May 2010 Archives

Wiring for machine sections that move at high speeds (upper section of an XY table, linear motor forcer coil, etc.) require special concerns to prevent accidents caused by wire breakage. Additionally, if the moving part carries additional devices such as air cylinders and linear encoders for position measurements all the cables must be handles properly so they do not hinder the movement. To do this, a cable carrier is typically used to manage the wiring to the moving body.

While the cable carriers offer increased reliability for moving wirings, it has its own moving deformation resistance that may negatively affect the motion accuracy of the system. Following points are to be considered.

(1) How to use a cable carrier

Mechanical engineers tend to rely on other professional for design and production of electrical items, but that may cause losses in product delivery time, cost, and standardization practices. Here, electrical enclosure interior designs are discussed.

(1) Item to include in electrical enclosures

	・	Circuit breakers
	・	Programmable logic controller (PLC)
	・	Power supply (PS)
	・	Motor driver (PMD)

The items above are connected using terminal strips (H49), DIN rails (H59), and wire conduits (W79) and mounted inside the enclosure.

(2) Interior layout of electrical enclosure

The enclosure interior layout should be made simple and easy for wire tracing. Choose an enclosure with ample capacity for the following reasons.

	1.	Provide ample space between each component for easy maintenance.
	2.	Anticipate future expansion possibilities when choosing the enclosure.

[Fig.1] and [Photo 1] show the layout of an enclosure of the catalog example. The terminal strips are labeled to prevent careless errors.

System configuration is discussed based on the single axis motor driven mechanism shown on [Photo 3] in Tutorial #040.

(1) System configuration explained

[Fig.1] is a system configuration of the mechanism composed of three sensors (home sensor and two overrun prevention sensors), motor, ballscrew, and linear guides, explained in tutorial #040.

Programs containing linear speed, distance, direction, etc are stored in the programmable logic controller (PLC). The motion controller converts the programs into motor control pulses and sent to the stepper motor via the motor driver. This is an open loop system since the system does not have a function to compensate for errors based on motor movement results.

(2) Increased system reliability

(1) Use of overrun prevention sensors

If the overrun sensor malfunctions, the table may collide with the ballscrew support unit and damage and loss accuracy may occur. Sensor reliability is of high importance in order to prevent any possible collision caused damages to the system. These sensors are wired as N/C (normally closed), same logic as emergency stop switches.

(2) Prevention of environmental electrical noise intrusion

Programmed pulse outputs may be negatively affected by electrical noises from surrounding environment. It is best to dedicate a separate power line for the system to isolate from noises and voltage fluctuations. If not possible to do so, avoid connecting electrically noisy equipment to the same power line as the system. Use twisted pair wiring for all pulse signal connections. Use cable shielding scheme if needed.

A homing routine for a rotating object using a slotted photo sensor is explained here.

(1) Method of homing for a rotating object

Rotating objects are homed in same ways the linear motion objects are homed. There are risks of overrun errors when homing rotary to linear converted objects, but there is no overrun risks when a rotation movement itself is utilized. The configuration only contains one home position sensor. As a sensor target, slotted photo sensor cams can be used. In this case the motor pulse counts are used for the control.
In addition, multiples of photo sensor cams can be used for a various angle settings to control stop positions.

[Photo 1] shows a combination of a slotted photo sensor and a slotted target cam. The system uses two sensors and two cam targets to detect the cam's angle.

[Photo 2] is an enlarged view of the slotted cams. Here, the slot width have been modified to accommodate the motor speeds. MISUMI can provide the cams with customized slot width, shape, number of slots, and shaft bores suit the customers needs for convenience.

(2) Managing sensor wirings

Care must be given to sensor wire management since many automation equipment troubles can be attributed to sensor wire problems. [Photo 3] shows a representative wire management example. Take in consideration the following to perform wiring work.

	・	Align pull direction from terminals to avoid wire strain.
	・	Secure wire where they do not interfere with operation and maintenance work.
	・	Label the wire with numbers to improve maintainability.
	・	Allow ample wire slack for cable carrier use.