Changes - Dynomotion

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5,384 bytes added, 22:03, 17 July 2016

/* Velocity, Acceleration, and Jerk */

<div class="yiv2818182665class" style="color: #000000; font-size: 13.3333px; font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; background-color: transparent; font-style: normal;">[[File:MaxVelAccelPower.png|none|link=|485x361px]] The Step Response Screen always performs 3rd order motion but 2nd order motion can be simulated by temporarily setting the Jerk to a huge value (1000X the acceleration value). In general reducing the Jerk value will result in longer times to perform motions, but often the improved smoothness will permit higher maximum accelerations and velocities to be used resulting in overall shorter motion times. An analogy might be how you might stop more quickly in a car, without skidding or spilling your coffee, by applying the brakes harder in a more gradual manner rather than slamming on the brakes. Make sure when testing the size of move is long enough for full acceleration and velocity are achieved. As a common mistake is to have Acceleration or Velocity Settings set to too high for your system but when testing a short move there is no indication of a problem. The plot mode of Velocity Output vs Time can be helpful to verify full Velocity is being achieved. </div>

<div class="yiv2818182665class" style="color: #000000; font-size: 13.3333px; font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; background-color: transparent; font-style: normal;">The idea is that a motor axis has torque and speed limitations and you should determine and understand what they both are for your system. Available motor torque drops off with higher speed. At some speed the motor will not even be able to generate any torque at all. Motor torque is required to accelerate the axis. We need to make sure that no combination of velocity and acceleration will ever cause the axis to fault/stall/fail. So there is often a complex set of speeds and accelerations that will work ok that form an envelope of workable speed and acceleration. So one nice thing to know is top speed possible. It is like taking your car out on a long straight highway and gradually accelerating to find the max speed is 100MPH. You will then know that ever attempting a speed over 100MPH will always fail. Also speed should probably be limited to something like 80MPH to allow for margin and to allow reasonable acceleration up to that speed. The reason for performing the top speed test at low acceleration is to avoid a failure because of not being able to accelerate at the specified rate. So we want to set the acceleration to a low value so it will not be a limiting factor when determining top speed. However if we set the acceleration so low with a short trip distance we will never go very fast and the speed test will be meaningless. This should be avoided. Change the Plot type to Velocity, Output, vs Time to clearly see if this is the case. For example to accelerate to 700,000 counts/sec at an acceleration rate of 700,000 counts/sec^2 would take 1 second. And then to slow back down would take 1 more second. So at least 2 seconds of motion would be required to get to full speed. A 100,000 count move (2 inches for a system with 50,000 counts/inch) takes much less time than that. Furthermore with a relatively low Jerk setting (1e6). This means the acceleration will be applied gradually over 0.7 seconds. This means an even much longer time and distance would be required to achieve top speed. Here is the method to follow to find 2nd/3rd order motion profile limits: (note every system is different and will vary based on resolution, motor size/type, mass, etc...)

'''1 - Find Max Velocity at low Acceleration and infinite Jerk'''

Choose a Velocity to test (ie V=100000 counts/sec) Set a moderately low Acceleration Time of 1 second (A = V/1sec = 100000 counts/sec^2) Set very high Jerk so Acceleration is applied almost instantaneously in 0.001 sec (J = A/0.001sec = 1e8 counst/sec^3) Test a very long move 20 inches if possible (for resolution 50000 counts/inch, 1,000,000 counts) Plot Velocity, Output, vs time to see if V is actually obtained If the motion was successful repeat all above with higher V, if not try lower V Once Max V is found reduce by some margin (ie 20-50%) Note you may decide on lower speed for other reasons (safety, vibration, noise, wobble, shock, etc....) '''2 - Find Max Acceleration for your system for the chosen Velocity'''

Using the V found above increase A (keeping Jerk high in the same manner) until the system fails/faults/excessive shock, etc... Once Max A is found reduce by some margin (ie 20-50%) These V and A Values are now known good values for your system and can be used for 2nd order motions such as KMotionCNC Trajectory Planner or Mach3 Motor tuning. Note units will need to be converted (counts to inches or mm, time from seconds to minutes depending on the App)

'''3 - Find optimal 3rd order Jerk limited motions.'''

The previous result from Step #1 and #2 used nearly infinite Jerk where Acceleration forces were applied nearly instantaneously. By reducing Jerk smoother motion should be possible. Set Jerk so it is applied over 20ms (ie for A=200000 J = A/0.020 = 1e7 Test to see if motions are smoother. With closed loop systems the Position Error Plot can be used to see if the motion has less error and is smoother. Open loop system will require you to hear the difference. Increase/decrease Jerk to find the optimal value. Note reducing Jerk a lot will of course make the system very smooth, but will also be much slower using less acceleration and velocity which is undesirable. So the highest Jerk possible should be found that still provides some smoothness. Reducing Jerk only will always make the system slower with less performance (but hopefully significantly smoother). It is usually then possible to increase Acceleration and Velocity somewhat to achieve even higher performance than what was possible with infinite Jerk while being as smooth or smoother.

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===Axis Resolution - counts/inch (or counts/mm)===

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