# Trajectories¶

For a motor controller supporting the use of a trajectory, and for which the support is effective in BLISS (cf writing a motor controller), trajectories can be defined and used to move axes in complex movements.

## Simple example¶

Here is a simple arc trajectory with two axis (X and Y). So axis X will have a cosin trajectory and axis Y a sin trajectory. In this example we want to move from 0 deg to 90 deg in 10 seconds.

First let define the alpha, 10 points from 0 deg to 90 deg:

import numpy
alpha = numpy.linspace(0,90,10) #deg


Then build position for the two axis:

x_positions = numpy.cos(numpy.deg2rad(alpha))


Then build the time array.

times = numpy.linspace(0,10,10)


At t0 == 0 and t9 == 10 seconds in 10 points.

In Bliss all trajectory are defined by the triplet PVT => P osition, V elocity, T ime. To achive this, use the PointTrajectory object to create the PVT array:

from bliss.physics import trajectory
pt = trajectory.PointTrajectory()
pt.build(times,{'X':x_positions,'Y':y_positions})


Create one trajectory object per each axis:

from bliss.common import axis

pvt = pt.pvt() # calculate pvt array
x_trajectory = axis.Trajectory(X,pvt['X'])
y_trajectory = axis.Trajectory(Y,pvt['Y'])


An finally create the TrajectoryGroup. This object group the movement for this two axis along the trajectory.

from bliss.common.motor_group import TrajectoryGroup
#Create the Trajectory group
group = TrajectoryGroup(x_trajectory,y_trajectory)
#load trajectories into the motor controller
group.prepare()
#linear move to the first point
#In this example will move X to 1 and Y to 0
group.move_to_start()
#move to the end of the trajectory.
#After this command X will be near 0 and Y near 1
group.move_to_end()


The calling sequence of this object must always be the same:

• call prepare first
• then move_to_start
• finally move_to_end

It’s possible to repeat the sequence but always in this order.