An engineering project designed, described and illustrated by Michael Adler
Industrial Robots are being used with increasing frequency in manufacturing technology for a wide range of processes on the factory floor and in processes where automation is required. They have become incorporated into computer integrated manufacturing, a concept which combines and links different engineering technologies among which are computer aided design, computer controlled production lines and computer controlled machine tools, computed inventory, automated raw materials and goods storage, retrieval and distribution.
Robots are found on the factory floor under widely different circumstances. They are used for parts handling, assembly and manipulation of tools, under a wide variety of conditions. They are computer controlled and re-programmable, multifunctional manipulators. They fill the gap between the specialised and limited capabilities of hard automation, and the extreme flexibility of human labour. The use of robots can bring about direct savings in production costs because of increased productivity and reduced materials wastage. Indirect benefits result from reduced inventory requirements and greatly improved product quality. In addition there are savings from flexibility in design and the costs of batch runs. They afford relief from time and energy consuming drudgery and even have the potential for performing tasks which are beyond human capability.
Robots are found in many configurations. They may assemble small components such as integrated circuits, or large parts in vehicle assembly. A wide variety of mechanical configurations are available with electrical, hydraulic or pneumatic drives. The end effector may be a gripper or an adapter for a variety of changeable tools. They may integrate with vision systems, and artificial intelligence.
Almost any article can be now be produced using automation techniques, which have brought irresistible gains, and new worldwide trends. The result is that whereas the first industrial revolution made possible the mass production of low quality goods, the new technology will provide goods of quality for everybody.
Robots exist in many configurations. In order to demonstrate the main characteristics of a robot to the best advantage, a design was chosen that had three degrees of freedom, providing a satisfactory work envelope, and which closely emulates similar devices found on the factory floor. A gripper was chosen as the end effector. The movements were to be powered by similar electric motors. Construction was to be sturdy enough to enable the device to be used for demonstration purposes at exhibitions, and to be of use as an educational tool in schools and universities.
The machine is built entirely from parts from the popular Meccano system which is widely available, and which consists of an assortment of metal strips and girders with holes punched to 1/2" centres, and a wide variety of compatible parts including rods, bearings and gears. This table top engineering system enables almost any machine or structure to be built, closely resembling the prototype, and the components can be re-used when their function in a particular project is completed.
A robot is particularly suitable for modelling in Meccano, as it has both structural and moving elements, demonstrates several mechanical principles, and it presents a challenge to produce a life-like and mechanically correct device, which could be instrumented in such a way that it could be controlled, as in the prototype, by computer.
The design should be both appealing to the eye, and be mechanically strong enough and stable enough to do a job of work, such as transfer articles from one position to another, as would be the case on the factory floor, for loading machine tools from a conveyor, or in an assembly process. Moreover it should be reasonably easy to build even by those unaccustomed to technigues of Meccano construction.
The instrumentation of the device presents a particular challenge, as a wide variety of electronic devices are available, their use depending on the reguirements of the robot, availability, cost and ease of integration with the computer. Account had to be taken of these instrumentation possibilities when the design was considered, and an open construction was therefore chosen which would make attachment and servicing least difficult and troublesome, and allow visualisation of the structural, mechanical and electronic elements.
Rigidity of construction free from distortion and shake when running light and under load was a prime consideration. For this purpose, strong H and T girder shapes were used, together with a rigid body and base.
A cylindrical design work envelope was chosen as the geometrical configuration. This had reach (extension and retraction), elevation and rotation. A mounting surface was provided on the end of the wrist for attachment of different end effectors, and a mechanical hand was built, with opposed gripping surfaces. This could easily be altered to another configuration, such as a magnet, or a device to hold a machine tool such as a drill.
The modular design of the robot allows easy assembly of the components once built, or the attachment of other parts which may evolve with further designs or uses. It should be seen as a multi- purpose device rather than a dedicated machine. Modular design may allow separate groups to build individual sections. Each component of the machine is clearly identifiable into anatomical parts, thus making its purpose clear for educational use.
The attachment of motors of different types and gear ratios is a simple matter, as the point of application of the drive is uncluttered. In this way, stepper or servo-motors could be attached, as well as reduction devices such as gear trains, epicyclic gears or harmonic drives. Hydraulics or pneumatics could be applied to the different movements.
Of the many characteristics of robots, those of positional accuracy, repeatability and reliability are paramount. Other characteristics are those of payload capacity, end of arm speed and memory capacity.
Feedback devices are essential to successful operation. The computer must not only be able to control motor function with regard to speed and direction, but also receive input signals from various monitoring devices to detect this motion. The simplest way of doing this is to use limit switches, which must be individually preset for each operation. When a limit switch is activated, a contact is closed and a signal is received at the controller which reacts by stopping the motion, or the device can be wired into the motor control circuit directly. A potentiometer could detect rotation to a limited degree, as could an impulse counter. A digital or optical encoder is better able to detect continuous rotation as well as speed and is able to provide extremely sensitive feedback, but requires considerable electronic application and computer programming. Linear motion can be detected by induction feedback, or by the use of previously described devices attached to actuators.
Most robots have dedicated computer devices networked to master controller computers, but for demonstration purposes, a personal computer is satisfactory. An input-output port and an electronic interface is required. Finally a computer programme to control repeatable movements must be written, which takes into account the input and output addresses, feedback and motor control signals and could compute speed and position. It could also be linked to other equipment such as conveyors, storage devices and manufacturing machinery.
Size: 18" x 13" x 9"
The robot is built in five modular sections: the base, the supporting body together with the horizontal frame, the horizontal linear actuator, the vertical linear actuator, and the end effector.
This is 12 1/2" square and I" high. Each of the sides is built up from two 12 1/2" angle girders, joined by a 12 1/2" flat girder to make a U-shaped section. These are then fastened to each other to form a square, incorporating six overlapping 12 1/2" x 2 1/2" strip plates to form a flat surface. There are five supporting girders underneath. Three of these are 12 1/2" angle girders, but the other two are built up from two 12 1/2" angle girders, joined together by a 12 1/2" flat girder. This specially strengthened area is used for the support and attachment of the roller bearing for the slewing motion.
The Body is built from four vertical 5 1/2" x 2 1/2" flat plates supported by four vertical 5 1/2" girders arranged as shown. Two horizontal 2 1/2" girders at each side strengthen the structure. Two 2 1/2" girders are bolted front and back at the bottom, and two 3 1/2" girders to the top. The latter support the horizontal frame. The horizontal frame consists of two U-shaped compound girders, one on each side, made from two 12 1/2" girders, joined by a 12 1/2" flat girder. These main side girders are joined together at the front by two 3 1/2" girders and a 3 1/2" flat girder, with a similar arrangement at the rear. Two 2 1/2" girders are attached inside the side frames 4 holes from the front, and two transverse 2 1/2" girders are bolted to them by their slotted holes. A 3 1/2" flat plate is bolted under the girders at the rear, and forms the floor of the counterweight. The front of this space is closed by a transverse 2 1/2" girder to which is bolted a 2 1/2" flat plate. After addition of the counterweight which consists of lead sinkers, the top is closed by a second 3 1/2" x 5 1/2" flat plate, and two 5 1/2" flexible strips which improve the appearance. This structure is bolted to the two 3 1/2" angle girders at the top of the base in the position indicated.
The roller bearing is built up from a ball thrust race, a ball thrust cage with balls, and a 3 1/2" gear wheel. The gear is fixed by long bolts to three stacked 4 1/2" x 2 1/2" flat plates, but separated from them by two short coupling on each bolt. If desired, a potentiometer can be attached to the bottom of the gear, its shaft protruding upwards through the bearing and fastened to the ball thrust race. Rotation of the body will thus rotate the potentiometer, and give feedback via analogue digital converter to a computer.
A 3" x 1 1/2" flat plate and a double arm crank are bolted to the top of the ball thrust race. The body is now attached to the top of this assembly. The bearing is now assembled, with the ball race between the gear and the ball thrust race. Either the potentiometer rod is used as the centre rod, or a separate 2" rod can be used, the parts of the bearing clamped tightly between collars. Bearings for the slewing rod are provided by the flat plate and a 1 1/2" perforated strip. The strip is fixed to the underside of the flat plate by 1 1/8" bolts. A 1/2" pinion is fastened to the rod between strip and plate, and meshes with the 3 1/2" gear.
A 1" gear wheel is fixed to the rod above the flat plate. A six gear motor set to the highest reduction, with a 7/16" pinion fastened to its output shaft is bolted to a 2 1/2" x 2 1/2" flat plate, which in turn is bolted to the back of the supporting body but separated from it by collars on each of the four bolts. The pinion on the motor meshes with the I" gear on the 2" rod. The entire assembly is now bolted to the base, the 4 1/2" x 2 /2" flat plates being bolted over the strong supporting girders underneath.
This consists of two parts, a static housing which provides bearings for the moving part and an attachment for the motor, and a moving section.
The moving section is a built up H-shaped girder. Two pairs of 12 1/2" angle girders are bolted together by their oblong holes to form U-sections facing away from each other. These are bolted to each other using two overlapped 12 1/2" flat girders on each side. Two 1/2" angle brackets are bolted to the front of the U-sections by their round holes, and a connecting 1 1/2" flexible strip is fixed to their free oval holes by two threaded pins. A 6 1/2" rack strip is bolted to the side of the girders, separated from them by two washers, and this meshes with a 1/2" pinion on the motor shaft.
Each side of the housing is made from two horizontal 3" flat girders, joined together by vertical 1 1/2" flat girders. They are connected together at the top by two 2" angle girders joined by a 2" flat girder with a similar arrangements below. 1/2" pulleys are used as rollers for the moving section, and they are fastened to 2" rods, the bearing for which are the free corner holes of the housing. The moving section is slid into the housing. Washers and collars are used where necessary to allow the pulleys to track on the free flanges of the moving section.
A 1/2" pinion mounted directly on the motor output shaft meshes with the rack strip on the moving section. The motor is a Richards 6 speed motor with integral epicyclic reduction gearbox set to the highest 60:1 reduction setting. Two pairs of overlapped 1 1/2" flat girders are bolted to the base, and 2 1/2" girders are attached to them. This assembly is then attached to the side of the housing via a 1 1/2" girder bolted to the housing, reinforced with a corner bracket, above and below.
There is room to attach two limit switches to the inside of the housing, and each is tripped by a small screw attached to the moving section.
This is built up from a 12 1/2" flat girder reinforced with a 12 1/2" perforated strip, the two being separated from each other by collars. A 6 1/2" rack strip completes this assembly. At the lower end, an attachment is added to receive the hand actuator. This attachment is built up from two 1 1/2" angle girders joined by two rod sockets. 1 1/2" corner brackets are bolted to each side, and the assembly is attached to the lower end of the vertical actuator. The two brackets are also joined together by a short coupling on a 1 1/8" bolt during this assembly. The housing for the vertical linear actuator is formed from two 3" angle girders joined to each other by two 2" flat girders.
Threaded pins are fasted at each corner of the angle girders, and 1/2" pulleys are free to rotate on each of them, held in place by collars.
The base of the motor is fastened to a 3" girder, and by a 1 1/2" flat girder to a second 3" girder. A 1/2" pinion is fixed to the motor output shaft, and the reduction gearing set to the slowest output speed. The assembly is then fastened to the housing, using a 1 1/2" corner bracket for bracing. The vertical actuator is slid into the housing and the rack meshes with the pinion on the motor shaft. The position of the threaded pins of the 1/2" pulleys is adjusted to prevent play on the flat girder.
Two limit switches are attached to the back of the housing, and 3/4" bolts on the vertical arm are positioned to close the switches where desired.
The end effector takes the form of a two-fingered gripper. The frame is constructed from two 5 1/2" angle girders, joined at one end by their oval holes by a 1 1/2" angle girder, and at the other by the base of the motor. The motor is set to its lowest output gear. A double arm crank is bolted across the middle holes of the girder and provides the bearing for the shaft on which the final actuator pinion is fastened. A I" corner bracket is fastened to one of the girders, but separated from it by two washers on each bolt. This provides a second bearing for an adjacent shaft. A second 1 1/2" perforated strip is attached 1 1/2" from the end opposite to the motor, and to this a limit switch will be attached.
The assembly is reinforced by attaching a 5 1/2" flat girder to each 5 1/2" angle girder, and the ends are closed by 1 1/2" flat girders. A framework is added to provide a bearing for the reduction gears, and an attachment to the vertical actuator. It is built up from two overlapped 2 1/2" flat girders top and bottom, attached to the outer sides of the flat girders, and connected together at their ends by a 1 1/2" x 1 1/2" flat plate and two 1 1/2" angle girders. Two threaded pins are fixed to the upper pair of flat plates, and these will be introduced into the rod sockets on the lower end of the vertical actuator. A I" corner bracket is bolted inside the flat plate to provide a bearing for a 2 1/2" rod, who's other bearing is the I" corner bracket first described. The train of reduction gears can now be assembled. A 1 1/2" bevel gear fixed to the 2 1/2" rod meshes with a 1/2" bevel on the motor output shaft. Also fixed on the rod is a 3/4" pinion. The pinion meshes with a 50 tooth gear fixed on a 1 1/2" rod which rotates in the double arm crank. A 7/16" pinion is also fixed to this shaft, and meshes with two 3 1/2" rack strips which are attached to each of the two fingers.
A limit switch is fastened to the 1 1/2" perforated strip attached to the 5 1/2" girders, and a 3/4" bolt on the adjacent finger will close it when the fingers are fully separated.
The two fingers slide towards and away from each other on two 6" rods which are retained in the end flat plates by collars. Double brackets on the base of the fingers slide on these rods. The base of each finger consists of a 1 1/2" x 1 1/2" flat plate to the back of which four double brackets are bolted. Construction differs between the fingers, because the pad of one of them is movable so that it can close a limit switch when under pressure. In the fixed finger a 1 1/2" angle girder is bolted to the opposite side of the flat plate. A 1 1/2" corner bracket is bolted to this girder, and it provides an attachment for the finger. The finger pad is built from two double brackets bolted to each other, and to which are attached two 1 1/2" angle girders by their elongated holes. The finger is attached to its base by the corner bracket, but separated from it by 3 washers on each bolt. A 3 1/2" rack strip is bolted to the lower pair of double brackets at the rear. A 1/2" bolt may be attached to the back of the rack strip to activate the micro-switch.
The other finger has the movable pad. The pad is built in almost the same way as previously described, but instead of being fixed to a corner bracket, it is free to move inside the flanges of a double angle strip which is bolted vertically to the base of the finger. Fish plates reinforce the slotted holes of the finger pads. A 1 1/2" corner bracket is bolted to a 1/2" angle bracket on the base, and a 2" rod with collars which also incorporates the corner bracket locates the movable finger in the double angle strip. A 3/4" bolt introduced through the end slotted hole of the top finger girder is fixed to the free corner hole of the corner bracket, and this acts as a limit stop for the finger. A limit switch may be bolted to the underside of the corner bracket, and its movable blade bears against the adjacent finger girder. This limit switch will close when the fingers are closed against one another, or when an object is grasped.
A second 3 1/2" rack strip is bolted to the rear of the two upper double brackets of this finger.
The finger are now ready for assembly to the hand. The upper of the two 6 1/2" rods is slid from one side, incorporating the two upper double bracket of each finger. Ensure that the fingers are equally fully open during assembly, and that their rack strips bear smoothly against the adjacent pinion. The second 6 1/2" rod can now be slide across, incorporating the lower double brackets on each finger. Collars locate the rods in position.
The base has already been joined to the body at the roller bearing. Now the horizontal actuator is joined to the horizontal frame, via the three threaded pins on the underside of the cage of the linear actuator which enter the three rod sockets on the frame. Large screws in the sockets are tightened to lock the cage securely. The counterweight lead sinkers can now be added and closed in. The vertical linear actuator is joined to the horizontal actuator. The two locating pins on the end of the horizontal actuator entering two matching rod sockets on the cage of the vertical actuator.
Finally, the end effector is joined to the end of the vertical actuator in a similar fashion.
Wiring to the motors, the limit switches and potentiometer should be brought to a plug attached to the base, from where they can be attached to a power supply and controlling electronics.
Michael Adler 1999
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