cwad
CWAD (pronounced quad) is a Clock With A Difference.
Before you read on, have a close look at the first photo and see if you can spot the difference. It is not hidden but quite plain to see.
When building a clock there are two critical aspects in the design. Firstly, there is the matter of gear ratios. Secondly, there is need for a device that allows independent concentric drive to each of the hands, ie, each of the rotational axes lie on the same straight line. To meet these requirements most designers of LEGO clocks use the turntable piece. A good example is LEGO's Cuckoo Clock. Meshing an 8t gear with the inner 24t gear of the turntable provides a useful 3:1 gear ratio and the central hole allows space for the minute hand axle. While browsing the Internet, I discovered other more compact ways to achieve the same result. This led me to the question: "Could a clock be built with CONCENTRIC hour, minute AND second hands?". The answer is yes and that is what makes this clock different. In fact I believe this to be a world first.
DESIGN
Design of this clock went through four main stages. The first three stages were carried out mostly on paper with minor prototyping to confirm some design elements.
Design Stage 1 - THEORETICAL DESIGN
This stage was concerned with working out the gear ratios. The required net ratios are:
second hand:minute hand = 60:1,
minute hand:hour hand = 12:1.
Looking at the factors for each of these numbers we have:
60 = 2 x 2 x 3 x 5,
12 = 2 x 2 x 3.
Each of these factors may be easily implemented with LEGO gears,but there is a problem.
Having drives for both the minute and second hand passing through the centre of the turntable leaves no space for driving the inner 24t turntable gear. Hence we are forced to use the turntable outer gear. The outer gear has 56 teeth and the factors of 56 are:
56 = 2 x 2 x 2 x 7.
The 7 is the problem. It cannot be made to fit in with the required ratios. While contemplating this problem it occurred to me that rather than trying to eliminate the annoying 7, I should look for a way to work with it. This led to the answer. Driving a 56t gear with an 8t gear gives a 7:1 gear ratio. If the 8t driving gear was itself driven from the output of a 1:7 gear ratio then the 7's cancel out and the net result is 1:1. This can be achieved with the following gear train:
in-56:8-8:56-out.
In this notation, gears are represented by their number of teeth, a hypen (-) represents a common axle and a colon (:)represents meshing gears.
This design stage was carried out a good many years ago when RCX was still king. It wasn't until mid 2008 that I got inspired to move forward.
Design Stage 2 - PRACTICAL DESIGN
This stage was concerned with resolving two conditions simultaneously. Actual gear combinations had to be chosen such that, firstly, each of the three outputs (hour, minute and second hands) rotated in the same sense, and secondly, they had to be concentric. From the factors mentioned above it can be seen that we need to implement the gear ratios 2:1, 3:1, 5:1 a number of times. Each of these ratios may be implemented in at least two different ways thus allowing for a large number of possible combinations. Luckily it only took a couple of tries to come up with a working answer. During this stage I also increased the complexity of the gear train by adding a double clutch mechanism for clock setting and a motor drive train to reduce the motor speed to a usable level.
Design Stage 3 - PHYSICAL DESIGN
This stage was concerned with working out the physical relationships between the gear axles and their supporting bearings.
Design Stage 4 - CONSTRUCTION DESIGN
Very early in the game I decided that building "from the bottom up" would be too difficult. Rather I chose a horizontal approach building "from the back forward". This involved creating a more or less common support module for each section and attaching it to a set of horizontal support rails. A lot of prototyping was done at this stage. Eventually I finished the clock and its programming in December 2008.
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The clock face. |
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Right hand side. |
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Rear view. The turntable at the back is the first stage of the 1:7-7:1 part of the drive train for the hour hand. The two 8t gears in the train are just visible in the centre top of the model. |
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Left hand side. The white lever in the middle switches the two clutches that direct the drive to either the second hand or the minute hand. |
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This shows the method used to drive the turntable used in the 1:7 part of the drive. This turntable is located at the back of the clock. The 24t gear is driven in the conventional manner by an 8t gear. |
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This top view shows most of the path of the drive train. The seconds drive (S) is reduced along the blue path to the minute drive (M). Further reduction along the green path results in the hour drive (H). The dashed sections of the path indicate the train has gone below the top level. This was necessary to accommodate the difficult spacing of twin 2:1 reductions (net ratio 4:1). |
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Detail showing how each hand was supported. The grey axle driving the minute hand traces a circular orbit around the black second hand axle. |
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This shows how the minute hand was driven. M is the final gear in the minute drive train. During normal running M is driven by the clutch ring A through gear L. In this photo the clutches are in the "minute setting" position so M is being driven by K. The assembly E is not part of the drive train. It merely extends the output past the obstructions visible behind assembly E. The rubber bands are not to hold it together but to stop the huge amount of backlash present in the clutch mechanism. X provides friction to prevent the minute hand falling when it reaches the "10 past" position due to the backlash in clutch A. H is the first gear in the hour hand drive train. Structures E and F freewheel around the second hand axle thus providing the minute hand with an axis concentric with the second hand. |
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Detail showing the double clutch structure. Clutch rings R1 and R2 are operated by levers L1 and L2 that move in opposite directions due to the 16t gears G. The rings are shown in the normal drive mode with R1 towards the viewer and R2 away from the viewer. R2 switches the motor drive D to the Seconds Drive gear during normal operation and to the Minutes Drive gear during the minute setting operation. Not shown is the minute output gear which is placed directly above M2 and is driven by M1 (see previous photo). Also not shown is the 16t gear at the beginning of the seconds to minutes drive train. Switching the clutches drives the minute hand directly from M2 and disengages the entire seconds to minutes drive train. Also note that the cam C is driven by the clutch switch mechanism. The cam pushes in a touch sensor thus signalling to the NXT that it should enter the time setting mode. |
CLOCK OPERATION
The clock runs in one of two modes - continuous or pulsed. Switching between the two modes is achieved by pressing the orange button on the NXT. The NXT internal clock is used as a reference for time accuracy and a servo like programming algorithm is used to keep the clock within +/- 2 seconds of the reference.
To set the clock the "clock set lever" must be moved to its second position. This does several things. It moves the first clutch so that the motor drive is redirected from the second hand to the minute hand. Simultaneously it moves the second clutch to disconnect the second hand from the drive train. If the second hand was not disconnected it would spin at a crazy rate while the minute hand was being adjusted. Lastly, a cam presses a touch sensor that signals the NXT to switch to time setting mode.
Setting the minutes (and hours) is achieved by pressing and holding either of the grey triangle buttons on the NXT. The right button moves the minutes forward rapidly and the left button moves them back. Pressing the orange button signals the NXT that minute setting is complete. The "clock set lever" is then returned to the normal running position. This restores drive to the second hand. The second hand is set in the same manner as the minute hand and when complete a press of the NXT orange button causes the clock to resume running. Each of these operations can be seen in the video.
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These NXT screen shots show the following: Top Left: During normal continuous operation.
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Click on the image to view the movie. |  |
