Accumulator : A robot mechanism designed to pick up a large number of similar objects

Conveyance Something that carries objects up from an intake into a storage chamber.

Compression The reduction in volume (causing an increase in pressure) of an object.

Phase Objectives After completing this phase, you will be able to: Design a basic mechanism to collect multiples of an object off the floor. Calculate the gearing necessary for an accumulator based on robot drivetrain speed. Describe the basic considerations of accumulator design.

Accumulators An accumulator is a robot mechanism designed to pick up a large number of similar objects. These mechanisms commonly utilize conveyor belts and rollers for their intake. The best accumulators have the following characteristics: A wide intake “mouth,” enabling pickup without precise robot positioning. The means to prevent jamming of objects after pickup.

A high-speed intake that enables a robot to suck up an object even at full speed. The ability to pick up multiple objects at the same time. The ability to pick up a large number of objects one after another without jamming or slowing down. The capability for picking up objects with size variances.

Examples of accumulators are shown here:

A component common in many accumulators is a conveyance system that carries objects from the intake up into a storage chamber. One simple version of this is a conveyor belt in front of a flat wall.

In this type of conveyor, the belt contacts the balls on one side and rolls them up the opposing surface. This relatively simple setup requires only one conveyor belt. However, this setup has some disadvantages. First, since the balls are rolling, they move through the accumulator at half the speed of the conveyor belt. Second, this setup is subject to jamming if two balls are picked up too closely together and they touch inside the conveyor. Because the back side of the ball in front is moving up while the front side of the ball in back is moving down, the balls can bind up and jam. A way around this jamming issue is to use independent rollers instead of one long conveyor belt.

This is the best of the setups. With two belts, the balls no longer roll, but move straight up the conveyor. It is almost impossible for two objects to touch, and if they do, jamming is less likely to occur. One downside of this system is the added complexity of using two belts.

Accumulator Gearing It is important to gear your accumulator appropriately. Ideally, the accumulator intake is geared so that it pulls an object in faster than the drivetrain at maximum speed. In a single-belt system, this means that the intake is geared in such a way that the linear belt speed is more than double the drivetrain’s top speed. In a two-belt system, the intake’s linear belt speed only needs to be more than the drivetrain’s top speed. When it comes to accumulator gearing, faster is almost always better – just make sure the accumulator can overcome the friction caused by pulling in the objects.

Compression and Elasticity As explained in Unit 8: Friction and Traction, friction is applied between two surfaces held together by normal force. For belts or rollers to pull in an object, there must be some force pressing the belt onto the object.

*Often this force is caused by the compression or elasticity of some part of the system. Sometimes the conveyor belting bends backwards and this “spring” is what applies the force on the object. *Other times, the object itself has some elasticity and deforms when it is sucked into the intake. And yet other times, additional elastic bands or springs are used to give the entire conveyor assembly some give, which enables it to deform when an object moves through it. In this case, the springs apply the normal force on the object. * Finding the correct balance of grip on an object is sometimes difficult, especially when building an accumulator designed to pick up multiple objects at the same time.

Example Accumulators

Build a collector

Joint A link between two rigid components, such as parts or subassemblies. A joint applies force from the first component on the second component.

Linkage Designed to convert some input motion into a different output motion, it typically consists of a series of rigid links. Each link has one or more joints that rotate freely, connecting the links. Typically, one link is fixed and cannot move, and one link is driven in some input motion.

Linkages are designed to convert input motion into a different output motion. A linkage typically consists of a series of rigid links. Each link has one or more joints which rotate freely, connecting the links together. Typically, one link is fixed and cannot move and one link is driven in some input motion. Linkages are a fundamental part of machine design because of their ability to create such a wide variety of output motions and their ability to alter the path, velocity, and acceleration of the input. Very precise and somewhat complicated motions can be designed using a simple linkage design. Linkage motions are extremely repeatable.

An example of a more complex linkage is shown here:

The simplest and one of the most common linkage types is the four-bar linkage. This is a closed-loop linkage system that can provide a wide variety of motion types. The most basic type of four-bar linkage is one in which the links are equal length and parallel to each other. You focus on this linkage type for the rest of this unit.

When the linkage travels through its motion, the output link remains parallel to the fixed link as shown.

The above robot utilizes a four-bar linkage to deploy its tools. The tools remain parallel to the floor at all times during their deployment.