Application Note #3412

Transcription

1 Application Note #3412 Smoothing Velocity Profiling (using IT and VT commands) Background: Moving a motor from one position to another involves accelerating the motor to a certain velocity, traveling at that velocity for a set, and then decelerating to a stop to a desired position. The Galil controller controls relative point-to-point moves of the motor using a trapezoidal velocity profile. This means that as the velocity changes from one value to another, the constant acceleration (or deceleration) value creates a linear velocity transition. A trapezoidal velocity profile is fine for most systems, but the sudden transition from zero to full acceleration can cause an unacceptable jerk in some mechanical systems. For these systems, the Galil controller has the ability to smooth the velocity profile. On current generation controllers - this smoothing of the motion profile is set with the IT command for all motion commands including JG, PR, PA, VP, CR and LI moves. Prior generation controllers use the VT command for smoothing coordinated motion moves (VM or LM mode). Example: Consider this simple motion program that commands a jog move on the X-axis: #A AC 4 JG 1 BGX EN Assume that the motor is at rest, the program above is downloaded into the program RAM of the controller, and then sending the XQ command to the controller runs the program. The motor will begin to accelerate from rest to the commanded jog speed of 1, counts per second. After that speed is reached, the acceleration stops and then the motor will run then continuously at 1, counts per second. A graph the velocity of the motor versus is shown below: Galil Motion Control, Inc. 27 Technology Way Rocklin, CA USA Ph: Fax:

2 Velocity 1 5 counts/sec.25 sec The linear profile of the velocity as the motor starts to spin and finally reaches the jog speed is referred to as a trapezoidal profile. Since the acceleration rate of 4, counts/sec 2 is constant, the motor reaches the 1, counts/sec jog speed in exactly.25 seconds. The acceleration versus graph is shown below: Acceleration 4, cts/s 2.25 sec The step shape of the acceleration versus is the source of the sharp discontinuities at the beginning of the move and at the point when the slew speed is reached at.25 seconds. These sharp corners can cause a sharp jerk in the mechanical motion; a preferable velocity profile would gradually roll on the acceleration and roll off when the desired speed is reached. This smooth roll on and roll off of acceleration is where the smoothing profile is needed. For our example program using the Jog mode of motion, the IT command can provide smoother transitions during the acceleration of the motor. The smoothing profiling is achieved by the use of an algorithm that smoothes the square pulse of acceleration with a single constant digital filter. Again, consider the motion program example above. If a single constant low pass filter is used to filter the acceleration step, the acceleration versus graph will look like the following: Galil Motion Control, Inc. 27 Technology Way Rocklin, CA USA Ph: Fax:

3 Acceleration 4, cts/s 2 This is the same effect that an electrical RC filter has on a step shaped voltage versus. The filtering of the acceleration versus will of course change the velocity versus into the following shape: Velocity 1 5 cts/s The amount of smoothing of the acceleration in our motion example is set with the IT command. Here is a table that describes the functionality based on the product line: Optima Series, DMC-18, DMC-21xx, DMC-18x2, etc: Command Effect IT smoothes AC, DC phase of motion due to JG, IP, PR, PA commands. VT smoothes VA, VD phase of motion due to VP, CR, LI commands. Current Generation Accelera Series (DMC-4x, 41x3, 18x6, etc ) Command Effect IT smoothes AC, DC phase of motion on JG,IP,PR,PA,VP,CR,LI commands. VT replaced with IT Galil Motion Control, Inc. 27 Technology Way Rocklin, CA USA Ph: Fax:

4 Using the IT command with Vector Motion on Current controllers The IT command affects vector moves on an individual axis basis for current generation controllers such that the velocity profile is smoothed individually rather than smoothing the vector velocity. This can help with abrupt changes such as a 9 degree corner however it will also lead to the corner being rounded. If a true 9 degree corner is desired without affecting the path shape, the IT command should not be used and instead the speed for the vector segments should be reduced using the <o >p values which are optional speed parameters for the VP, CR or LI segments. Example 1 Default. Here is the graph of velocity for the X and Y axes on a 9degree vector move with no IT smoothing: #A IT1,1; default VA1 VD1 VS5 DP, WT1 VMXY VP1, VP1,1 VE BGS AMS EN Example 2 using IT.1,.1. Here is the graph of velocity for the X and Y axes on a 9degree vector move with IT smoothing: Galil Motion Control, Inc. 27 Technology Way Rocklin, CA USA Ph: Fax:

5 The following shows how the shape of the corner is affected (IT smoothing shown in dark red, no IT smoothing shown in light red): Y Axis X Axis Example 3 Corner Slow Down. Here is the program and velocity graph using corner slow down and default IT 1,1. Note: this method does not cause rounded corners the shape of the pattern will be followed exactly. #A VA1 VD1 VS5 DP, WT1 VMXY VP1,> VP1,1<5 VE BGS AMS EN Modeling the Smoothing function The S curve digital filter is modeled in the z plane by this equation: F ( z z ) z e T / where T is the sample, fixed at 2 milliseconds on the controller, and is the constant. The IT command sets the pole of the S curve filter: Galil Motion Control, Inc. 27 Technology Way Rocklin, CA USA Ph: Fax:

6 1 IT e T / Solving the above equation for the constant, and substituting the 2 millisecond sample for T:. 2 ln( 1 IT) The equation above shows how the IT setting affects the S curve filter s constant. The IT setting on the controller defaults to 1., which results in an infinitely short constant and no filtering effect. Decreasing IT causes the constant to increase, and the acceleration and deceleration is then smoother. Substitute VT for IT in the above equation to determine the vector acceleration constant. The constant of the S curve filter changes the amount of to ramp up and ramp down the acceleration in our motion example. If the S curve constant were, then the acceleration ramps up and down nearly instantaneously: Acceleration 4, cts/s 2.25 sec If the IT value is decreased the constant will increase, and the amount of for the acceleration to reach the set value will also increase: Acceleration 4, cts/s 2 The amount of for the acceleration to ramp up is approximately equal to the constant multiplied s 2.2. Since the equation for the constant is exponential, the ramp is an approximation. Experimentation is the only method to obtain an exact value. The amount of for the acceleration to ramp up to the set value and the for the acceleration to ramp down from the set value back to zero are equal Galil Motion Control, Inc. 27 Technology Way Rocklin, CA USA Ph: Fax: