Electro-mechanical actuator system, for instance for adjustable furniture

The present invention relates to an electro-mechanical actuator system for instance for adjustable furniture as stated in the preamble of claim 1. This type of actuator systems is for instance known from WO 02/29284 A1 to

Linak A/S, WO 01/17401 A1 to Linak A/S and WO89/10715 A1 to Dewert concerning a linear, a rotary- and a dual actuator, respectively. Actuator systems especially for height-adjustable tables are also known, cf. for example WO 02/39848 A1 to Linak A/S and EP 1 079 511 A1 to Linak A/S, concerning a linear actuator and a power supply/control unit, respectively.

In recent years a marked interest for electrically adjustable furniture has arisen followed by a parallel product development to reduce the prices of the furniture to a price level making them available to the common man.

To comply with the demand for price reductions of the products, the recognition that the furniture is only occasionally adjusted and the fact that the actual adjustment is quite rapidly done, is being utilized. On these grounds, power supplies and motors are chosen, which at the prescribed maximum load of the furniture are loaded to absolute maximum performance, and a brief overload is even permitted. Hereby, smaller and more inexpensive low-voltage DC-motors and power supplies can be chosen. In return a duty cycle is introduced, which means that the actuator is only permitted to run for a certain amount of time in the course on an hour. A duty cycle of for instance 10 means that the actuator can only run for 6 minutes in the course of an hour to prevent superheating.

As a safety measure a thermal fuse, which cuts off the current to the motor if the temperature exceed a predetermined critical temperature is often mounted in connection with the transformer, cf. for instance DE 200 08 192

U1. A problem with thermal fuses is that successive damages can be inflicted on the rest of the control unit before the fuse is activated.

The purpose of the invention is to provide a more accurate and effective protection from superheating.

This is achieved according to the invention in that the electro-mechanical actuator system is constructed according to claim 1 , where the power supply and/or the electric control unit comprises at least one thermal diode, where the voltage is inverse proportional to the temperature and calibrated so that a given voltage is converted into a given temperature, and where the control unit is arranged so that the current for the motor of the actuator is cut off at a predetermined critical temperature. Diodes are small, inexpensive and relatively accurate components which can be mounted on a circuit board, and they can be located where the temperature conditions are expected to be critical. To begin with, it is decisive that the current for the motors are cut off if a superheating should occur, so that successive damages also to the actuator system are avoided. It is of course not possible to know exactly why a superheating of the actuator system has occurred but unlike the thermal fuse it is possible to gather certain information as to where in the system the superheating has occurred. It is noted that superheating is also here registered as a result of mechanically developed heat, and thermal diodes can deliberately be placed accordingly.

In an embodiment the voltage over the diode is not directly used, but is used for a pulse generator which is traced by a micro processor. However the voltage over the diode still determines the cyclic time. A long period corresponds to a low temperature while a short period corresponds to a high temperature. The micro processor measures the cyclic time at start-up (for instance tests during production), and uses it as basis. The temperature is hereafter calculated as a change in the cyclic time. The basic defects and the variations of the diodes are hereby removed from the tracing system.

A special advantage by the invention is that a random number of diodes can be coupled in parallel, which in that way provides more measuring points - an extra measuring point is merely a diode at a moderate cost. It is of course

not possible to distinguish between the individual diodes but normally it does not matter where the system is too hot as it will shut down anyhow.

An embodiment for the invention will be explained more fully below with reference to the accompanying drawing, in which:

Fig. 1, shows a diagram of a embodiment according to the invention.

The embodiment shown in the drawing comprises four diodes D1,D2,D3,D4, arranged in parallel, connected to a power source and connected with a multimeter. The diodes are located strategically in the actuator system, where the system is traditionally known to be superheated as a result of overload, also as a result of defects. Such a strategic location can for instance be in connection with FET-transistors for controlling the actuator. If the area surrounding for instance the diode D2 becomes hotter than the area surrounding the other diodes, the voltage will fall as a result of the fact that the resistance in the diodes falls. This is recorded by the multimeter V, and on the basis of this signal the current for the motor can be cut off.

Though the description of the invention departs in furniture, it is evident, that the invention can be used in connection with other applications of actuator systems.