Position vs Velocity mode

Hello Don,
you seem like you are an expert in servos field, so I will try my luck with this question:

I have both delta B3-L and B3-Ethercat drives
B3-L (step-dir) are currently hooked up to my machine using mesa card 7i95t and they are tuned via delta software to practically zero following error when checking scope in delta software (except for some minor when starting and stopping. like 0.006mm on 5mm ball screw).
However I am unable to accomplish the same results on EtherCAT drives, even though I compared all parameters. The filters are all off or minimum, but i have approximately 12000PUU following error on EtherCAT drives even though the motors are running freely without any load - they are only on the test bench.
The only difference is load inertia ratio which is ~5 when on machine and 0 in case of unconnected motors.
Might it be, that unconnected motors are unable to be tuned properly?

Donb9261 wrote:

Of course.

Well that is simple yet complex to explain. First, is the following error in the drive a match to the one derived in LCNC. The TP is in the drive so LCNC is not the ideal place to see the true FE.

In position mode, the controller should only need to send position commands as needed. It does not need to even know whether the following error exists given the actual position is dynamically controlled locally in the drive. Now, there are considerations in excess error. Time delay of the cyclic updates. Properly sized servo motors for the load(this is vital to ensure you have a 125% power to load ratio - see final note below), and the FF forward parameters which will ensure enough torque is available to maintain P/T or velocity until the loop position becomes zero. The position control loop receives a command to move 1" at 10 IPM. It will then take that 1" and use the Delta to start positioning the servo via the position FB loop to drive the motor from 1" to zero. The error is calculated within the Delta. The job of the Position loop is to make it to zero in the T requested. If it sees that the T is being stretched it will request more torque to compensate and if to early it will remove torque to comp that. The issue can be in many cases that the LI or load inertia is too great in both ACC/DEC states and continuous travel.

Most higher level drives have several params that adjust the servo response time, power levels, and FF(Aka Loop Gain). These drives also have dynamic auto tuning and if the input calcs for your inertial loads are accurately calculated will allow the auto tuner to account for the calculated load while tuning for you. But, that is the caveat. You must ensure that you have properly calculated the true inertial load of your axis.

Final Note -

The axis has a load characteristic before it actually starts cutting. Cutting requires additional torque to ensure the loop is maintained else you may have a perfectly tuned machine and fined out later while milling a slot that the error is pretty wide and may even see issues with blending or ovular interpolations. It is key to ensure that you know the static load(unloaded machine not cutting) and the dynamic load(machine moving the static while cutting adding a variable load). The math can be a bit much but there are many calcs out there in the ether to help. Once you know that value of what is specifically required, as a rule engineers like myself add 10-25% additional motor KW to ensure that the motion can maintain P/T and make targets without under shoot or overshoot and minimize oscillations at idle.

So if my calc says I need .75kw to handle S and D loads, then I install a 1kw servo. Keep my FF low, and use minor filter adjustments to cancel out ACC/DEC response at higher V.

Following error is more a measure of the servo response not the command response of the controller if in ECAT mode. ECAT can be used in closed loop of course, but that is redundant and increases chances of issues of imbalance of power. ECAT wants to separate concerns. Controller decides where the servo should go, tells the servo go here, the servo does it's job and next. If the servo has an issue, it has its' own ability to stop before disaster. Like I said, ECAT is better at point and shoot than have a controller try to manage everything. That would be like racing a Ferrari while hooked to a tow truck. It works, but the tow truck(controller) is never going to be a Ferrari(servo).

Hope that helps.

PS - Closed loop stepper drives and motors are not Ferrari's. More like a Lota. Lol. But, you can put a spoiler on a Lota and it looks pretty cool.


 

Hello gentelmen,

it is very interesting reading over this topic around timing and control modes etc... but I can strongly recommend to not calculate servo powerrate to power factor of servo drive. I can recommend to calculate torque or intertia mass(which are in the end torque) which are in the end current...but no every servo drivers running at same voltage and there schould be difference finally at power output, which make a sense for final user... old japanesse servos running on low voltage but high current... nowday are solutions different

I have talking about open servo solution with local Siemens 840D expert the he recommend me maximum 1/4 of motor as reserve for final solution... It is great compromise about final tunning and overoscillation factor...

regards

I doubt it. It is a hard question to answer without seeing the settings for both sides. The B3 manual is a very lengthy one. I would start with ensuring the electronic gear ratios are set correctly and that the values match on both LCNC and the drive.

I am not too familiar with Delta products to be honest.

I would check that the motor type settings are correct, then EGR values. If the results are the same, then you may need to send a tech support ticket to Delta with the drive model and motor model with encoder type.

If you are running the motors unconnected to LCNC and the results are drawn from the Delta tuning software then I suspect the motor/drive settings are not correct. PPR for inc encoders or bit specs for abs encoders must be set correctly. Also verify the drive/motor current is compatible. I assume these have no brakes on the motors that are still engaged.

Servo current is always calculated from the the end effect torque requirements and load inertia.

Velocity, weight, rigidity, and friction coefficients must be calculated +=2%. There are other factors to consider such as gear, belt, ballscrew driven axis. Whether one uses a precision gearbox to increase torque.

In the case of CNC, the cutting tool will add additional torque requirements but are variable sometimes needing higher response rates to remain on P/T path. In these cases, an appropriate safety factor for torque requirements is needed. The cutting will either try to pull the tool or push the tool which the servo must respond to. If I incorrectly size to a machine without the cutting tool consideration the drive may not have the excess power to control the dynamic load and will cause indeterminate cutting conditions especially through an interpolation move where one axis cannot stay in sync due to the dynamic load condition.

The drives available today have higher dynamic response rates. In many cases they have a 3x peak load that can be accessed in the dynamic range for a n time value based on the drive characteristics. But, this type of drive is more expensive.

Regardless, every drive has a specific set of characteristics and every motor has the same. The calculations for the axis are the same and your choice should be based on your specific application. In general, if the true dynamic load is unknown, it is best to choose a power range above the known values to account for your actual application. 125% of net need is usually a solid safety factor to assist with the dynamic loads for CNC machining.

If using Chinese servo motors and drives, I have found that in most cases the values given for the actual drive/motor combo are well, over blown. I always use a 135% to 150% safety factor for these motors to account for the variability of the actual specs.

Do your homework before you buy. Know your specific application. If unsure, then many servo OEM's will assist with the calculations if you can give them relatively accurate values for your machine axis. The math is not easy for sure.

zmrdko wrote:

Thank you for explaining. So basically we are better off with position mode. Because servo drive is better at handling position commands from linuxcnc. I am running 8kHz linuxcnc servo thread which is the same as 8kHz cycle in B3 drives. The following error showed in servo drive is basically zero with proper tuned servo drive. The linuxcnc shows following error 0.22mm at 3000mm/s speed and 0.44mm at 6000mm/s and 0.88 at 12000mm/s. Does it mean that I should just set MINFERROR in linuxcnc with some room (so that I dont get following error errors), because the the following error which lonuxcnc shows is not the true following error? I will come back with screenshots and exact values later.

hello zmrdko,

your thread in the linuxcnc are running at oversample 8kHz .. what is your confing for them ? hardware and else.. ethercat has which time period ?

Donb9261 wrote:

I doubt it. It is a hard question to answer without seeing the settings for both sides. The B3 manual is a very lengthy one. I would start with ensuring the electronic gear ratios are set correctly and that the values match on both LCNC and the drive.

I am not too familiar with Delta products to be honest.

I would check that the motor type settings are correct, then EGR values. If the results are the same, then you may need to send a tech support ticket to Delta with the drive model and motor model with encoder type.

If you are running the motors unconnected to LCNC and the results are drawn from the Delta tuning software then I suspect the motor/drive settings are not correct. PPR for inc encoders or bit specs for abs encoders must be set correctly. Also verify the drive/motor current is compatible. I assume these have no brakes on the motors that are still engaged.

 

Delta products are great but servos and VFDs are really budget solutions.. delta asda with underestimated EEPROM which you have to overwrite before you want to use the ethercat is joke... if you want to use that as PTP manipulator or plasma with no expert tunning - ok... but for critical applications I do not recomend it ... there is more sophisticated solutions on the market