DE202004009637U1 - Hydraulic cylinders monitoring system e.g. for monitoring position of a piston, has ultrasonic sensors divided into segments, and located on cylinder walls and applied using self adhesive carriers - Google Patents

Abstract

The position of a piston (4) within cylinder is measured by attached ultrasonic sensors (2a,b). The ultrasonic sensors can be flexible such as piezo film sensors and are divided into segments. The sensors are located on the cylinder walls and applied using self adhesive carriers. The sensors face the cylinder wall (1) and the position monitoring is obtained through an impulse echo. The segments of the ultrasonic sensors form a coded series in direction of motion of the piston and are connected together by a multiplexer and follow a successive length increment. The segments share a common electrode for the piezoelectric substrate.

Description

The The invention relates to a device for monitoring the position of a piston movable in a pressurized cylinder.

On Such a measuring system is used, for example, for pressure storage (also energy storage) used. These are used, for example, in forming machines.

But also on conventional ones Hydraulic cylinders are position determination and monitoring of the end positions necessary.

So far there are a variety of different physical solutions to this Problem. Some solutions require additional Attachments, such as measuring rods, the movement of which is recorded (optically, inductive or resistive).

magnetic field sensors on the hydraulic cylinder, however, sometimes require modifications to the piston for additional Magnets. Systems are known which monitor the position of the piston and drive through the cylinder to achieve repeatability have to.

mechanical Limit switches with switch rod and permanent magnet only allow a maximum piston speed of up to 0.5 m / s.

Furthermore there are still devices that keep the piston position continuously Show.

To wire rope measuring systems with rotary potentiometers or magnetic flaps connected. The restrictions regarding the piston speed also apply here.

newer Position measuring systems record the position of the piston with ultrasonic measuring systems. such Systems must usually screwed into the cylinder base. Through special Impedance adjustments for use in hydraulic oils the ultrasonic transducers for optimized these systems.

The reliability this system is severely limited by the temperature dependence. The ultrasound speed is strongly temperature dependent. On the Operating temperature range can be errors up to 30% or more occur. For example, one type of oil changes its Speed of sound from 1550 m / s at –20 ° C to 1250 m / s at + 80 ° C. Farther is the viscosity of the hydraulic oil also strongly temperature-dependent. The increased viscosity in turn causes an increased damping the ultrasonic waves. So that the range at low temperatures of these systems restricted.

to Minimizing the temperature error will take several measures proposed.

So can Reference measuring sections must be built into the hydraulic oil. This will a reference reflector firmly connected to the sensor in the hydraulic piston built-in. Other ultrasonic measuring systems use a second one Ultrasonic sensor for compensation. Even with an additional one Temperature measurement tries to minimize this possibility of error.

In JP 10-224946 and JP 10-241707 are such Position detectors for hydraulic cylinders described.

To JP 11-240365 is a temperature sensor over a Analog-to-digital converter connected to the microprocessor that made up the determined transit time the corrected distance of the piston to Ultrasonic sensor determined.

The the above-mentioned temperature error of the ultrasonic measurement is at rapidly moving hydraulic systems are just one of several shortcomings. conditioned the transit time of the ultrasonic pulses results in an additional one Error for the exact positioning of the hydraulic pistons.

following is described in more detail using an example of these basic shortcomings. Be given a hydraulic cylinder or a hydraulic accumulator with a piston speed of 3.5 m / s and a driving length the piston from the ultrasonic sensor to the measuring point of 2.5 meters. The reflected signal would at an oil temperature of approx. 80 ° C and an ultrasonic speed of 1250 m / s a time of Take 4 milliseconds from sending to receiving. If the repetition frequency of the transmission signals is between 10 Hz and 100 Hz the piston is already a distance covered from 35 mm to 350 mm to have.

To This error, which only results from the position measurement, comes with further errors Mistakes, so for the signal processing time and the response time for the hydraulic valve.

The Positioning the piston is definitely a significant one Error charged.

Becomes the path length for the The ultrasound signal can be significantly shortened absolute measurement errors can be reduced.

If the position measurement is carried out on the side of the cylinder, the position sensors usually protrude laterally outwards. Known sensors have previously only been mechanically mountable using clamps or welded-on fastening points. Furthermore, their dimensions expose them to possible damage.

The These sensors are only measured point by point and are usually only limited to one position.

The The invention is based on the movement of the hydraulic cylinder by a device for position measurement with one or more to detect piezoelectric elements. This piezoelectric Elements are attached to the side of the hydraulic cylinder. there these elements are flexible and adapt to the curvature of the Hydraulic cylinder.

Further advantageous embodiments of the invention result from the Features of the subclaims. In the following, the invention is to be described on the basis of exemplary embodiments become. Show it:

1 2 shows a cross section of a hydraulic cylinder with 2 sensor arrangements,

2 1 shows a schematic representation of a sensor arrangement on the surface of a hydraulic cylinder,

3 a sensor arrangement with position code,

4 a sensor arrangement with detailed sensor electronics,

5 a cross section of a sensor arrangement on the hydraulic cylinder.

In 1 is the arrangement of the ultrasonic sensors 2a and 2 B on the cylinder wall 1 of the hydraulic cylinder. The position of the piston 4 in the cylinder by interrupting the ultrasonic signal 3a be determined. The ultrasonic sensor working in pulse-echo mode 2a receives the echo from the opposite cylinder wall 1 , In a known manner, the gate for the received echoes is set in such a way that only significant signals are evaluated. When the position determined by the respective element of the ultrasonic sensor is reached, the ultrasonic signal is interrupted. The piston position can of course also be caused by the reflection of the ultrasound signal 3b on the piston rod 5 be monitored.

The one on the cylinder wall 1 attached ultrasonic sensors have surfaces that can be shaped to the wall curvature. In the 1 are the elements of the ultrasonic sensors 2a and 2 B shown schematically.

For the flexible ultrasonic sensors 2a and 2 B Piezo film sensors, PVDF film sensors or flexible composite transducers made from a mixture of cast resin and ceramic are suitable, for example.

2 shows a schematic representation of a sensor arrangement on the cylinder wall 1 a hydraulic cylinder in which the piezoelectric elements 8a . 8b and 8e are arranged as individual elements in rows so that they enable coded information about the sensor position. The coding used can be in a generally known form, for example as a cray code. The lane-wise arrangement of the segments is advantageously carried out on an adhesive carrier layer 21 , This backing layer 21 can be, for example, an ultrasound transducer coated with adhesive made of a 1-3 composite.

Coded matrix arrangements of ultrasonic transducers on an adhesive PVDF film can also cover the entire surface of the cylinder wall 1 of the hydraulic piston at the positions to be detected.

The design of the position-sensitive ultrasonic transducers requires only small overall heights. This means that no design changes to the hydraulic cylinders are necessary. For the mechanical protection of these ultrasonic transducers, a protective plate adapted to the curvature of the hydraulic cylinders or a similarly designed protective cover that extends over the carrier layer is sufficient 21 is arranged. The space between the carrier layer 21 and the protective cover (not shown) can be filled in a known manner with a layer that does not conduct the ultrasound.

3 shows an embodiment with 4 tracks ( 9a . 9b . 10a and 10b ) of piezoelectric elements. These are operated via a switch 17 driven. The piezoelectric elements are used, for example, by an ultrasound transmitter 13 driven. This ultrasound transmitter 13 (Pulse generator) can be used to control several piezoelectric elements. The ultrasonic echoes received by the piezoelectric elements are generated by several ultrasonic receivers 14 amplified and the microprocessor 15 fed. The control of the transceiver switch 17 is also done by the microprocessor 15 , The position signal 16 is on the microprocessor 15 tapped.

The tracks can be used to improve the spatial resolution 9a and 10a spatially to the tracks 9b and 10b be spaced. The distance should be selected so that there is no crosstalk from the reflected echo of the a tracks to the b tracks and vice versa.

4 shows another way of obtaining position signals. Here are the piezoelectric elements 9a and 9b from the ultrasound transmitter 13 controlled in parallel. The received echo signals become separate ultrasound receivers 14 fed. The triggered signals are, for example, via a decoder / demultiplexer 18 (1-out-8) encoded in 3-digit binary numbers. In a known manner, many position values can be obtained with limited input channels of the microprocessor. With special ASICs, simple 6-bit decoders in a matrix structure for querying the piezoelectric elements can also be arranged on the carrier. A possibility that is not shown further would be the use of cascaded circuits (for example: 74HC138). The diverse possibilities of decoding are well known

5 shows a cross section through a hydraulic cylinder. On the cylinder wall 1 are the a-tracks 9a . 10a and b tracks 9b . 10b , For better signal separation, these traces are staggered on the wall circumference so that the echo signals 19a and 19b after reflection on the piston rod 20 , do not cross-talk to the adjacent track.

Claims (8)

Device for monitoring the position of a piston movable in a pressurized cylinder by means of ultrasound sensors for pulse-echo operation, which are attached outside the cylinder, characterized in that the ultrasound sensors are divided into segments, on the side facing the cylinder wall, which have moldable surfaces, and on the entire surface Fit the cylinder wall. Device according to claim 1, characterized in that the segments are applied to a self-adhesive carrier layer. Device according to claim 1 to 2, characterized in that the segments of the ultrasonic sensors have a length code form in the direction of movement of the movable piston. Device according to claims 1 to 3, characterized in that that the segments made of the piezoelectric substrate with at least are connected to a multiplexer. Device according to one of the preceding claims, characterized characterized in that several segments share a common electrode for the possess piezoelectric substrate. Device according to one of the preceding claims, characterized characterized in that the segments are arranged in a coded sequence are. Device according to one of the preceding claims, characterized characterized in that the segments of successive length increments have a lateral offset that is greater than the diameter of the evaluable amplitude of the reflected signal lobe of the echo, for crosstalk to avoid the received reflections. Device according to one of the preceding claims, characterized characterized that the multiplexer and other signal-generating and signal processing circuit elements with the segments one Form unity.