The hook is made from brass rod bent into shape and attached to a pulley block. The pulley wheels used here and in other tools are included in the pulley set from Tamiya™.
Two views of the hook, show it attached to a pulley block. The sides of the block are bolted together by two bolts at the top, with 6 mm spacers between them.
A special mechanism raises and lowers the hook. The same mechanism is used to raise and lower the gripper. The principle of this is illustrated below. The winch motor M3 is at the left end of the y-frame. From the winch the cord runs under the x-frame and is tied to the right end of the y-frame.
The travelling pulley mechanism. The upper two pulleys are mounted in a box on the x-frame. The lower pulley is in the pulley block that carries the hook. As the frame moves from left to right the height of the hook above ground is unchanged.
The hook is raised by winding in some of the cord. It is lowered by unwinding. But if the frame is moved without winding or unwinding the cord, the cord simply passes through the system and the hook stays at the same height. The photo below shows the mechanism in detail.
The travelling pulley mechanism with its front panel removed from the four long bolts at its corners. The rear panel is bracketed at a right angle to the tool's base panel so that it is vertical beneath the x-frame.
The cord is seen threaded through the mechanism as in the photo on p. 309.
The robot needs to know two facts about the hook: whether it is carrying a load and its height above the working surface. This information is obtained in a simple way by the two microswitches, labelled MS1 and MS2 in the photo. MS1 is used to detect if the hook is carrying a load. The axle of the right-hand pulley passes through holes drilled in the front and rear panels. The axle of the left-hand pulley is mounted on a lever which is pivoted at one end. This lever is held up by a spring. The upper end of the spring is bolted to one of a row of holes in the rear panel. The tension in the spring is adjustable by the choice of hole. The tension is set so that the spring supports the lever when there is no load, but allows the lever to be pulled down when a given load is present.
When the lever is pulled down it presses against the actuator lever of MS1, which closes the switch, sending a signal to the microcontroller.
MS2 is seen end-on in the figure. When the pulley block is raised as far as possible it presses up against the lever of MS2. This closes the switch, indicating that the hook has reached its upper limit. From this position a short timed burst on the winch lowers the hook to a reasonably predictable height.
There is no limit switch to indicate when the hook has dropped to ground level. However, suppose the hook is known to be carrying a load, as indicated by MS1. If the hook is then lowered, MS1 opens when the load touches the ground. The hook has reached the level at which the robot should free the hook from the load by moving the x-frame to one side. This is an instance of how software can substitute for the lack of a limit switch.
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