Soft Landing Motion Control: C# Implementation & Key Parameters

ByHANI_AUTOMATION HANI

In high-precision industrial automation, the way a machine stops is often more critical than how it moves. Traditional motion control methods maintain high speed until very close to the target, causing inertial shock, vibration, positioning errors, and even damage to parts or workpieces. Soft landing is an advanced control strategy that uses intelligent speed planning and smooth transitions to dramatically improve stopping performance and reliability.

What is Soft Landing?

Soft landing is a motion control method based on a segmented velocity profile. The positioning process is divided into two main phases: a high-speed approach phase and a low-speed precision stop phase. At a predefined switching point, the system transitions from high speed to a very low, controlled speed, minimizing kinetic energy and achieving a near-impact-free stop. This technique is essential in applications like precision assembly, semiconductor bonding, and optical alignment, where force control at the endpoint is critical.

Key Parameters for Soft Landing Control

Implementing effective soft landing requires careful tuning of several parameters, supported by robust algorithms:

Parameter Description
FORCE_SPEED The velocity used during the soft landing phase. Typically set to a low value to control contact force and ensure smooth stopping.
MOVELIMIT The maximum speed allowed in the final stage. This limits motion energy and prevents overshoot.
Switching Point The position where the system transitions from high speed to low speed. It must be optimized based on inertia, response time, and load characteristics.
S-Curve Acceleration Using an S-curve profile ensures continuous, smooth velocity changes, avoiding jerk-induced oscillations and noise, and improving positioning accuracy.

Typical Implementation with Motion Commands

On modern motion controllers, soft landing is often implemented using compound move commands. For example, a MOVESP instruction can be split into two segments:

  • First segment: High-speed approach using FORCE_SPEED and high acceleration to cover distance quickly.
  • Second segment: Low-speed final approach limited by MOVELIMIT, with S-curve transition to the target.

This approach allows direct configuration of the soft landing zone, end speed, and transition point, offering great flexibility for different applications.

C# Code Example for Soft Landing

Below is a simplified C# snippet demonstrating how to set up a two-stage soft landing move on a motion controller. The code sets axis parameters, defines two move segments with different speeds, and triggers the motion.

private void button6_Click(object sender, EventArgs e)
{
    if (g_handle == (IntPtr)0)
    {
        MessageBox.Show("Not connected to controller!", "Tip");
    }
    else
    {
        int[] axislist = { m_axisnum };
        float[] poslist1 = { Convert.ToSingle(textBox21.Text) };
        float[] poslist2 = { Convert.ToSingle(textBox31.Text) };

        // Set axis parameters
        zmcaux.ZAux_Direct_SetUnits(g_handle, m_axisnum, Convert.ToSingle(C_AxisUnits.Text));
        zmcaux.ZAux_Direct_SetSpeed(g_handle, m_axisnum, Convert.ToSingle(C_AxisSpeed.Text));
        zmcaux.ZAux_Direct_SetAccel(g_handle, m_axisnum, Convert.ToSingle(C_AxisAcc.Text));
        zmcaux.ZAux_Direct_SetSramp(g_handle, m_axisnum, Convert.ToSingle(C_Axis_sramp.Text));
        zmcaux.ZAux_Direct_SetLspeed(g_handle, 0, Convert.ToSingle(C_Axis_lspeed.Text));
        zmcaux.ZAux_Direct_SetMerge(g_handle, 0, 1);

        // Trigger oscilloscope
        zmcaux.ZAux_Trigger(g_handle);

        // First segment: high-speed approach
        zmcaux.ZAux_Direct_SetForceSpeed(g_handle, m_axisnum, Convert.ToSingle(textBox22.Text));
        zmcaux.ZAux_Direct_MoveSp(g_handle, 1, axislist, poslist1);
        zmcaux.ZAux_Direct_MoveLimit(g_handle, m_axisnum, Convert.ToSingle(textBox23.Text));

        // Second segment: soft landing low speed
        zmcaux.ZAux_Direct_SetForceSpeed(g_handle, m_axisnum, Convert.ToSingle(textBox32.Text));
        zmcaux.ZAux_Direct_MoveSp(g_handle, 1, axislist, poslist2);
        zmcaux.ZAux_Direct_MoveLimit(g_handle, m_axisnum, Convert.ToSingle(textBox33.Text));
    }
}

Practical Tuning Steps

To achieve optimal soft landing performance, follow these steps:

  1. Connect to the controller and initialize the bus and axes.
  2. Set pulse equivalent, speed, acceleration, and S-curve time based on the drive and mechanics.
  3. Perform homing according to the drive’s zero-return mode.
  4. Plan the soft landing trajectory: define the high-speed distance and speed, and the low-speed distance and speed.
  5. Optionally, implement torque monitoring to stop motion if a force limit is exceeded, protecting the workpiece.

Real-World Example

Consider a precision gluing application: the first segment moves 40 mm at 300 mm/s, ending at 50 mm/s. The second segment moves the final 10 mm at a constant 50 mm/s, then decelerates to zero. The S-curve ensures no jerk, and the low final speed prevents adhesive splashing or misalignment.

Benefits of Soft Landing Control

  • Reduced mechanical shock: Minimizes wear on leadscrews, belts, and couplings.
  • Higher positioning accuracy: Eliminates overshoot and settling time.
  • Improved process quality: Essential for delicate operations like pick-and-place, dispensing, and pressing.
  • Lower noise and vibration: Smooth velocity transitions reduce acoustic noise and structural vibrations.

By integrating soft landing into your motion control system, you can significantly enhance machine performance and longevity. The combination of FORCE_SPEED, MOVELIMIT, and S-curve profiles provides a powerful toolkit for precision stopping in any industrial automation application.

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