While it is possible to control a hydraulic scissor lift using Navigator today, the method employed relies very heavily on the PID loop to compensate for predictable changes in the geometry of the lift and other measurable system variances.
The common feedback from end users of hydraulic lifts include;
*the transition off the helper cylinder is too bumpy for the artist
*poor targeting or excessive 'hunting' at the end of a move
*overshooting the target altogether
*poor speed control under varying conditions
One possible solution to these issues is to scale the non-linear output to a data lookup table.
See the attached data table image.
The lift speed demand is placed in the vertical axis and the current lift position is placed in the horizontal axis.
Points are carefully selected based on the characteristics of the lift. For example, multiple position points are selected around the helper cylinder transition to provide the tuner the ability to smooth out excessive bumps. The axis process streams the position and speed demand to the program.
The incoming speed demand and position is parsed out and 4 relavent data points are selected by the program. These 4 points go through a mathematical function called bilinear interpolation and a corrected output is created and sent to the proportional valve.
A test was performed on a rental "Justin HD" scissor lift and valve stack. A CX8090 was programmed usning TwinCAT to handle the lookup table and control the propportional valve.
Please find the attached motion graphs. The first graph represents a typical Navigator axis tuned using the axis output spline. The spline can only provide a technically correct output for a very small portion of the lift's travel. Throughout the rest of the move, the PID is shifting the output.
The next attached graph is the same lift run using the lookup table. It could be further smoothed out by increasing the number of points on the table, but much better control was achieved. It is true that both graphs show the commanded and physical speed lines basically lining up but look at the position error traces. The lookup table operated lift offers PE (position error) that is about 1/5 or less (and that hovering around zero) the error of the spline controlled axis.
The proposed lookup table could be accessed either next to the existing spline in the axis properties or it could exist as an IO function.
Please sign in to leave a comment.