November 2017 Archives

The efficiency is an indicator of the ability to perform a given task within a specified time. To achieve high efficiency, the following three issues must be considered:

1.Improvement of production capacity per hour (reduction of production cycle time)
2.Improvement of machine utilization rate within a prescribed period
3.Waste elimination

To guarantee the product quality, the production method adopted should produce little variance in precision (it must have so called 'process capability') so that the quality can be assured without an inspection. The following section uses laser machining as an example to describe how to reflect the requirement 1 in designing high efficiency equipment.

(1)Equipment design with improved production capacity per hour (reduced production cycle time)

・Workpiece processing generally consists of a) processing to add values to products (main tasks) and b) non-processing work (incidental tasks). This section describes how to make the main tasks a) more efficient.

・To add several straight lines to a workpiece using the laser beam, there are two ways. You can use a single laser beam to irradiate multiple areas by moving the workpiece (Fig. 1a), or you can also shorten the processing time by splitting a laser beam into several beams to irradiate multiple areas at once (Fig. 1b).

・To use multi-shot processing shown in b), it is necessary to adopt a technology that can equally distribute several laser beam components from a single source.

・The laser beam will be split into horizontal and vertical polarization components using optical parts, such as a polarization beam splitter and a quarter wavelength plate. (Fig.2)

(1) Issues in designing high-speed equipment

- The following points must be kept in mind when developing an automation device that drives at a high speed:
a) Selecting an appropriate actuator that supports high-speed driving and how to use it efficiently
b) Characteristics of the mechanical components that support high-speed driving

- Since the two issues a) and b) are correlated, it is critical to incorporate both of them when developing an optimal design for high-speed equipment.
- If the two issues are handled separately, the engineer working on b) tries to develop a high-precision and high-rigidity mechanism, which tends to be a heavier structural object, while another engineer working on a) will develop an expensive system as he plans to adopt high-precision control and high-power driving design.
- To develop an optimal design by considering the both issues, the equipment development can be carried out in line with the concept introduced hereinafter. By following this concept, it is possible to develop a lightweight yet highly rigid design by incorporating both the actuator's motion characteristics and optimal mechanical structure.

Optimal equipment concept for high-speed design ---- High-speed equipment realizing minimum energy consumption

(2) Vital points in designing high-speed equipment

- The following six points are important in designing high-speed mechanical design:

1. Adopt a rib structure for the movable components to the extent without affecting the rigidity.
   Description: Develop a lightweight design for the movable body without decreasing the strength.
2. Keep the point of effort and the center of gravity on the movable body as close as possible, and minimize the effect of the moment of inertia.
3. Simplify the structure and reduce the number of the components of the transmission drive mechanism to develop a highly rigid structure, thereby increasing the servo stiffness.
   Description: Reducing the number of components used for the motion area (especially the number of joints between the actuator and the movable body) can prevent servo stiffness from being degraded and can also minimize unstable vibration during control, etc.
4. For the triple axis drive using the X, Y, and Z shafts, the load placement must be optimized, such as by moving the load applied on a shaft used for high-speed driving to the other shafts.
   Description: To use the Y-axis for high-speed driving, design the structure so that a large portion of the load will be applied to X-axis and Z-axis, while reducing the load on the Y-axis movable body.
5. Safety measures for limiting a driving range of the high-speed movable part
   Description: Installing runaway stoppers (Fig.1) or a safety cover for the movable body
6. Select flexible and highly reliable wiring for connecting the drive actuator
   Description: For the mechanism used for repeated reciprocating motion, the actuator joint of the movable body is prone to fracture. Thus, selecting an appropriate cable carrier (Fig.2) is critical. If an inappropriate cable carrier was adopted, it hinders the cable carrier motion, which may degrade the positioning accuracy or cause cable disconnection or abrasion of carrier components (made of plastic). Ultimately, unexpected problems, such as the equipment contamination by wear debris, could occur.

In the midst of adverse market conditions, the automation devices must be manufactured at a higher standard in terms of quality, cost, and delivery time. The requirements of making the automation device high-speed not only involve increasing the productivity per hour, but also include machining at a higher density than the existing method in order to upgrade the product quality, such as the appearance. The high-speed equipment is also required for achieving this high-density production within a short period. This volume explains the high-speed equipment design of automation devices.

(1)Issues in designing high-speed equipment

The following points must be kept in mind when developing an automation device that drives at a high speed:
a)Selecting an appropriate actuator that supports high-speed driving and how to use it efficiently
b)Characteristics of the mechanical components that support high-speed driving
This volume describes the details on a)
Selecting an appropriate actuator that supports high-speed driving and how to use it efficiently.

a)Selecting an appropriate actuator that supports high-speed driving and how to use it efficiently

・Adopting a high-power drive actuator is not the only point we must be aware of here. We also need to select a configuration plan where the servo stiffness becomes high so that the control stability can be secured during driving.
*Selecting an appropriate drive actuator (rotary motor + ball screw/belt, etc., linear motor, and so on)
*Selecting an appropriate high-speed drive control method (open/closed loop, acceleration and deceleration control, etc.)
*Motor heat insulation measures (heating control, heat shielding, heat cooling, etc.)

・In this case example, the components consisting of a linear motor and closed loop control are adopted for precision positioning control. An air cooling method is adopted as the heat insulation measures for the linear motor. The aluminum cooling fins having high heat conductivity (Fig.1) are installed at the motor portion.

・Since the motor coil unit generating heat is attached to the bottom of the moving table, the heat from the coil unit may cause the mechanical components to deform. To prevent the heat deformation, a heat shielding plate is inserted between where the motor coil unit and the moving table are connected. (Fig.1)

・To improve the actual efficiency of machining per hour, high acceleration and deceleration control must be adopted so that the motor reaches at the maximum speed within a short period. However, without selecting an acceleration and deceleration control program allowing smooth transition, a large moment of inertia (shock) applies when the motion stops, which degrades the durability of the movable parts.

1) Characteristics of granite stones

- While the granite stones adopted for the large XY table have the following advantages, including 1. Minimum degradation with age, 2. Small thermal expansion rate, 3. Vibration dumping properties, they also have the weakness in the bending rigidity since the material strength is smaller than that of metal.

- Therefore, we needed to develop a technology of enhancing the rigidity in the structure using granite stones. This volume introduces the high rigidity structure for large tables.

2) High rigidity structure for large equipment design

- Fig.1 shows the structure diagram of the linear guide mechanism adopted for an XY table designed in the general machining dimensions (machining area is equivalent to A4 to A3 size.) Since the pneumatic hydrostatic bearing is adopted, it has the bearing shape as shown in Fig.1.

- If you increase the equipment size without modifying the structure, the load will cause a considerable deformation of the table top between the two bearings. As a result, the following problems will occur:

(1) Error in linear motion
(2) Bending of workpiece caused by deformation of the table top
(3) Vibration caused by the unstable pneumatic bearings resulting from deformation of the table top

- In this case, the deformation volume (deflection) in the center of the drive table increases in proportional to the cube of distance between the two guide rails. (Fig.2)

y=W・L³/48E/I
W:Weight, L:Length of beam, E:Young's modulus, I:Second moment of area

- In this case example, an auxiliary rail is installed between the two sliding guides to minimize deflection caused by the table weight. (Fig.3)

- The auxiliary guide in the middle does not work as a bearing for linear motion. It simply supports and reinforces the moving table from the bottom.

- For large automation devices, notice that the flatness of the base plate or slight inclination will have an effect on the assembly precision.