Although the weight of the nozzle head is only around 4 lbs., the actual motion profile needed to achieve 12 inches of effective constant speed at 72 cycles per minute is a challenging task. In order to provide a constant speed over 12 inches, at a 72 cycle/min rate, the following motion parameters were calculated:

Therefore the acceleration is approximately 3G! Based on the high speed and acceleration rate, and the overall reliability requirement for this high throughput, high duty cycle application, it was determined that a direct drive linear servo actuator would be the best solution. In addition, a control architecture had to be selected that would:

• Allow the high speed and acceleration as calculated above
• Not introduce any additional dwell time, while executing the motion profile accurately.
• Provide for a simple operator interface that would allow a single potentiometer to dynamically control speed
• Provide overall machine and safety control

The actuation was created with a linear tubular servo motor, a linear encoder for servo position feedback, and a servo drive capable of running the linear servo motor utilizing encoder feedback. This created an actuation system without any mechanical linkages that would introduce wear, especially in a harsh sandblasting environment with small abrasive particles in the air. The air gaps between the motor moving parts and the linear encoder moving parts are large enough to avoid any particle buildup and potential clogging.

The machine and motion controller that would be able to meet all requirements was Control Technology Corp.’s latest Model 5300 Blue Fusion controller with the following configuration:

• 4-axis slot rack with main CPU card and power supply
• A dual axis servo card with +/- 10V analog reference and encoder feedback
• A 16 digital in / 16 digital output card
• An 8 channel 16-bit analog input card
• A 16 channel 16-bit analog output card

In addition to being extremely compact, it also had many additional features that facilitated the setup and provided future expandability. The overall system wiring is at right:

Some of the hardware facilities that made system wiring efficient:

• Controller 5V source capable of 2A to power linear encoder which draws 0.5A
• Analog input card has 10V output source to power potentiometer
• Motion card has 24V isolated inputs to interface directly with PNP limit switch sensors.
• Digital I/O card can interface with servo drive TTL logic and also with dry contact switches and push buttons.
• Motion card 24V outputs can drive LEDs in panel buttons.

The system programming was highly simplified through the QuickBuilder programming environment. First of all, servo loop tuning was straight forward by using the ServoTune panel in QuickBuilder (see left):

Not only are all the servo loop gains accessible from within a single panel, the servo loop itself is also enhanced by a high performance velocity loop utilizing a Kalman filter for optimal speed estimation. Although all gain parameters can be adjusted individually, by reformulating the gain relationships based on the loop type (PID or PDF), the required bandwidth, and the amount of damping, the overall tuning is more comprehensive and desired results can be obtained more quickly. This is especially important in a direct drive, low friction system such as this linear motor application. In order to ensure that no additional dwell time was introduced between consecutive moves, the motion sequence block (MSB) responsible for the back and forward motion was run in the foreground (as opposed to background). This ensures path planning at the hard servo loop rate with no delay or variability in the motion trajectory. The ability to dynamically vary speed based on an analog input value (from a potentiometer) typically requires some kind of routine that reads an input, then scales this value to a speed value, and then (hopefully) sets a new speed value – during motion. In order to make this speed adjustment seamless and smooth for the operator, this needs to be done quickly and often, in turn taxing the controller processing power. This particular aspect of the system was solved with virtually no code! As part of the controller resources, the user has the ability to dynamically adjust the time base of the motion trajectory. When set to 1.0 (the default value), the target generator’s “time” is un-scaled. When set to a value between 0.0 and 1.0, the target generator’s “time” is slowed down, effectively generating lower velocities. When set to 0.0, motion stops. To further eliminate actual code, a virtual PID loop was configured; with the analog input as feedback and the axis time base as output:


By applying the proper scaling to feedback and output and by using a simple proportional gain of 1 (to effectively feed through the input), the time base is directly and dynamically set by the potentiometer with absolutely no code in the program. In addition, this concept works all the way down to zero speed, with no potential for creep and no effect on the rest of the program! In addition, the multi-tasking nature of the Model 5300 Blue Fusion controller allows for simple addition of events, which run in the background, to handle any safety and interrupt driven mechanisms.

A few additional “bonus” benefits of this Blue Fusion architecture are:

• Ability to interface with touch panel HMI
• Ability to electronically “slave” to an encoder in order to link the reciprocating motion to the motion of the part to be sand blasted (the part is fed on a conveyor belt)
• Ability to add data acquisition and logging (e.g. number of cycles, hours of operation…)
• Ability to add remote diagnostics and warning message generation (the main CPU has a built-in mail server which can send an e-mail in case of specific user configurable events).