In an automated assembly system, all the action typically comes from above. A base part is moved into position, and then some sort of tool (a robot, screwdriver or press, for example) moves down to perform some sort of action (place a part, install a fastener or insert a plug).
As a result, it’s essential for the system to know exactly where the base part is. If the position of the base part is off by even a millimeter, the robot might misplace the part, the screwdriver might miss the hole, or the press might be unable to insert the plug.
A rotary encoder cannot measure the position of a linear stage, at least not directly. What it actually measures is the position of the motor shaft. The linear position of the stage can then be calculated based on the pitch and diameter of the ballscrew. How closely the calculation matches the actual position of the stage depends on the amount of backlash and other mechanical error in the system.
To obtain the exact position of the stage, regardless of any play in the system, a linear encoder is needed. There are several technologies to choose from, but the two most common are optical and magnetic.
Optical linear encoders work much like their rotary cousins. But, instead of reading light signals passing through a rotating disk, optical linear encoders read light signals that pass through or reflect off a graduated linear scale made from glass or steel. The scale usually has two parallel tracks. The main, or incremental, track consists of thousands of finely spaced lines. These are what the encoder counts to measure movement. The other track consists of intermittently spaced reference marks, which enable the encoder to determine its absolute position.
A magnetic linear encoder relies not on a graduated scale, but on a magnetic tape laminated to a steel strip. Absolute positional information is magnetized onto the tape in a sequential code. This information is enhanced by interpolation of sine-cosine signals that are provided on an additional incremental track on the tape. A noncontact magnetic sensor is mounted to the stage. The sensor reads its exact position at it moves over the tape.
Whether optical or magnetic, integrating linear encoders into a linear motion system takes time and expertise. The new IMScompact integrated measuring system from Bosch Rexroth makes that process easier.
With its sensor system completely integrated in the ball runner block, the IMScompact determines the exact position of a linear stage with a repetition accuracy of ±1 micron. IMScompact enhances the range of linear guides and is available in sizes 15, 20 and 25, making the measuring system for ball rail systems a compact and cost-effective extension for smaller machines. Integration into the runner block ensures high protection against electromagnetic interference, which means IMScompact is also suitable for use in combination with linear motors. The measuring system is not subject to wear, either incremental or absolute. It increases precision and dynamics, particularly for handling applications or general automation systems.
With magnetic tape adhered along the entire length of the linear guide, the IMScompact determines the position of the runner block by means of noncontact measurement, with an absolute precision of ±20 microns per meter and a repetition accuracy of ±1 micron at speeds of up to 5 meters per second.
The runner block’s steel body shields the integrated encoder against magnetic interference like a Faraday cage, allowing the integrated measuring system to provide exact values even in combination with linear motors. With its contact-free, zero-wear measuring principle and insensitivity to dust, impact and vibrations, the IMScompact is particularly durable. The dimensions are the same as those of standard systems, except the cable outlet is on the side of the runner block, which means the IMScompact can be integrated into existing designs without changes to the construction. Complete integration into the runner block makes additional attachments unnecessary, reducing system costs and avoiding interfering contours.
The measuring system is equipped with an incremental 1Vpp or TTL interface and an absolute SSI encoder. The measuring length is up to 17.8 meters with several rail sections, and commissioning is facilitated by the use of standard components. Additionally, linear guides can be configured online. While the absolute version is always supplied with a connector, users can also specify if they want an open cable end for the incremental variant. The cable works with cable track chains and the cable length is selectable. As an accessory, Bosch Rexroth offers an external digital display, which can be used to measure component lengths in fixture construction.
For more information on the IMScompact, click here.