GB2090470A - Piston position sensing device - Google Patents

Abstract

A sensing device for determining that the spool or piston 32 of a hydraulically operated valve has reached the end of its travel comprises a plug 10 which is inserted into the valve body 30 and has its contact 12 earthed through the piston 32 to the body of the valve 30 at the end of each stroke. The duration of each cycle may thereby be monitored and an alarm produced in the event of the duration of a cycle exceeding a pre-determined limit. <IMAGE>

Description

SPECIFICATION Spool valve cycling sensor The invention relates to a sensor for determining that the spool or piston of a hydraulically operated valve has reached the end of its travel or to determine that a cycle has occurred, (although not limited thereto). The device is particularly, though not exclusively, suitable for monitoring the operation of single line valves of progressive type, commonly known as dividers.

The monitoring of single line dividers presents particular problems when these valves are operating continuously at high cycling rates. The conventional methods of monitoring at low cycling rates is by means of microswitches which are operated by rods attached to the reciprocating spools or pistons. These rods protrude through the end of the valve via packing glands.

Hydraulic packing are subject to wear with consequent leakage of lubricant which may become sufficiently severe to cause underlubrication of vital machinery. Additionally the piston rods reduce the effective area of the piston therefore the pressure must be increased to produce the same effective thrust.

The volume of discharge is also reduced by an amount equal to the space occupied by the rod. Most hydraulic seals create drag which is proportional to the pressure therefore a compensating increase in pressure results in a further increase in drag.

However, the life of a microswitch rated at 106 operations is less than three weeks when operating continuously at 60 cycles per minutes which is a typical cycling rate on oil recirculation systems. This type of switch or any other form of mechanical switch cannot therefore be used.

Even if the life of a microswitch were sufficient for this application a special housing for the switch would be necessary to withstand the high temperatures, humidity and acidity encountered on paper mills and some steel mills. Furthermore the space taken up by the switch presents problems in the limited space which is usually available.

The same objection applies to all externally mounted switches of whatever type. Reed switches operated by magents attached to piston rods require pressure-tight non-ferrous housings. Proximity switches of inductive or capacitive type require similar housings. Being small signal devices they also require closely coupled amplifiers to produce a sufficiently strong signal to withstand induced noise. A minimum of three electrical conductors is usually required. The same objections apply to photo electric switches, piezo electric switches and strain gauges. The cost of providing and installing these devices is also a considerable factor particularly where a large number of dividers are involved.

According to the present invention, there is provided a device for use in a monitoring system for a lubrication system which contains proportioning or metering valves with reciprocating pistons, the device serving to sense the position of a piston of a valve and comprising a contact insulatedly mounted within a plug and operative to come into contact with the piston at predetermined positions, the low electrical resistance between the piston and the body of the valve resulting in the contact being effectively earthed to the valve body when the piston is in said predetermined positions.

The invention detects the position of the divider piston by using the piston itself as an earth contact. Although the piston is surrounded by lubricant which would seem to act as an insulator the electrical resistance between the piston and the body of the valve has been surprisingly found to be quite low.

The greatest measured resistance on valves with many years of service was 3 ohms. This is a sufficiently low resistance to pull a 1 2 V DC 20 mA signal line down to 60 mV. Using C-MOS logic gates a resistance of even 300 ohms-would be quite sufficient to guarantee a low logic level. The piston can thus be regarded as an earth contact.

When contacts are used for switching low level DC signals, they usually require plating with precious metals to prevent oxidisation of the surfaces. This is not necessary in this application because the contacts are immersed in oil which prevents both arcing and oxidisation. A test installation on site has, to date, completed more than 50 X 106 cycles, with no sign of contact degradation; In a preferred form of the device the piston is of hardened steel and the contact is of stainless steel.

The invention will now be described further with reference to the accompanying drawings, in which: Figure 1 shows a section through a plug to be screwed into the tapping on the lubrication valve which is normally occupied by the standard closure plug.

Figure 2 shows a connector with attached signal cable fitted to a plug.

Figure 3 is a cross-sectional view of one of the sections of a typical divider with plug and connector fitted.

Figure 4 illustrates a basic circuit for monitoring the operation of continuously cycling dividers.

In Fig. 1 an externally threaded plug body 10 houses a central rod 1 2 which is radiused at its inner end to form a contact. The outer end 14 is shaped to fit a connector. The rod is retained within the body of the plug by rigid insulating medium 1 6 such as epoxy resin.

Fig. 2 shows a right angle snap fit connector 20 of heat resistant thermoset material fitted to the end of a cable 24.

In Fig. 3 the steel divider body 30 is bored to receive a three landed piston 32. The bore is tapped at both ends to suit a standard closure plug 34 and the special closure plug 10 as shown in Fig. 1. The plug is fitted with the right angle connector 20 shown in Fig. 2 In Fig.4 the signal cable 24 provides and input signal to a monostable circuit 36 the square wave output from which is used to reset and retrigger a missing pulse generator 38 the output of which goes low if a pulse is not received within the period set by a rotary switch 40. A low output will illuminate an indicator 42 and will also operate an alarm relay 44 via a common alarm line 46. The square wave output from the monostable 36 is also used to briefly illuminate an indicator 48 to indicate that a cycle of the divider has occurred.

One such circuit as shown in Fig. 4 is required for each monitored divider. In a preferred form of the device ten identical circuits are incorporated onto a printed circuit board. A standard 3U sub-rack can accomodate a number of such p.c.b's. plus a power supply module of conventional design.

No screening of the signal cables is necessary. The cables are effectively earthed when the piston is against the stop and as the piston moves away from the stop the voltage on the cable will rise through a high logic level triggering the monostable which can only be reset by a succeeding low logic level.

The cables are thus immune to noise; Should a cable become detached then the missing pulse generator will operated the alarm. The circuit is thus totally fail safe because only the correct signal at a sufficient frequency can prevent the alarm operating.

Although described particularly with reference to the monitoring of continuously cycling single-line dividers, the device is also well suited for monitoring other types of lubrication systems. For example dual line lubrication systems, which operated intermittently, can most conveniently be monitored by detecting the position of the pistons. The sections in a dualine valve manifold operate individually unlike single line dividers wherein each piston is hydraulically interlocked so that the pistons operate in a fixed sequence. It is often necessary to monitor all the sections in a dualine manifold serving vital bearings. This is traditionally effected by means of micro switches operated by piston rods but the mounting of a number of micro switches in protective housings onto one manifold is difficult and access for wiring these switches is very limited. Use of the devices of this invention would overcome these objections.

The subsequent savings on cost makes the monitoring of large numbers of valves a viable proposition. This is particularly relevant to the increasing use of computers for process control and monitoring.

The device will operate reliably at 5 V and the signal lines can therefore be interfaced directly or more usually via one or more multiplexers to a computers 1/0 port.

Claims (6)

1. A device for use in a monitoring system for a lubrication system which contains proportioning or metering valves with reciprocating pistons, the device serving to sense the position of a piston of a valve and comprising a contact insulatedly mounted within a plug and operative to come into contact with the piston at predetermined positions, the low electrical resistance between the piston and body of the valve resulting in the contact being effectively earthed to the valve body when the piston is in said predetermined positions.

2. A device as claimed in Claim 1, wherein the contact acts as an end stop for the piston and the piston is held against the contact by hydraulic pressure.

3. A monitoring system comprising a device as claimed in Claim 1 or Claim 2 and an electronic timing circuit responsive to the changes in the electrical resistance between the contact and the valve body and operative to initiate an alarm in the event of the time between consecutive strokes of the piston exceeding a predetermined threshold.

4. A monitoring system as claimed in Claim 3, wherein the change in resistance is further utilised to provide a visual indication of each cycle of every monitored valve.

5. A monitoring system as claimed in any preceding claim, wherein the change in resistance is monitored by a microprocessor based system.

6. A monitoring system constructed, arranged and adapted to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.