Patent Description:
From <CIT>, a system according to the preamble of claim <NUM> is known. A control has a timer that provides a string of electrical pulses to a solenoid. With each pulse, the solenoid applies pressurized air to a cylinder piston, thereby opening a needle valve and permitting coating material to flow past the needle valve. The needle valve is closed for durations of time between pulses, and the coating material is ejected from a dispensing needle in response to closings of the needle valve. <CIT> also discloses a fluid dispenser having a dispensing valve movable between open and closed positions for controlling the flow of fluid from the fluid dispenser. A digital control provides on/off states, but no state with reduced current.

<CIT> describes a medical fluid machine, e.g. an apparatus for dialysis, comprising pinch valves using solenoids to occlude a piece of tubing within a desired or programmed time within a valve control sequence for the dialysis machine. The solenoid has a solenoid coil and an armature, wherein a spring pushes the armature closed when the coil is not energized. The power supplied to the solenoids is switched between two power levels, a first level to actuate the solenoid and, after receipt of a negative-going spike, i.e. once the armature begins to move, the power to the solenoid coil is reduced. That is, the solenoid requires more power to counter the force of the spring to begin movement than it does to hold the solenoid armature against the spring force once the armature is fully actuated.

<CIT> discloses an electromagnetic dispenser for dispensing viscous heated fluid, such as hot melt adhesives. In order to reduce the operating temperature of the coil, it is suggested to reduce the current passing through a coil once the plunger has moved to its full open position, thus reducing the heat generated by the coil windings.

Viscous material dispensers having dispensing pumps for dispensing electronic materials operate in a variety of manners. Some well-known dispensing pumps use servo motors to drive a rotary auger, while some dispensing pumps use linear servo motors to drive a piston. Other dispensing pumps do not use an electric servo motor, but instead rely on other means for actuation. One such dispensing pump, which is disclosed in <CIT>, includes a dispensing valve or unit that operates by moving a piston away from a seat using pneumatic pressure, thereby compressing a spring, and then releasing the pneumatic pressure to allow the spring to accelerate the piston back against the seat. With this dispensing unit, a droplet of material is forced out of an orifice at the seat as the piston contacts the seat. In such a dispensing unit, a solenoid valve is typically used to control the flow of air (or other gas) into and out of the piston chamber.

It is well known in the dispensing industry that a dispenser will respond differently when cycled one drop at a time as compared to the response when it is cycled repeatedly to produce a periodic series of drops in rapid succession. In particular, it is well known though not well understood that the first drop or even the first few initial drops in a periodic series of drops may differ from the balance of the drops in the periodic series. For example, the initial drops may contain a lesser mass of material than subsequently deposited drops.

<FIG> illustrate two known solenoid drive circuits used to energize a coil. Prior art solenoid drive circuits generally use an on/off drive circuit to energize the coil. Some prior art circuits (see <CIT> to Gieffers) have improved on the simple on or off states by adding another discrete level of drive voltage (or current). This secondary drive level is utilized to reduce the current needed to maintain the coil in the energized position at a level less than that used to rapidly transition the solenoid from the un-energized to the actuated position (i.e., a holding current that is lower than the pull-in current). This lower drive level serves to save energy and reduce coil heating when compared with the simpler on/off drive circuits, and may improve the speed of operation by minimizing the time required for the solenoid field to collapse when the solenoid is turned off.

The present invention is directed to a system for dispensing material on a substrate as defined in claim <NUM>.

Embodiments of the system further may include a transconductance pulse-width modulated amplifier. The amplifier may be connected to the solenoid coil through passive electrical filter components. The solenoid coil of the solenoid valve may be connected as a load for the amplifier. The solenoid coil of the solenoid valve may be connected either directly to the amplifier or through intermediate filter components. A time required for the magnetic field of the solenoid coil to collapse and turn the solenoid valve off can be further shortened by commanding a slightly negative current for a very short period of time to more rapidly and actively drive the field strength to a near zero off state condition. A time required for current to build up in the solenoid coil of the solenoid valve may be inversely proportional to an available supply voltage. The amplifier may permit the use of a supply voltage high than a rated voltage of the solenoid coil of the solenoid valve. The dispensing unit may be configured to dispense viscous material on an electronic substrate.

For the purposes of illustration only, and not to limit the generality, the present disclosure will now be described in detail with reference to the accompanying figures. This disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The principles set forth in this disclosure are capable of other embodiments and of being practiced or carried out in various ways. Also the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," "having," "containing," "involving," and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.

Various embodiments of the present disclosure are directed to viscous material dispensing systems, devices including dispensing systems. Embodiments disclosed herein are directed to techniques for dispensing material on an electronic substrate by a dispensing pump that is configured to control a current flowing in a coil of a pneumatic solenoid valve at a desired level.

<FIG> schematically illustrates a dispenser, generally indicated at <NUM>, according to one embodiment of the present disclosure. The dispenser <NUM> is used to dispense a viscous material (e.g., an adhesive, encapsulent, epoxy, solder paste, underfill material, etc.) or a semi-viscous material (e.g., soldering flux, etc.) onto an electronic substrate <NUM>, such as a printed circuit board or semiconductor wafer. The dispenser <NUM> may alternatively be used in other applications, such as for applying automotive gasketing material or in certain medical applications or for applying conductive inks. It should be understood that references to viscous or semi-viscous materials, as used herein, are exemplary and intended to be non-limiting. The dispenser <NUM> includes first and second dispensing units, generally indicated at <NUM> and <NUM>, respectively, and a controller <NUM> to control the operation of the dispenser. It should be understood that dispensing units also may be referred to herein as dispensing pumps and/or dispensing heads. Although two dispensing units are shown, it should be understood that one or more dispensing units may be provided.

The dispenser <NUM> may also include a frame <NUM> having a base or support <NUM> for supporting the substrate <NUM>, a dispensing unit gantry <NUM> movably coupled to the frame <NUM> for supporting and moving the dispensing units <NUM>, <NUM>, and a weight measurement device or weigh scale <NUM> for weighing dispensed quantities of the viscous material, for example, as part of a calibration procedure, and providing weight data to the controller <NUM>. A conveyor system (not shown) or other transfer mechanism, such as a walking beam, may be used in the dispenser <NUM> to control loading and unloading of substrates to and from the dispenser. The gantry <NUM> can be moved using motors under the control of the controller <NUM> to position the dispensing units <NUM>, <NUM> at predetermined locations over the substrate. The dispenser <NUM> may include a display unit <NUM> connected to the controller <NUM> for displaying various information to an operator. There may be an optional second controller for controlling the dispensing units. Also, each dispensing unit <NUM>, <NUM> can be configured with a z-axis sensor to detect a height at which the dispensing unit is disposed above the electronic substrate <NUM> or above a feature mounted on the electronic substrate. The z-axis sensor is coupled to the controller <NUM> to relay information obtained by the sensor to the controller.

Prior to performing a dispensing operation, as described above, the substrate, e.g., the printed circuit board, must be aligned or otherwise in registration with a dispenser of the dispensing system. The dispenser further includes a vision system <NUM>, which, in one embodiment, is coupled to a vision system gantry <NUM> movably coupled to the frame <NUM> for supporting and moving the vision system. This embodiment is also illustrated in <FIG>. In another embodiment, the vision system <NUM> may be provided on the dispensing unit gantry <NUM>. As described, the vision system <NUM> is employed to verify the location of landmarks, known as fiducials, or components on the substrate. Once located, the controller can be programmed to manipulate the movement of one or more of the dispensing units <NUM>, <NUM> to dispense material on the electronic substrate.

Systems and methods of the present disclosure are directed to dispensing material onto a substrate, e.g., a circuit board. The description of the systems and methods provided herein reference exemplary electronic substrates <NUM> (e.g., printed circuit boards), which are supported on the support <NUM> of the dispenser <NUM>. In one embodiment, the dispense operation is controlled by the controller <NUM>, which may include a computer system configured to control material dispensers. In another embodiment, the controller <NUM> may be manipulated by an operator. The controller <NUM> is configured to manipulate the movement of the vision system gantry <NUM> to move the vision system so as to obtain one or more images of the electronic substrate <NUM>. The controller <NUM> further is configured to manipulate the movement of the dispensing unit gantry <NUM> to move the dispensing units <NUM>, <NUM> to perform dispensing operations.

Embodiments of the present disclosure are directed to a dispensing unit, such as dispensing units <NUM>, <NUM>, having an actuator control circuit configured to control a current flow in a coil of a pneumatic solenoid valve at (virtually) any desired level. Specifically, with reference to <FIG>, an actuator drive circuit, generally indicated at <NUM>, is configured to drive the current in a solenoid coil <NUM> of a solenoid valve, generally indicated at <NUM>, with a PWM transconductance amplifier <NUM>, which uses an input voltage to command and control a current in a load. Although a transconductance amplifier <NUM> is shown and described herein, other types of amplifiers may be used to achieve the results achieved with the transconductance amplifier. In one embodiment, the solenoid coil <NUM> of the solenoid valve <NUM> is connected as a load for the amplifier <NUM>, and is connected either directly to the amplifier or through intermediate filter components <NUM>, such as high frequency choke inductors. In other words, the current control is part of an analog control system, rather than the digital (on/off) control systems typically found other coil drive systems, and this additional control is utilized to better achieve the overall system objectives. For example, the benefits of a reduced holding voltage may also be realized with the analog amplifier control, without requiring any special or additional circuitry.

As shown, the amplifier <NUM> is coupled to the controller <NUM> to control the operation of the actuator control circuit <NUM>, and more particularly the solenoid valve <NUM>, which is comprised of the solenoid coil <NUM> and a pneumatic valve <NUM>, with the solenoid valve being configured to drive the operation of the dispensing unit <NUM>, <NUM>. The solenoid valve <NUM> is configured to control the flow of air to an air cylinder <NUM> coupled to a piston <NUM>, which is pneumatically driven from a lower (first) position to an upper (second) position. The piston <NUM> engages a valve seat <NUM> to dispense material on the substrate <NUM>. Specifically, the solenoid valve <NUM> is configured to control air flow to and from the air cylinder <NUM> and the piston <NUM>. The controller <NUM>, which is coupled to the amplifier <NUM>, is configured to generate a command signal to the amplifier to control current in the solenoid coil <NUM>.

By providing full analog control over a current waveform of the solenoid coil <NUM>, many characteristics of the solenoid valve <NUM> are modified or tailored to better meet the particular needs of dispensing systems. Maintaining the solenoid coil <NUM> in the on state with a reduced holding current minimizes the time required for the solenoid field to collapse when the solenoid valve <NUM> is turned off. For the actuator control circuit <NUM> of embodiments of the present disclosure, the time required for the magnetic field of the solenoid coil <NUM> to collapse (i.e., to turn the solenoid off) can be further shortened by commanding a slightly negative current for a very short period of time (a few hundred microseconds, for example) to more rapidly and actively drive the field strength to a near zero (off state) condition.

Another benefit of the actuator drive circuit <NUM> relates to the fact that the time required for the current to build up in the coil or other inductor is inversely proportional to the available supply voltage (di/dt = V/L). In on/off prior art systems, the resistance of the coil winding and the rated supply voltage are chosen such that power dissipated in the solenoid coil <NUM> will be within a rating of the coil. Using the current control amplifier <NUM> permits the use of a supply voltage higher than the rated voltage of the solenoid coil <NUM>. For example, driving a 24V coil from a 48V supply (rather than from a 24V supply) might permit the current in the solenoid coil <NUM> to build twice as rapidly, but the current will build to twice the rated current, and thus the power dissipation would be four times the nominal value. By using the current control amplifier <NUM>, the desired current can be commanded at the full rated value, and the amplifier can use the full available supply voltage to rapidly build the current in the solenoid coil <NUM>, yet limit the current once the commanded value is reached.

It should be noted that in this context, an "analog control system" is intended to include digitally controlled amplifiers, such as those that use digital-to-analog converters (DACs) and analog-to-digital converters (ADCs) to provide pseudo-analog control through digital circuitry, and which are able to generate hundreds or thousands of small discrete levels within a given range. For example, a <NUM>-bit DAC can provide <NUM> discrete levels, which may be referred to as "analog control" to distinguish it from bilevel or tri-level on/off control systems. Furthermore, while the known systems referenced above may use bipolar junction transistors (BJTs) or field effect transistors (FETs) to control the current in the solenoid coil <NUM>, and while such transistors can be also used in amplifiers, or might themselves be considered amplifiers, their use in on/off circuits, such as those in the known systems, is characterized by operating the transistor in either a saturated (on) condition or in a cut-off (off) condition. This mode of operation is meaningfully distinct from the function of the amplifier <NUM>, the flexibility and advantages of which are described herein. In amplifiers, such as the PWM transconductance amplifier <NUM> used in embodiments of the present disclosure, the transistors are switched rapidly back and forth between the saturated state and the cut-off state to minimize power dissipated in the transistors. However, this switching takes place at a frequency high enough that substantially all of the energy at the switching frequency and its harmonics may be filtered before reaching the load. The remaining direct current (DC) and lower frequency energy passes through the filter to the load. In this configuration, even though the transistors are utilized only in their on or off states, the function of the amplifier subsystem provides the function and virtues of an analog amplifier without the drawbacks, and in particular, without the limitations of the known on/off systems.

A significant commercial advantage of using the amplifier <NUM> to drive the solenoid coil <NUM> is realized when one recognizes that many existing dispensing machines drive the dispensing unit as a servo motor mechanism. By utilizing this existing infrastructure, deployment of the solenoid-controlled, pneumatically-driven pump to the field is greatly facilitated. The solenoid valve <NUM> may be connected to the same connector that normally supports a servo-driven pump, and the changes to the system to accommodate the different pump types are minimized. The use of known solenoid control systems in a system designed to also support servo-motor controlled dispensing pumps would necessitate the presence of both the amplifier for servo motor control, and also a separate solenoid coil drive system, the combination of which would be needed to support the driving of both pump types.

Further not claimed embodiments are directed to a method of minimizing the differences in drop dispensing behavior between the dispensing of a first drop or first few initial drops in a sequence and the dispensing of subsequent drops in the sequence. The observation that the power dissipated in a coil varies with cycle frequency, that a temperature of the coil varies with power dissipated in the coil, that a winding resistance of the coil changes with coil temperature, and that in turn, the response of the coil may vary with the winding resistance of the coil. Reference can be made to the following:
Cycle Frequency → Power Dissipated → Coil Temperature → Winding Resistance → Solenoid Response.

When the coil is actuated for the first time after sitting idle for an extended period of time, the coil will be at a different temperature than it will be for subsequent drops dispensed periodically. Thus, if the coil temperature can be more closely controlled, then the response of the coil can be made to be more consistent. Embodiments of the method further include using an amplifier to drive a solenoid coil such that turn on and turn off times are faster than would be achieved with prior art drive methods.

In one embodiment, the method is capable of controlling the current flow in the coil of the pneumatic solenoid valve at (virtually) any desired level. Specifically, the method includes driving a current in a coil with a PWM transconductance amplifier of a dispensing unit, which uses an input voltage to command and control a current in a load. As mentioned above, other types of amplifiers may be used to achieve the same benefit. In one embodiment, the coil of the solenoid valve is connected as the load for the amplifier, and is connected either directly to the amplifier or through intermediate filter components, such as high frequency choke inductors. With the amplifier arrangement, when the valve is not actively being actuated, the current may be controlled to flow at a low steady state level. This level may be chosen to be low enough to not actuate the valve, but yet enough to keep the coil warm. In other words, the current control is part of an analog control system, rather than the digital (on/off) control systems typically found in known coil drive systems, and this additional control is utilized to better achieve the overall system objective. The benefits of a reduced holding voltage may also be realized with the analog amplifier control.

Some known systems recognize the advantages of maintaining the temperature of a solenoid coil (see <CIT>). However, such known system use short pulses to dissipate some energy in the coil without energizing the solenoid to the on position.

In one method, the coil is designed for 24V, and has a resistance of about <NUM> ohms @ <NUM>, resulting in a current of <NUM> Amps. When the coil warms up, the resistance will increase and the current will be reduced. A controlled actuation current of <NUM>. 4A causes the valve to actuate at substantially the same speed as with higher currents, and thus minimizes unnecessary winding heating. Once in the actuated state, the coil will maintain its actuated position until the current is reduced below about <NUM> mA. Below this level, this particular coil will return to a de-energized state. Furthermore, maintaining a current at a level well below this holding current threshold will not cause the valve to return to the energized state.

There are beneficial effects of increasing the idle temperature of the coil with an idle current of about <NUM>. This low "background" current generates enough heat to warm the coil above ambient, yet it does not interfere with normal operation of the coil. In practice, the level of the idle current required to generate the same steady-state temperature in the coil will be dependent upon ambient conditions, the thermal time constant of the coil, the normal periodic actuation rate of the coil, and other similar factors and variables.

When comparing the position versus time response of a poppet in a pneumatic valve, the presence of a low level "idle current" in the coil can substantially reduce or even eliminate the difference in first actuation response. Referring to <FIG> and <FIG>, the third trace represents a motion of a solenoid poppet. It can be seen that the response of the valve in <FIG> is much more consistent, having corrected the separate trace seen in <FIG>, which is due to the first pulse response.

As shown in <FIG> and <FIG>, the first trace represents current command, the second trace represents trigger, the third trace represents poppet position, and the fourth trace represents piston position. These traces were captured with a digital oscilloscope in persistence mode, with traces from ten events overlaid upon one another. As seen in the third set of traces in <FIG>, one event response is different than the others in the set of sweeps - this is the first event after a period of inactivity (i.e., first drop). It can also be seen in the fourth trace that the piston responds differently to this change in the poppet response. In <FIG>, idle current in the coil has been introduced, and in the third set of traces, representing the poppet response, it is apparent that all of the poppet response curves are substantially identical. The remaining slight variation in piston response is due to other sources.

Claim 1:
A system for dispensing material on a substrate (<NUM>), the system comprising:
- a dispensing unit (<NUM>, <NUM>) including a dispensing piston (<NUM>), the dispensing piston (<NUM>) being pneumatically driven from a first lower position to a second upper position;
- a solenoid valve (<NUM>) coupled to the dispensing unit (<NUM>, <NUM>), the solenoid valve (<NUM>) being configured to control air flow to and from the dispensing piston (<NUM>), the solenoid valve (<NUM>) including a solenoid coil (<NUM>);
- an amplifier (<NUM>) connected to the solenoid coil (<NUM>); and
- a controller (<NUM>) coupled to the amplifier (<NUM>), the controller (<NUM>) being configured to generate a command signal to the amplifier (<NUM>) to control current in the solenoid coil (<NUM>),
characterized in that
the controller (<NUM>) is configured to provide a full analog control over a current waveform of the solenoid coil (<NUM>) of the solenoid valve (<NUM>) and is configured to maintain the solenoid coil (<NUM>) in an on state with a reduced holding current compared to the current in its actuated position to minimize a time required for the solenoid field to collapse when the solenoid is turned off.