Touch measurement system

A touch measurement system includes a touch probe for detecting touching of a workpiece by a touch probe stylus and including at least one sender for emitting an infra-red light in response to detection of the touching, and a receiving unit including a housing having a window permeable to an infra-red light and provided with an electro-conductive coating for shielding an interior of the housing from electro-magnetic interferences, and an infra-red light receiver for receiving the infra-red light emitted by the sender of the touch probe, and a unit for processing the infra-red light received by the receiver, with both the receiver and the processing unit being located in the housing interior.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a touch measurement system for measuring a workpiece and including a touch probe for detecting touching of the workpiece by a touch probe stylus and having at least one sender for emitting an infra-red light in response to detection of the touching, and a receiving unit including a housing having a window permeable to an infra-red light, an infra-red light receiver located in the housing interior for receiving the infra-red light emitted by the sender of the touch probe, and a unit likewise located in the housing interior for processing the infra-red light received by the receiver.

2. Description of the Prior Art

Touch measurement systems of the type described above are used for determination of a relative position or surface coordinates of a workpiece or a tool of a machine tool or of a coordinate-measuring machine upon touching. On the touch probe, at least one sender for emitting infra-red light in response to the touching is arranged. The infra-red light, which is emitted by the sender, is transmitted to a spaced from the touch probe, receiving unit that converts the received infra-red light into a trigger signal and transmits the trigger signal to the motion control that controls the displacement of the touch probe.

A touch measurement system of the type discussed above is disclosed in German Patent DE-34 22 180 C2. The receiving unit has a housing the opening of which is closed by a window permeable to infra-red light. In the window, there is arranged an infra-red filter for screening or eliminating visible light.

The touch measurement systems of the type described above should meet higher and higher requirements to their reliability. On one hand, the touch measurement systems should be able to function with the distance between the touch probe and the receiving unit increasing more and more and, on the other hand, the touch measurement systems become subjected to ever increasing influence of interference-creating devices, e.g., powerful motors.

Accordingly, an object of the present invention is to provide a touch measurement system with an increased functional reliability.

SUMMARY OF THE INVENTION

This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing the housing window with means that shield the housing interior from electro-magnetic interferences.

Advantageously, the shielding means is formed as an electro-conducting coating which is provided on the inner side or surface of the window.

According to an advantageous embodiment of the present invention, the housing has a base body connected with a reference potential, and the window has an attachment flange connectable with a circumferential mounting flange provided on the base body for connecting the window with the base body. With the electro-conducting coating also covering the inner end surface of the attachment flange, the coating becomes electrically connected with the base body in an assembled condition of the base body with the window.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Touch measurement systems according to the present invention cooperate with motion control of different machines such as, e.g., machine-tools or coordinate-measurement machines.FIG. 1shows a machine-tool2which is controlled by a numerical control1, with a touch measurement system supported thereon. The touch measurement system shown inFIG. 1includes a touch probe3and a receiving unit4.

For measuring a workpiece5, the touch probe3is displaced relative to the workpiece5, with the movement of the touch probe3being numerically controlled by the control1of the NC machine tool2. For displacing-the touch probe3, there is provided at least one drive6that is connected with the numerical control1. The displacement of the touch probe3is sensed by a position measurement device7. When a stylus31of the touch probe3touches the workpiece5, a touch signal is generated in the touch probe3. The touch signal is generated in per se known manner by a motion sensor, e.g., formed as light barrier, which detects deviation of the stylus31. The touch signal is transformed by a sender32in an infra-red light A and is transmitted cordlessly to the receiving unit4. The receiving unit4has at least one infra-red receiver41for receiving the infra-red light A. The infra-red light A is transformed in a triggering signal AT that is transmitted to the control1leading to shut-down of the drive6and to transfer or storage of an instantaneous position P that was sensed by the position measurement device7at a point of time the workpiece5was touched by the stylus31.

The receiving unit4is shown in details inFIGS. 2-4. The receiving unit4includes a housing42in the interior of which the infra-red receiver41is located. The housing42has an electrically conducting base body43, which is connected with a reference potential E, and a window44which is formed of a plastic material or glass and through which the infra-red light A can penetrate. The window44has a shield in form of an electro-conducting coating45for protecting the inner space of the housing42from an electro-magnetic radiation. As it is particularly shown inFIG. 4, the coating45is provided on the inner side of the window44and is, thus, protected from environmental influences. The coating45extend to an inner side of the attachment flange46of the window44, which is adjacent to the base body43, covering the flange inner side. Therefore, in an assembled condition of the window44with the base body43, the coating45has a good electrical contact with the outer end surface of a mounting flange47of the base body43. Thus, the coating45becomes electrically connected with the reference potential E.

The coating45of the window44is so selected that it is permeable for infra-red light having a wave length of about 880 nm without any noticeable losses, but shields from electro-magnetic interferences. The electro-magnetic interferences are interferences defined in accordance with specific technical standards and, in particular, are electro-magnetic fields with a frequency range from 80 to 1000 MHz. The window44is formed preferably of a plastic material, and the electrically conducting coating45is formed of a metal oxide, in particular, of an oxide of a semi-conducting metal. Alternatively, the coating can be formed of silver with a thickness of several nm. In order to improve the transmission of the infra-red light, the coating is advantageously formed of several reflection-inhibitive layers. Preferably, an anti-reflection coating is used.

In the embodiment shown in the drawings, the coating45covers the entire surface of the window44. However, it can cover only a portion of the surface of the window44and be formed as a grid45.1as shown inFIG. 5, or in form of any other suitable pattern, and form, together with the base body43, a Faraday cage for the infra-red receiver41arranged in the interior of the housing42.

The base body43is formed as a stable metal body. However, the base body can also be formed of a plastic material with inclusion of additive that make the base body electro-conductive. Alternatively, a plastic base body can be coated with an electrically conductive coating.

When the touch measurement system1according to the present invention is used with NC-machine-tool2, it must meet greater requirements to its reliability. It should be insured that the drive6, upon the stylus31touching the workpiece5, reliably shuts down. The infra-red light A, which is emitted by the receiving unit4, should be rapidly and correctly detected. With an operational space of the machine tool2increasing more and more, the transmission path between the touch probe3and the receiving unit4also increases more and more. On the other hand, drives6become more and more powerful, which results in a continuous increase of the range of the generated electro-magnetic interfering radiation. The present invention permits to eliminate the influence of this interference radiation on the receiving unit4. The expenses associated with filtering of the output signal of the infra-red receiver41are insignificant, and the interference-free output signal can be amplified. This insures a reliable detection of the infra-red light over a long transmission path and its transformation into a triggering signal. For transformation of the output signal of the infra-red receiver41in a triggering signal AT, the processing unit48is provided with at least one amplifying element48.1which is located in the shielded, from interferences, housing42.

In order to increase its sensitivity, the receiving unit4is provided with several infra-red receivers41the output signals of which are jointly transmitted to the processing unit48for forming the triggering signal AT. In order to detect the infra-red light A, which is emitted upon the stylus31touching the workpiece5, as soon as possible, the plurality of the infra-red receivers41are arranged parallel to each other. Thereby, a high input current, which results from the parallel connection of the infra-red receivers41, appears at the input of the amplifying element48.1, so that the ratio between the input current and the noise of the amplifying element48.1is high.

The receiving unit4, which is shown inFIGS. 2-3, includes several senders49for emitting infra-red light B though the window44in a direction toward the touch probe3. For receiving the infra-red light B, at least one infra-red receiver33is arranged in the touch probe3. In front of the receiver33, there is provided a intra-red light-transmitting window34that has an electrically conducting coating35which shields the interior of the touch probe3from the electro-magnetic radiation. Advantageously, the window34with its coating35is also located in front of the sender element32, and the coating35of the window34of the touch probe3is connected with a reference potential E for carrying away the interference voltage. The infra-red light B can be used in a per se known manner for a low-current operation of the battery-operated touch probe3.