GB2313170A - A linear motion conveyor having an intermittently actuated damper - Google Patents

Description

2313170 DAMPER FOR LINEAR MOTION CONVEYOR This invention relates to linear

motion conveyors and more particularly to dampers for use on a linear motion conveyors.

Linear motion conveyors typically include a tray having a supporting surface for the goods to be transported. Generally, the tray is moved slowly in the forward direction and is then pulled rearward at a faster rate. This allows the goods disposed upon the tray to be transported in a forward direction.

As an example, U.S. Patent No. 5,351,807 to Svejkovsky discloses a universal or u-joint structure for driving a linear motion conveyor. The action of the u-joint causes the shaft as well as the tray and counterweight used in the linear motion conveyor to accelerate and decelerate.

Specifically, as shown in Figure 1, means 10 for imparting differential motion is shown comprising a motor 12, a gear box speed reducer 14, a universal or u- joint or Hookes joint 16, a constant rotational speed shaft 18, and a variable speed shaft 20. The u-joint 16 interconnects the constant rotational speed shaft 18 with the variable speed shaft 20. The rotational speed of shaft 20 varies twice per revolution. The variable speed shaft 20 drives a 1:2 ratio mechanism 22 that may be any type of fixed ratio drive mechanism, such as a gearbox, a chain and sprocket driveffl or a timing belt 24 and pulleys 26 and 28. As a result, the speed of shaft 30 varies once per revolution because of the 1:2 ratio mechanism. The conveyor speed ratio may be adjusted by varying the angle 32 between the axes of the shafts 18 and 20.

As a result,, components such as the gearbox, the u-joint, KH C:\$P=\ffiDblIO4.WU 13 Auquat 1996 2 and the ratio mechanism for driving the shaft for imparting notion to the tray may experience a potentially detrimental mechanical knock. That is, the driving forces to accelerate and decelerate the tray may cause undesirable mechanical knock. Svejkovsky discloses, for example, the use of a mechanical / hydraulic damper to help prevent the mechanical knocking. However, prior art dampers are turned on for both the forward and rearward moving of the tray. This may result in the use of more power for operating the linear motion conveyor because more power may need to be used in the rearward motion to overcome the damping forces of the conventional damper. Similarly, other means 10 for imparting differential motion, such as the use of elliptical gears as disclosed in U.S. Patent No. 3,253,700, may result in potentially detrimental mechanical knock caused by the acceleration and deceleration of the tray. Therefore, it is desirable to have a damper for use with a linear motion conveyor which is only activated selectively during the acceleration and deceleration of the trough in order to reduce power consumption and mechanical knock.

In accordance with the present invention there is provided a linear motion conveyor comprising a drive for moving a trough in a forward direction at a slower speed and in a backward direction at a faster speed for moving goods along said trough and a damper assembly arranged to act upon the drive, the damper assembly being selectively activated at a f irst time in a crank shaft cycle and deactivated at a second time in the crank shaft cycle.

The present invention also provides a linear motion conveyor comprising a drive having differential motion means and a damper assembly arranged to act upon the drive, the damper assembly being selectively activated at a first time in a crank shaft cycle and deactivated at a second time in the crank shaft cycle.

3 According to the invention a method for conveying materials comprises the steps of providing differential motion to a trough, measuring a speed for driving the trough, determining whether the speed is greater than or equal to a set point, and providing an output from a damper assembly to a shaf t when the speed is greater than or equal to the set point.

Preferred features of the invention are in the dependent claims or will be apparent from the following description of illustrative embodiments made with reference to the drawings in which:-

Figure 1 is a view of a prior art exciter f or a vibratory conveyor.

Figure 2 is a top view of a linear motion conveyor of the present invention.

Figure 3 is a top perspective view of the drive for the linear motion conveyor of fig.2.

Figure 4 is an end view of elliptical gears showing cooperation between the teeth of the respective elliptical gears.

Figure 5 is an end view of an elliptical gear.

Figure 6 is a side elevation view of a first embodiment of the damper assembly.

Figure 7 is a block diagram of an alternative embodiment for activating the damper assembly.

Figure 8 is a plot of crank torque versus time.

Figure 9 is a plot of motor speed versus time.

4 Figure 10 is a f low chart diagram of a proportional integral differential control algorithm.

Figure 11 is a top perspective view of an alternative embodiment for the drive of the present invention.

Figure 12 is a front view of an alternative embodiment of the damper assembly.

Figure 13 is a side view of the damper assembly of Figure 11.

Description of the Preferred Embodiments

Referring now to Figure 2, there is shown a top view of the linear motion conveyor 50 of the present invention. The linear motion conveyor 50 comprises a trough 52, a counterweight 54, and a drive 56. The drive 56 may be disposed on a drive support 58. The drive support 58 may have bores 60 to allow the drive support to be secured to a support stand, not shown, as is commonly done in the industry.

Referring also to Figure 3, the drive 56 comprises a motor 62. The motor 62 may, for example, be a one horsepower, 1800 rpm, 230/460 volt ac induction motor. Preferably, the induction motor 62 has an average operating speed of 1750 RPM. The drive 56 further comprises a worm gear reducer 64 which may, for example, be a 7.5:1 reduction worm gear.

The drive 56 also comprises a differential motion means 66. The worm gear reducer 64 may be connected to the differential motion means 66 by a flexible shaft coupling 68, a key 70, and a first shaft 72. The flexible shaft coupling 68 may for example be a model 6A37 coupling manufactured by zero Max of Minneapolis, Minnesota allowing for misalignment of gear reducer 64 and shaft 72. The key 70 positively transmits torque from the gear reducer 64 to shaft 72, as is standardly done in the industry. The first shaft 72 may, for example, be a shaft formed of 1045 steel having a diameter of 1.5 inches (38.1mm).

Referring also to Figure 4, the differential motion means 66 comprises two elliptical gears 74, 76. The first shaft 72 is connected to the f irst elliptical gear 74. Further, a second shaft 77 is connected to the second elliptical gear 76. The elliptical gears 74, 76 have teeth 73, 75 which engage one another. Due to rotation imparted by the motor 62 and worm gear reducer 64 to the first shaft 72 and, in turn, the first elliptical gear 74, rotation is imparted to the second elliptical gear 76 due to the engagement of the teeth of the respective gears 74, 76. This, in turn, causes a shaft 77 to rotate at a differential speed. The maximum speed of shaft 77 is 1.56 times the average speed; the minimum speed being the average speed divided by 1.56, where, for example, the average speed is 233.3 RPM. The shaft 77 rotates phase adjuster 102 by a key 100 which in turn rotates the shaft of torque transducer 106 by means of key 104. The shaft of the torque transducer 106 rotates coupler 110, which is similar to coupler 68, by means of a key 108. coupler 110 is connected to a crankshaf t 78. The torque transducer 106 such as model 01224-023 made by Sensor Developments, Inc. measures torque between keys 104 and 108 and exports a signal that can be translated to a signal that can be displayed on a personal computer screen.

Referring also to Figure 5, there is shown an example of the dimensions of the elliptical gears 74, 76 where a 1.5748 inch (40.00Omm) diameter first shaft 72 and second shaft 77 are employed. The geats may have a bore diameter 80 also of 1.5748 inches. Further, the gears may have a dimension 82 of 3.0469 inches (77.39lmm), a dimension 84 of 1.9531 inches (49.60lmm), a dimension 86 of 1.7379 inches (44.143mm), dimensions 88 and 90 of 2.4395 inches (61.963mm), and dimensions 92 and 94 of 0. 1875 inches (4.7625mm).The gears 6 may also be, for example, 0.5 inches (12.7mm) in thickness. A standard 0. 375 inch (9.525mm) key seat 94 is formed in the gears to effectively transfer torque to the input gear 74 from said first shaft 72 and from output gear 76 to said second shaft 77 using a 0.375 inch (9.525mm) key as is commonly done in industry.

The elliptical gears may, for example, be used to generate a 2.4336:1 angular velocity ratio in the driven gear when the driver gear is driven at a constant speed. Preferably, the gears 74, 76 are enclosed with a crank mechanism in a housing containing oil. Further, bearings for the gears 74, 76, such as Link-Belt tapered roller bearings model number FC3U2MON, as well as retaining caps may be used for keeping the bearings in place.

Referring to Figures 2 and 3, the first shaft 72 may be connected to an Acushnet shaft coupling 96 which is, in turn, connected to a BEI rotary shaft encoder 98. The standard shaft coupling 96 connects shaft 72 and the rotary encoder 98 allowing for any shaft misalignment. The BEI encoder may be used to verify any speed changes and may be used instead of the tachometer 170 as described in relation to Figure 7.

Two pillow block bearings 112 and 114 are disposed about the crank assembly 115. The crank assembly 115 comprises a key 116, a drive eccentric assembly 118, a flanged cartridge bearing 120, and drive arm assemblies 122, 123. Referring to Figure 3, the first drive arm assembly 122 may be secured to the counterweight 54 and the second drive arm assembly 123 may be secured to the trough 52 by use of fasteners 126 and 128, respectively. The key 116 transmits torque f rom the crank shaft 78 to the drive eccentric assembly 118. The drive eccentric assembly 118 may comprise dual 0.5 inch (12.7mm) eccentrics offset one hundred eighty degrees out of phase. Further, the key 108 at the end of the crank shaft 78 received in coupler 110 may be thirty five degrees out of phase from 7 each of the eccentrics. The flanged cartridge bearings may, for example, be a model FC-B22447E/2 bearing manufactured by Link-Belt. Further, bolts 124 may transmit force to the crank arm assemblies 122,123.

A sensor or electronic timing switch 130, such as a Candy Model P switch manufactured by Candy Manufacturing, Co. of Evanston, Illinois may be disposed near one end of the crank shaft 78. The candy switch 130 senses the rotation of a metal plate 131 disposed at the end of the drive shaft 77. The candy switch 130 is connected to a control 132 via line 134. The control may be a 12 volt power supply such as Radio Shack's model 22-120A by Micronta, and a General Instruments model LM44DOO switching relay or equivalent and an adjustable dc power supply.

Referring to Figure 6, there is shown a first preferred embodiment of the damper assembly 136. The damper assembly 136 comprises a speed increaser/torque amplifier (i.e., a chain and sprocket assembly) 140 having a first sprocket 142 disposed around the crank shaft 78, a second sprocket 144 disposed around a shaft 146, and a chain 148 disposed around the first and second sprockets 142 and 144. The speed increaser/torque amplifier 140 may, for example, provide an increase ratio between three to six. The shaft 146 is mounted f ree to rotate by the use of a mounting bracket 150. The damper assembly f urther comprises a controllable viscosity fluid damper 152 which is preferably a magnetorheological brake disposed around shaft 146.

The magnetorheological brake 152 may be a series of five MRB-2107 brakes manufactured by Lord Corporation disposed in series about the shaft 146. Alternatively, the magnetorheological brake 152 may, for example, be made in accordance with U.S. Patent No. 5,492,312 assigned to Lord corporation. For example, such a brake may have a maximum torque of sixteen ft-lbs (21.7Nm), a maximum speed of 900 rpm, 8 a maximum power dissipation of approximately 1 000 Watts, and an input power of twelve Watts (one Amp at twelve Volts). Where, for example, a larger capacity magnetorheological brake 152 is used, such as a magnetorheological brake made in accordance with U.S. Patent No. 5f492, 312 having a maximum torque of one hundred fifty ft-1bs (203Nm), a maximum speed of 900 rpm, a maximum power dissipation of approximately 1 000 Watts, and an input power of thirty six Watts (three Amps at twelve Volts), there would be no need to use a speed increaser/torque amplifier 140 and thus the brake 152 could be connected directly to the crank shaft 78. The brake 152 is activated by control 132, shown in Figure 3, by line 138.

In order to dissipate any heat generated by the brake 152, cooling fins 154 may be disposed adjacent to the brake 152. Further, fan blades 156 may also be disposed near the brake 152 to help cool down the brake.

The sensor or candy switch 130 would be used to signal the control 132 to turn on the brake 152 at a first time in the cycle and then to turn it of f at a second time in the cycle. Specifically, the control 132 would activate the brake 152 during the negative crank torque for the motor 62 and would inactivate the brake 152 during the positive crank torque f or the motor 62. The crank torque is the torque delivered by, for example, a key at the end of the crank shaft to the crank shaft 78. That is, for example, the torque delivered by key 108 to the crank shaft 78.

Preferably, the brake is activated such that no negative torque on the motor results. The point at which this occurs may be termed the set point. That is, the approximate point in time when the brake 152 is activated such that no negative torque results in the motor 62 is deemed to be the set point. The set point may be ascertained, for example, by viewing the motor 62 torque on a personal computer with the aid of a torque transducer as is commonly done in the industry.

9 Preferably, the set point is at a time slightly before the crank torque goes negative in value. The brake 152 would be deactivated at a time slightly after the point in time that the crank torque would otherwise go from negative to positive had the brake 152 not been activated. The process would repeat for each motor cycle. As a result, the brake 152 would be activated when the trough 52, counterweight 54, and crank shaft 78 are decelerated (i.e., when the crank torque would otherwise go negative in value).

Referring now to Figure 7 there is shown a block diagram for an alternative embodiment for activating the brake 152. similar components have been labelled similarly for purposes of clarity. In this embodiment, a tachometer 170 as is commonly used in the industry is provided in order to determine the speed of the motor 62. The output of the tachometer is provided to control 132. The control 132 may be a Proportional Integral Differential Controller as is commonly used in the industry. The output of the control 132 is provided via line 138 to brake 152 as is similarly shown in Figure 3.

The desired output of the brake 152 may be approximated by the following relationship:

(outputz")(sReed,r,= - set pointl + K(speed acceleration), (speeday= - set point) where outputmx is the maximum output of the brake or damper 152, speed.= is the current speed of the motor 62, set point is the approximate speed of the motor 62 at a point in time when the brake 152 is activated such that no negative torque results in the motor 62, speedaym is the synchronous speed of the motor 62, speed acceleration is the rate of change of speed of the motor 62, and K is a constant. The value of the constant K is determined such that the output brake value results in a zero or slightly positive value for the crank torque. This may be done by viewing the output of the torque transducer on a personal computer as is standardly done in the industry to determine that the value of the crank torque remains positive throughout the crank cycle. The output of the brake 152 cannot exceed the maximum brake output allowable f or the brake. Therefore, if the value determined by the above noted equation results in a value f or the brake output to be larger than the maximum allowable brake output, then the brake should be activated such that it provides its maximum allowable output. The tachometer 170 may, for example, calculate the motor speed instantaneously and the control 132 would adjust the brake output according to the equation every millisecond, for example.

As an illustrative example in applying the above noted equation, referring also to Figures 8 and 9, there are shown plots of crank torque versus time and motor speed versus time, respectively. As shown in these figures, the synchronous motor speed (i.e., where zero motor torque occurs) is approximately 188 rad/sec. At the time of 0.845 seconds and 0. 85 seconds, the motor speed is 178 rad/sec and 183 rad/sec, respectively. The set point as discussed earlier is the approximate speed of the motor 62 at a point in time when the brake 152 is activated such that no, negative torque results in the motor 62. Preferably, the set point is the speed of the motor at a time slightly before the crank torque curve of Figure 8 goes negative in value. As a result, the set point may be 180 rad/sec. Further, the maximum brake occurs when twelve volts dc is applied to the brake 152 and the minimum brake occurs when zero volts dc is applied to the brake 152. Moreover, the value of the differential constant may be 0.005 Volts.seC2 /rad. The time values of.845 and.85 seconds were picked from the curves close to the set point in light of the fact that the brake 152 is applied shortly in advance of the set point. Typically, the current speed of the shaft would be measured approximately every millisecond. The difference between the current speed reading and the last speed reading would be the current acceleration.

The speed acceleration is determined by the equation:

speedum, - speedti., = 183rad/sec - 178rad/sec = 1000 rad/seC2 (time2 timel) 0.85sec- 0.845sec As a result, the desired brake output would be calculated by the above noted equation as:

(12yoltsl(183radlsec - 180radlsec) + (0.005Volts.secl/rad)(1000rad/ sec'). (188rad/sec - 180rad/sec) Therefore, the desired brake output would be 9.5 volts.

Referring now to Figure 10, there is shown a control algorithm that may be used in order to apply the proper voltage to the brake 152. After the program is activated as noted by block 172, the speed of the motor 62 is calculated as noted by block 174. The set point may then be determined as noted by block 176. As stated before, the motor speed may be determined by tachometer 170 and the set point may be determined by finding the motor speed such that the motor 62 does not experience negative torque. The motor speed and set point are then compared as noted by decision block 178. if the motor speed is less than the set point, then no brake output is required which is set to zero in block 180. If the motor speed is greater than or equal to the set point, then the brake output is provided in accordance with the above noted equation, as illustrated by block 182. The control 132 provides the necessary voltage level to the brake 152 for providing the desired brake output. This process repeats continuously as long as the unit is operating. Further, the set point is the same for each conveyor cycle.

Now referring to Figure 11, there is shown an alternative embodiment of the drive of the present invention. Similar items have been labelled similarly for purposes of clarity.

12 in discussing the embodiment illustrated in Figure 3, the differential motion means 66 was described as comprising two elliptical gears. However, as illustrated in Figure 11, the differential notion means may comprise a u-joint 16 as discussed in connection with the means for imparting differential notion 10 in Figure 1. Alternatively, the differential motion means 66 may comprise any other suitable means for imparting differential motion.

Referring now to Figures 12 and 13, there is shown an alternative embodiment of the damper assembly 136. The damper assembly 136 shown in these f igures is an electromagnetic brake 190 comprising a partial metal disc 192. The disc 192 comprises a bore 194 and a cut-out portion 196, the bore 194 sized such that the disc may be secured by welding or other appropriate securing means to the crank shaft 78. The electromagnetic brake 190 further comprises two electromagnets 200 at opposite sides of the disc 192.

The disc 192 may, for example, be formed of aluminum and may have an uncut diameter of fourteen inches (356mm) and a thickness of 1/16 inch (1. 59mm). Preferably, the cut-out portion 196 is 270 degrees such that the braking torque is applied for 90 degrees of rotation each conveyor cycle.

The cut-out portion 196 may be dimensioned such that no braking torque is applied when the crank torque is positive. The electromagnets 200 may, for example, be EB-222-A magnets manufactured by FMC Corporation. The electromagnets 200 may be disposed 0.125 inches (3.175mm) from the top edge of the disc 192 as measured from the uncut portion of the disc and 0. 1 inches (2.54 from the front and rear face of the disc 192. Preferably, fifteen Amps of current flow through each of the electromagnets 200. The cut-out portion 196 may be a 270 degree cut-out whose inward points 202 may be two inches (50.8mm) from the center of the disc.

3 In operation, when the crank torque is positive, there would preferably be no electromagnetic forces acting upon the disc due to the fact that at that point in time, the cut-out portion 196 would be disposed between the electromagnets 200. However, where the crank torque would otherwise go negative, the disc 192 would be disposed between the electromagnets 200. As a result, the electromagnetic forces acting upon the disc 192 would slow down the rotation of the disc 192 and, in turn, apply a braking force to the crank shaft 78. Therefore, the damper assembly 136 would be selectively activated depending upon whether or not the electromagnets 200 were disposed between the cut and uncut portions of the disc 192 during a given crank shaft 78 cycle.

It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural details without departing from the principles of the invention. Therefore, the appended claims are to be construed to cover all equivalents failing within the true scope of the invention.

14

Claims (18)

Claims:

1. A linear motion conveyor comprising: a drive for moving a trough in a forward direction at a slower speed and in a backward direction at a faster speed for moving goods along said trough; and a damper assembly arranged to act upon the drive, said damper assembly being selectively activated at a f irst time in a crank shaft cycle and deactivated at a second time in said crank shaft cycle.

2. A linear motion conveyor comprising: a drive having differential motion means; and a damper assembly arranged to act upon the drive, said damper assembly being selectively activated at a f irst time in a crank shaf t cycle and deactivated at a second time in said crank shaft cycle.

3. A conveyor as defined in claim 2 wherein said differential motion means comprises a u-joint for imparting differential motion to a trough.

4. A conveyor as defined in claim 2 wherein said differential motion means comprises an elliptical gear for imparting differential motion to a trough.

5. A conveyor as defined in any of claims 1-4 wherein said damper assembly is activated when said crank shaft has a negative torque value and is deactivated when said crank shaft has a positive torque value.

6. A conveyor as defined in any of claims 1-5 wherein said damper assembly comprises a controllable viscosity fluid damper.

7. A conveyor as defined in claim 6 wherein said damper assembly comprises a magnetorheological damper.

M C:\UECS\W081104.WRI 13 AU 1996

8. A conveyor as defined in any of claims 1-5 wherein said damper assembly comprises an electromagnetic damper.

9. A conveyor as defined in claim 8 wherein said electromagnetic damper comprises a partial disc and an electromagnet disposed near a side of said disc.

10. A method for conveying materials comprising the steps of: providing differential motion to a trough; measuring a speed for driving said trough; determining whether said speed is greater than or equal to a set point; and providing an output from a damper assembly to a shaft when said speed is greater than or equal to said set point.

11. The method of claim 10 wherein said step of measuring comprises measuring a speed of a motor.

12. The method of claim 11 wherein said damper assembly output is approximately equal to:

(output.") (sReedc:,= - set Roint) + K(speed acceleration), (speedsy= set point) where: output is the maximum damper output, speedc= is.the current speed of the motor, set point is the approximate speed of the motor at a point in time when the damper is activated such that no negative torque results in the motor, speeday= is the synchronous speed of the motor, speed acceleration is the rate of change of the motor speed, and K is a constant and wherein the brake output does not exceed the maximum brake output.

13. The method of any of claims 10-12 wherein said damper 16 assembly is a controllable viscosity fluid damper or an electromagnetic damper.

14. The method of any of claims 10-12 wherein said electromagnetic damper comprises a partial disc and an electromagnet disposed near a side of said disc.

15. The method of any of claims 10-12 wherein said damper assembly is a magnetorheological damper.

16. The method of any of claims 10-15 wherein said step of providing differential motion comprises providing a u-joint or an elliptical gear for imparting differential motion to said trough.

17. A linear motion conveyor and damper assembly substantially as described with reference to or as shown in figs. 2-10 or fig. 11 or figs. 12 and 13 of the drawings.

18. A method of driving a conveyor substantially as described with reference to figs. 2-10 or fig. 11 or figs. 12 and 13 of the drawings.

18. A method of driving a conveyor substantially as described with reference to figs. 2-10 or fig. 11 or figs. 12 and 13 of the drawings.

J1 Amendments to the claims have been filed as follows claims:

1. A linear motion conveyor comprising: a drive for moving a trough in a forward direction at a slower speed and in a backward direction at a faster speed for moving goods along said trough; and a damper assembly arranged to act upon the drive, said damper assembly being selectively activated at a f irst time in a crank shaf t cycle and deactivated at a second time in said crank shaft cycle.

2. A linear motion conveyor comprising:

drive having differential motion means; and damper assembly arranged to act upon the drive, said damper assembly being selectively activated at a f irst time in a crank shaf t cycle and deactivated at a second time in said crank shaft cycle.

3. A conveyor as defined in claim 2 wherein said differential motion means comprises a u-joint for imparting differential motion to a trough.

4. A conveyor as defined in claim 2 wherein said differential motion means omprises an elliptical gear for imparting differential motion to a trough.

5. A conveyor as defined in any of claims 1-4 wherein said damper assembly is activated when said crank shaft has a negative torque value and is deactivated when said crank shaft has a positive torque value.

6. A conveyor as defined in any of claims 1-5 wherein said damper assembly comprises a controllable viscosity fluid damper.

7. A conveyor as defined in claim 6 wherein said damper assembly comprises a magnetorheological damper.

M C:\UECS\U091104.WU 13 A 1996 8. A conveyor as defined in any of claims 1-5 wherein said damper assembly'comprises an electromagnetic damper.

9. A conveyor as defined in claim 8 wherein said electromagnetic damper comprises a partial disc and an electromagnet disposed near a side of said disc.

10. A method for conveying materials comprising the steps of: driving a trough for movement in a forward direction at a slower speed and in a backward direction at a faster speed for moving goods along said trough; measuring the driving speed of said trough; determining whether said speed is greater than or equal to a set point; and providing an output from a damper assembly to a shaft when said speed is greater than or equal to the set point.

11. The method of claim 10 wherein said step of measuring comprises measuring a speed of a motor.

12. The method of claim 11 wherein said damper assembly output is approximately equal to:

(output...) (speed,:. set point) + K(speed acceleration), (speedsyr.c set point) where: outputmax is the maximum damper output, speed.. is the current speed of the motor, set point is the approximate speed of the motor at a point in time when the damper is activated such that no negative torque results in the motor, speedsync i 5 the synchronous speed of the motor, speed acceleration is the rate of change of the motor speed, and K is a constant and wherein the brake output does not exceed the maximum brake outIDU,,- - 13. The method of any of claims 10-12 wherein said damper assembly is a, controllable viscosity fluid damper or an electromagnetic damper.

14. The method of any of claims 10-12 wherein said electromagnetic damper comprises a partial disc and an electromagnet disposed near a side of said disc.

15. The method of any of claims 10-12 wherein said damper assembly is a magnetorheological damper.

16. The method of any of claims 10-15 wherein said step of providing differential motion comprises providing a u-joint or an elliptical gear for imparting differential motion to said trough.

17. A linear motion conveyor and damper assembly substantially as described with reference to or as shown in figs. 2-10 or fig. 11 or figs. 12 and 13 of the drawings.