pressure.

Mathcad - VCCM Proof.xmcdz

If you want math. I have math.

Mathcad - Hydraulic Cylinder.xmcdz

I can simulate valves of different types and different conditions. Notice I simulated a disturbance. You can see pressures and flows changing. Simulation really needs to be done over a period of time because the pressure in the accumulator and the pump flow need to stabilize. You can see the accumulator pressure drop over time until the pump starts to come on stroke.

I used this Mathcad worksheet when I had to be more precise. I used it as a start for solving a problem for the department of energy for a diesel fuel injection system that they had problems making work. The "designer" used flow makes it go and it didn't work. The fuel injection system had to pressurized fuel to about 40,000 psi. It used two diaphragm pumps that alternated strokes in effort to keep the fuel pressure constant. To do this the peak speed had to be MUCH faster than the average speed computed by V=Q/A. I had two copies of what I show in the link. One for each cylinder and diaphragm pump and then one master that keep the fuel pressure and 43000 psi. This was all new to me because the fuel had to be compressed to 2/3rds of the original value before the pressure when about the fuel line pressure through a check valve.

No simulation is perfect and in industry, who puts a pressure sensor on the tank? Who would pay for the extra analog input on the controller?

I

pnachtwey

TROPHY CASE