A linear actuator

The present invention relates to a linear actuator of the type which is based on a spindle, as defined in the introductory portion of claim 1.

Actuators of this type are generally known and used. By way of example reference may be made to WO 98/30816 A1 to Linak A/S, which relates to an actuator with an activation rod guided in an outer tube, and DK 174 457 B1 to Linak A/S, in which the spindle nut is intended for direct attachment to the application. An example of an application is found e.g. in EP 488 552 A1 to Huntleigh Technology pic, which relates to a hospital bed with several actuators coupled to a common control box containing a power supply and a control. A special embodiment of actuators for domestic beds with two spindles, an integrated power supply and control is known from EP 372 032 B2 to Dewert Antriebs- und Systemtechnik GmbH & Co. KG.

A spindle with an associated spindle nut is a central element of these ac¬ tuators. In case of spindles of a non-self-locking type, they are provided with a brake, or, alternatively, a transmission with a low efficiency is se¬ lected, so that the transmission is self-locking. A perfect operation of these elements is critical to the operation of the actuator. A typical transmission is a worm gear which is of plastics owing to noise, but on the other hand it is more exposed to wear than a worm gear of metal. In order to stop the acti- vation rod of the actuator, the actuator is provided with end stops. It may be end stop switches which are activated by the spindle nut or an outer pipe. Another form of end stops is a mechanical stop for the spindle nut. Still an¬ other form of end stops is based on optical or magnetic encoders which count the revolutions of the spindle. When the nut engages the mechanical stop, the motor current is increased considerably and is interrupted by the control electronics. Again, a safe function of these elements is conditional

upon a perfect function of the spindle and the spindle nut. Indirect damage may also occur because of wear of the seal between the activation rod and the outer end of the surrounding protective tube. Penetrating dust and moisture also act to wear out the spindle and the spindle nut, just as end stops and other components are likewise exposed to wear-out and mal¬ function.

To prevent damage to individuals and equipment, the actuators may be equipped with various safety measures, such as safety nut, spline function, electrical overload protection, etc.

An example of a safety-critical application is a patient lifter which acts like a crane and can lift patients out of beds, chairs, etc. In such an application, it is critical that the actuator does not fail suddenly.

The manufacturers of actuators guarantee a certain service life against compliance with mounting specifications and service instructions. However, it has been found that the actuators are used far beyond the guaranteed service life, where the risk of failure is the greater the longer they are used. Nor is it unusual that the actuators are used for greater loads than the pre¬ scribed maximum load, or in an environment for which they are not in¬ tended.

In the event of an unfortunate occurrence or direct accidents, it may be dif- ficult to elucidate the concrete conditions of the actuator which have caused the event. All that one has to go by is various marks which make it possible to recover the product specifications and the production time at the manu¬ facturer's. First and foremost, it is a matter of avoiding unfortunate occur¬ rences and accidents, but to achieve this it is decisive that the manufactur- ers get an adequate basis for improving the quality and the safety of the actuators. In the second place, allocation of responsibility is also involved,

which, in turn, influences the insurance conditions and contracts. In this connection it is observed that the cost level plays a decisive role. If the selling price is too high, it is seen that safety measures having no im¬ mediate effect are not adopted. It is noted that for cost reasons the actua- tors are constructed such that they can momentarily accept a certain over¬ load, a constraint in the form of an intermittence, typically 10%, being si¬ multaneously imposed, which means that an actuator may only run for 6 minutes out of an hour.

The object of the invention is to provide an actuator, where a more ade¬ quate basis for evaluating the state of the actuator may be obtained.

This is achieved according to the invention in that the actuator is equipped with a memory unit for collecting at least one set of data from the operation, indicating how long and how hard the actuator has worked.

The information, which is saved/collected, may be collected by a micro¬ processor likewise disposed in the actuator and be saved in the internal memory, if so desired. Alternatively, a microprocessor may be disposed in a controller elsewhere in the system, e.g. in a control box arranged externally relative to the actuator, and collect and save this information in the actuator via a form of bus. It is ensured hereby that the information follows the ac¬ tuator in case of replacement, rebuilding, etc. Further, a form of physical memory is conceivable, where the user "draws on" the memory on the ba- sis of e.g. the consumption of power and elapse of time.

The invention is contemplated for use together with a collection of a plural¬ ity of parameters, which might e.g. be, but are not necessarily restricted to, power, time, temperature, load, voltage, speed.

A message to the user to the effect that the product is approaching wear-

out, may take place in that the actuator, on its own, provides information in the form of visual or acoustic signals, lacking functionality and the like, but may alternatively also be done by a form of service reading which may be made in periodic intervals by service staff, or more regularly by a connected controller which itself contains a form of signaller.

It is important to note that it is not necessarily possible beforehand to set up the criteria of the wear-out level. It is conceivable that just a collection of data takes place, and that the evaluation of these data then takes place at a later time. This will typically be the case at the start of a production of an entirely new application. It is conceivable that, then, the wear-out criteria will be available only after the product has been on the market for some time.

Embodiments of the invention will be described more fully below with refer¬ ence to the accompanying drawing, in which:

Fig. 1 shows a perspective view of the actuator,

Fig. 2 shows a longitudinal section through the actuator along the line A-A ₁

Fig. 3 shows a perspective view of the actuator provided with a control box.

As will appear from the drawing, the main components of the actuator are a cabinet 1 , a motor 2, a worm gear 3, a spindle 4, a spindle nut 5, an activa¬ tion rod 6, an outer pipe 7 and a rear attachment 8. The motor is a re¬ versible electric motor, typically a 24V or 48V DC motor.

As will appear from fig. 3, the actuator may be equipped with a control box 9 containing control electronics as well as a power supply, where 10 desig¬ nates the mains cable. The cable 11 from the actuator is connected with

one of the outputs 12 of the control box, but further actuators as well as hand controls for the other outputs may be connected.

A memory unit is arranged in the actuator for the collection of data from the operation, indicating how long and how hard the actuator has worked. These data are collected by a microprocessor disposed in the control box.

To evaluate the state of the actuator, it is possible to use the power as a parameter, which may be measured in a simple manner across a serial re- sistor in the power path to the motor. The size of the power is an indication of how severely the actuator is loaded. Another important parameter is the period of time during which the power has been switched on, which may be obtained directly from the microprocessor.

Another important parameter is the temperature, which is obtained from a temperature sensor arranged in the actuator or in another manner. It may hereby be recorded whether the actuator has been exposed to steep in¬ creases in temperature, e.g. as a consequence of abnormally severe load¬ ing.

An alternative or supplement to the foregoing is the provision of strain gauges for measuring the load. These may e.g. be arranged in connection with the spindle bearing.

A further alternative is to sum up the revolutions of the spindle, which may be carried out by a mechanical counter, optical or magnetic encoders, scanning of the commutator pulses of the motor. In special applications, the actuator may be provided with a linear slide potentiometer positioned in parallel with the spindle, and also the signal from this may be used as a parameter for the evaluation of the state of the actuator.

To signal wear-out of the actuator, this and/or the control box may be equipped with light emitting diodes, which signal wear-out or optionally the degree of wear-out by means of a colour code. More expediently, however, the signal generator is arranged in the hand control, where it is directly vis- ible.

The invention thus provides a linear actuator, where it is possible to make an evaluation of the instantaneous mechanical state of the actuator on the basis of at least one set of data from the operation, indicating how long and how hard the actuator has worked. In the first instance, the invention is contemplated for actuators fresh from the factory, but may also be imple¬ mented in connection with already existing actuators, of course, typically in connection with an overall service of a group of applications, e.g. all the patient lifters of a nursing home. It might e.g. be incorporated in the rear attachment, which is typically replaceable and moreover extends into the cabinet.