BALANCE WITH ACTIVE HEAT FLOW CONTROL

Abstract

A balance (30) is a single unit with a weighing pan (21) enclosed in a weighing compartment (22). A housing (23) adjoins the weighing compartment. The balance housing contains a weighing cell compartment (24) enclosing a weighing cell, an electronics compartment (25) containing electrical and electronic circuit elements, a thermoelectric heat pump module (27) and a heat flow controller (28). The balance is equipped to determine a net heat flow (P.sub.net) inside the housing in the direction from the weighing cell compartment to the electronics compartment. The heat flow controller uses the net heat flow as a control input to regulate the the thermoelectric heat pump module, arranged inside the housing. The control input is used to generate an active heat flow (P.sub.A) with magnitude and direction for holding the net heat flow at a level equal to the rate of heat dissipation produced inside the weighing cell compartment.

Claims

1. A balance configured as a single unit and comprising: a weighing compartment; a weighing pan enclosed in the weighing compartment; a balance housing, adjoining the weighing compartment and comprising: a weighing cell compartment, enclosing a weighing cell; an interstitial space, separated from the weighting cell compartment by a first interior wall; and an electronics compartment, containing electrical and electronic circuit elements, separated from the interstitial space by a second interior wall; a means for determining a net amount of heat flow P.sub.net inside the balance housing from the weighing cell compartment in the direction of the electronics compartment and for providing a signal corresponding to the magnitude and direction of the net amount of heat flow; a heat flow controller, in the balance housing; arranged to receive the signal as a control input; a thermoelectric heat pump module, configured as a Peltier module arranged in the interstitial space with a first side thereof thermally connected to the first interior wall and a second side thereof thermally connected to the second interior wall, the Peltier module operating in a heat pumping mode with the first side removing heat and the second side producing heat to generate an active heat flow P.sub.A with a magnitude and direction to hold the net amount of heat flow P.sub.net at a level that essentially equals the rate of heat dissipation produced inside the weighing cell compartment, where the Peltier module is regulated by an electric current driven therethrough from the heat flow controller; and insulating material, filling the interstitial space surrounding the Peltier module with an amount of passive heat flow P.sub.i occurring through the insulating material; such that P.sub.net is defined by the equation:
P.sub.net=P.sub.AP.sub.i.

2. The balance of claim 1, wherein: the Peltier module serves as the means for determining P.sub.net by being temporarily switched from the heat-pumping mode to a thermoelectric generator mode in which: when a temperature difference T.sub.I exists across the insulating material between the first interior wall and the second interior wall, the Peltier module generates a voltage signal U.sub.Seebeck according to the relationship
T.sub.I=k.sub.2U.sub.Seebeck and the heat flow controller receives the voltage signal U.sub.Seebeck, as the control input and, using the further relationship:
Pi=k.sub.1 T.sub.I=k.sub.1 k.sub.2 U.sub.Seebeck determines P.sub.net.

3. The balance of claim 1, further comprising: a third interior wall that adjoins the weighing cell compartment; a boundary layer of insulating material having a thermal resistance R.sub.th, arranged between the third interior wall and the first interior wall; and a pair of temperature sensors, placed, respectively, on the third interior wall and on the first interior wall; such that the pair of temperature sensors serve as the means for determining P.sub.net by sending at least one temperature signal as the control input signal to the heat flow controller for calculating the heat flow through the boundary layer, which represents the net heat flow P.sub.net.

4. The balance of claim 3, wherein: the first of the pair of temperature sensors measures a temperature T.sub.1 of the third interior wall and the second of the pair of temperature sensors measures temperature T.sub.2 of the first interior wall, and each of the temperature sensors sends a temperature signal to the heat flow controller, where the net heat flow P.sub.net is calculated according to the equation: $P_{\mathrm{net}}=\frac{T_1-T_2}{R_{\mathrm{th}}}.$

5. The balance of claim 3, wherein: the pair of temperature sensors comprises a pair of thermocouples, connected anti-serially with the junctions placed, respectively, on the third interior wall and on the first interior wall, so that the pair of thermocouples measure a temperature difference in which the temperature of the first interior wall is subtracted from the temperature of the third interior wall, and the pair of thermocouples send a temperature difference signal T to the heat flow controller, where the net heat flow Pnet is measured according to the equation: $P_{\mathrm{net}}=\frac{.Math..Math.T}{R_{\mathrm{th}}}.$

6. The balance of claim 3, further comprising a second Peltier module, arranged to operate in the thermoelectric generator mode to do the following steps: measure a first temperature T.sub.1 of the third interior wall and a second temperature T.sub.2 of the first interior wall; generate a temperature difference signal T with a magnitude and direction determined by subtracting the second temperature from the first temperature; and send the temperature difference signal T to the heat flow controller, and where the heat flow controller calculates the net heat flow P.sub.net according to the equation: $P_{\mathrm{net}}=\frac{.Math..Math.T}{R_{\mathrm{th}}}$

7. The balance of claim 1, wherein: the electronics compartment is divided into a weighing electronics chamber for containing temperature-sensitive, primarily analog electronic circuits and a digital electronics chamber for containing primarily digital and power circuits that are less temperature sensitive.

8. The balance of claim 7, further comprising: a third interior wall, adjoining the weighing cell compartment; a boundary layer of insulating material having a thermal resistance R.sub.th; and a fourth interior wall, such that the boundary layer of insulating material is between the third interior wall and the fourth interior wall and the weighing electronics chamber is located between the fourth interior wall and the first interior wall; and a pair of temperature sensors, placed on the third interior wall and the fourth interior wall, the pair of temperature sensors operating as the means for determining the net heat flow P.sub.net by measuring the respective temperatures and sending at least one temperature signal to the heat flow controller, where the heat flow through the boundary layer is calculated and is used to represent the net heat flow P.sub.net out of the weighing cell compartment.

9. The balance of claim 1, further comprising: additional electronic elements that dissipate heat are incorporated in, or attached to, the weighing cell compartment, such that the rate of heat dissipated by the additional electronic elements is calculated and/or measured and a signal representative thereof is sent as an additional input to the heat flow controller in order to regulate the net heat flow (P.sub.net) as required to remove the increased amount of heat due to the additional electronic elements; wherein the additional electronic elements comprise at least one of: an auxiliary display panel and an electronic out-of-level sensing device.

10. The balance of claim 1, further comprising: cooling fins, arranged on an exterior wall portion of the balance housing, the cooling fins exposed to the ambient atmosphere.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The balance according to the invention will be described hereinafter through embodiments shown schematically in the drawings, wherein

[0033] FIG. 1 shows a modular balance according to the state of the art;

[0034] FIG. 2 shows a balance according to the invention seen from the front;

[0035] FIG. 3 represents a first embodiment of the invention with active heat flow control between the weighing cell compartment and the electronics compartment;

[0036] FIG. 4 represents a second embodiment of the invention with active heat flow control between the weighing cell compartment and the electronics compartment;

[0037] FIG. 4a shows an alternative heat sensor arrangement for the embodiment of FIG. 4; and

[0038] FIG. 5 represents a third embodiment of the invention, wherein the interior of the balance housing is divided into a weighing cell compartment, a weighing electronics chamber, and a digital electronics chamber.

DETAILED DESCRIPTION OF THE DRAWINGS

[0039] FIG. 1 shows a state-of-the-art balance 1 which is manufactured by the assignee of the present invention. The temperature-sensitive parts of the balance 1, i.e. the weighing compartment 5 and adjoining weighing cell compartment 4 are joined together in a weighing module 2. The analog and digital electronics as well as electrical parts that produce most of the power dissipation are joined together in a separate modular unit 3, commonly referred to as weighing terminal, which includes in this case the touch screen display 6 and the electronics module 7. As mentioned previously, the modular layout of the state-of-the-art balance 1 provides a satisfactory solution to mitigating the thermal problems caused in an analytical balance by the power-dissipating electronics and electrical parts, but the modular configuration is inherently more expensive and more space-consuming than a balance that is configured as a single unit.

[0040] Therefore, the task presented itself to combine the weighing module 2 and the electronics module 7 of the prior-art balance 1 in a single unit as represented by the balance 10 in FIG. 2, which shows a perspective frontal view of the balance 10 according to the invention as it presents itself to a user, with the cylindrical weighing compartment 12 and the door handle 13 in front, adjoined by the balance housing 14 with cooling fins 15, adjustable leveling feet 16, and power- as well as interface connector sockets 17 at the rear. The parts which in the prior-art balance 1 were assigned to the weighing cell compartment 5 and the electronics module 7 are now enclosed in the balance housing 14. An optional, auxiliary weight display 18 is mounted on the balance housing directly above the weighing compartment 12. The main user interface of the balance 10, analogous to the touch screen display 6 of the prior-art balance 1, is still configured as a separate module (not shown in FIG. 2). The rationale for keeping the main user interface a separate module is to protect the balance 10 from thermal and mechanical disturbances which could be caused by the hands of an operator when pressing the keys on a touch screen or other input device if the latter were integrally incorporated in the balance 10.

[0041] FIG. 3 illustrates a first embodiment of a balance 20 according to the invention in the single-unit configuration of the balance 10 of FIG. 2, with a weighing pan 21 enclosed in a weighing compartment 22 and with a balance housing 23 adjoining the weighing compartment 22, wherein the balance housing 23 encloses a weighing cell compartment 24 containing a weighing cell (not shown in the drawing), an electronics compartment 25 containing electrical and electronic circuit elements (not shown), a thermoelectric heat pump module 27, typically a Peltier module 27, and a heat flow controller 28. The Peltier module 27 is sandwiched between a first interior wall 33 and a second interior wall 35, and the interstitial space 34 surrounding the Peltier module 27 is filled with an insulating material 37. The interior walls 33 and 35 are made of a heat-conducting material, so that the first interior wall 33 has an essentially uniform first temperature, and the second interior wall 35 has an essentially uniform second temperature.

[0042] FIG. 3 further illustrates the heat sources and heat flows in the balance 20. As mentioned previously, the weighing cell of an analytical balance is very sensitive to temperature, and the generation of heat in the weighing cell compartment 24 is therefore kept to a minimum, typically of the order of 0.1 W as indicated graphically in the drawing. In contrast, the heat generation in the electronics compartment in balances according to the invention is typically of the order of 3 W. As a result of the larger heat dissipation and therefore higher temperature in the electronics compartment 25, a passive heat flow P.sub.I occurs from the interior wall 35 through the insulating material 37 to the interior wall 33. The Peltier module 27, on the other hand, removes heat from the interior wall 33 and generates heat on the side of the interior wall 35, thus producing an active heat flow P.sub.A in the opposite direction of the passive heat flow P.sub.I. The difference P.sub.AP.sub.I=P.sub.net represents the net heat flow in the direction from the weighing cell compartment 24 to the electronics compartment 25. Finally, the arrows P.sub.O represent the heat flow from the electronics compartment 25 to the outside by way of the cooling fins 36.

[0043] In the embodiment of FIG. 3, the Peltier module 27 is alternatingly switchable between two operating modes. In a first operating mode, the Peltier module works as the heat-pumping module that generates the active heat flow P.sub.A as described above. In a second operating mode, the Peltier module works as a sensor for a temperature difference between the first side and the second side of the Peltier module. If a temperature difference T.sub.I exists between the first side and the second side of the Peltier module, the latter generates an electrical signal in the form of a voltage representative of the temperature difference T.sub.I and sends it to the heat flow controller 28.

[0044] The generation of an electrical signal in the form of a voltage in response to a temperature difference T.sub.I is referred to as Seebeck effect. From the measured voltage U.sub.Seebeck the temperature difference can be calculated as

T.sub.I=k.sub.2U.sub.Seebeck,

wherein the factor k.sub.2 depends mainly on properties of the Peltier module that can be determined in the product development of the balance.

[0045] Next, based on the temperature difference T.sub.I, the heat flow P.sub.I through the insulating material 37 can be calculated as

P.sub.I=k.sub.1T.sub.I,

wherein the factor k.sub.1 depends on the dimensions and properties of the insulating material 37 and can be determined experimentally in the product development of the balance. Finally, the heat flow P.sub.net is determined as the difference of the heat flow P.sub.A that occurred previously in the heat-pumping mode of the Peltier module minus the heat flow P.sub.I determined from the temperature difference T.sub.I through the last equation above, i.e.

P.sub.net=P.sub.AP.sub.I.

Based on the net heat flow P.sub.net, the heat flow controller 28 determines the active heat flow P.sub.A to be generated by the Peltier module 27 after it will have been switched back into heat-pumping mode. The Peltier module 27 is regulated by the heat flow controller 28 to generate an active heat flow P.sub.A of a magnitude and direction to keep the net heat flow P.sub.net at a controlled level that is necessary to remove the heat produced inside the weighing cell compartment 24.

[0046] If additional power-dissipating parts are added to the weighing cell compartment 24, such as for example the optional, auxiliary weight display 18 shown in FIG. 2 which uses about 0.2 watt, or an electronic level indicator which uses about 0.1 watt, the power dissipation originating from the weighing cell compartment 24 increases commensurately and can further be variable. As it affects the required level of heat flow P.sub.net that needs to be maintained by the heat flow controller 28, the additional power consumption needs to be calculated or measured and provided to the heat flow controller 28 as an additional input.

[0047] In contrast to the foregoing embodiment of FIG. 3, where the Peltier module 27 serves both as a heat pump and as an indirect means to determine the heat flow P.sub.net, the balance 30 in FIG. 4 uses a separate means dedicated only to the determination of the net heat flow P.sub.net. As illustrated, the heat flow determining means is configured as a pair of temperature sensors 31, 32 placed on opposite sides of a flat boundary layer 29 of insulating material of a known thermal resistance R.sub.th. The temperature sensors 31, 32 measure, respectively, a first temperature T.sub.1 in the third interior wall 38 on the side of the boundary layer 29 that faces towards the weighing cell compartment 24 and a second temperature T.sub.2 in the first interior wall 33 on the side of the boundary layer 29 that faces towards the electronics compartment 25. Based on the electrical signals received from the sensors 31, 32, the heat flow controller 28 calculates the heat flow through the boundary layer 29 (which equals the net heat flow P.sub.net=P.sub.AP.sub.I) according to the equation:

[00003]$P_{\mathrm{net}}=\frac{T_1-T_2}{R_{\mathrm{th}}}.$

[0048] FIG. 4A shows an alternative to the temperature-sensing arrangement of FIG. 4. The means to determine the heat flow through the boundary layer 29 includes a pair of anti-serially connected thermocouples wherein two thermocouple materials A and B are joined in a sequence A-B-A and the junctions A-B and B-A (shown, respectively, as 31, 32) are placed on opposite sides of the boundary layer 29. Such a thermocouple pair 31, 32 can measure the temperature difference T.sub.1T.sub.2 directly and send a corresponding signal to the heat flow controller 28.

[0049] FIG. 5 illustrates a further embodiment of a balance 40 according to the invention, which differs from the balance 20 of FIG. 3 and the balance 30 in FIG. 4 in that the electronics compartment 25, is now divided into a weighing electronics chamber 41 containing temperature-sensitive, primarily analog electronic circuits and a digital electronics chamber 42 containing primarily digital circuits and other power-dissipating elements. The parts of the balance 40 that are identical to those of the balances 20 and 30 are identified by the same reference symbols as in FIGS. 3 and 4.

[0050] Within the balance 40 of FIG. 5, the weighing electronics chamber 41 borders on the boundary layer 29 across a fourth interior wall 39, while the opposite side of the weighing electronics chamber 41 is delimited by the first interior wall 33. The digital electronics chamber 42 occupies the end of the balance housing 23 beyond the second interior wall 35. Analogous to the balance 30 of FIG. 4, the heat flow through the boundary layer 29 is the quantity being determined by means of the temperature sensors 31, 32, based on which the heat flow controller 28 regulates the active heat flow P.sub.A generated by the Peltier module 27.

[0051] FIG. 5 further illustrates the heat flows occurring in the balance 40 as a result of the dissipation of electrical power into heat. As in the previous examples of FIGS. 3 and 4, the weighing cell inside the weighing cell compartment 24 dissipates typically of the order of 0.1 watt, resulting in a small heat flow from the weighing cell compartment 24 through the boundary layer 29 into the weighing electronics chamber 41. Typically, the primarily analog electronics in the weighing electronics chamber 41 dissipate 0.5 watt, while the circuits inside the digital electronics chamber 42 dissipate 2.5 watt, resulting in commensurately large heat flows P.sub.O to the outside by way of the cooling fins 36 and P.sub.I towards the weighing electronics chamber 41 by way of the insulating material 37 that fills the interstitial space 34 around the heat pump module 27. The heat pump module 27 generates the active heat flow P.sub.A as dictated by the heat flow controller 28 to keep the heat flow P.sub.net at the required level.

[0052] While the invention has been described through the presentation of several specific embodiments, it is considered self-evident that numerous additional variants could be developed based on the teachings of the present invention, for example by combining the features of the individual embodiments with each other and/or by exchanging individual functional units of the embodiments against each other. For example, the alternative means for determining the heat flow P.sub.net as illustrated in FIG. 4A, or the concept of using the Peltier module alternatingly as a heat pump and as a sensing device for a temperature difference T.sub.I could obviously be applied also to the embodiment of FIG. 5.