What is a Cartesian Robot?
Cartesian Robot Basics: (see Considerations in Selecting a Cartesian Robot)Cartesian robots are made up of multiple linear actuators configured so that the resultant motion of the tip of the configuration moves along 3 mutually orthogonal axes aligned with each of the actuators. Cartesian robots are either 2 or 3 axis machines. They can also be combined with rotational axes to form more complex systems. The major benefit of a Cartesian robot is that the position of the robot can easily be programmed without extensive geometric calculations or trigonometric calculations as would be needed for an articulated robot arm or even a SCARA robot. Examples of Cartesian robots are like the following:
Examples of Cantilevered Cartesian Robots
|XYZ Cartesian Robot
|XY Cartesian Robot
|XYZ Cartesian Robot
Isel USA offers such Cartesian robots in various configurations.
Another benefit of Cartesian robots is that the work envelope or reach of the robot is well defined and can easily be visualized. This is very helpful if safety guarding will be required for the robot application. The other important benefit of a Cartesian robot is that the work area is open and parts can easily be placed in the work area from 3 sides including the “front” where the operator risk of being struck is minimized. It is worth pointing out that articulated or SCARA robots have spherical or cylindrical work envelopes which require guard fencing that is much more difficult to fabricate. If simple rectangular framing is used with the SCARA or articulated arm robot then the guard envelope becomes much larger than the work envelope and it could be more difficult loading workpieces into robot work envelope.
Cartesian coordinates are named after the French mathematician and philosopher René Descartes (1596–1650), who introduced the coordinate system to show how algebra could be used to solve geometric problems. That is why the term “Cartesian” is usually capitalized.
Benefits of Cartesian Robots
- Easy to calculate and program position
- Work envelope easily defined and enclosed for safety
- Generally much higher payloads than SCARA or articulated arm robots
- Much larger work envelopes can be formed by selecting appropriate linear actuators
The Cartesian GANTRY robot overcomes this limitation. Gantry robots also move the individual actuators along mutually orthogonal axes. The chief design difference is that a “GANTRY” robot has the 2 parallel axes or supports on the bottom axis and thus eliminates the cantilevered design and torque on the base axis. The payload and reach limits can now easily be extended in the design without having to use extremely oversized linear actuators. Some examples of Cartesian gantry robots are:
The Cartesian Gantry Robot on the left is driven by 2 independant but electrically linked motors that are coordinated or synchronized to achieve motion. The example on the right has a single motor drive down the center of the bottom axis but it has the rails separated to provide stability in the motion, This separation of the rails eliminates racking and provides support at both ends of the gantry for the load on the Z axis. This symmetric support shares the load and converts the TORQUE load of the cantilevered robot into a DIRECT VERTICAL LOAD on the base axis support bearings. It is well worth noting that virtually all linear bearings on a single rail are excellent at supporting direct vertical loads but are limited at supporting torque loads. That is why heavy duty linear actuators have 2 parallel rails and 4 linear bearings - this configuration takes a torque load and converts it to a pair or equal and opposite vertical loads on the linear bearings.
Isel USA offers 6 different sizes of the flatbed style Cartesian Gantry Robot and numerous other configurations to address various design considerations.
The benefit of the Cartesian gantry robot is that the open cantilever configuration is replaced by an axis that is supported at both ends of its travel. This then means that both the payload and the “gantry” axis travel can be significantly increased without the need for stronger, more expensive linear bearings and actuators. The tradeoff for the gantry design is that the open robot work area is now less accessible. The workpiece can still be loaded in the robot work envelope from the left or right side relatively easily but loading from the front will require the robot to move off to the side. The parallel axis also takes up part of the robot work envelope. Also note that the benefit of increased robot reach and payload is achieved at the cost of an extra actuator on the bottom axis as well as the need for a motion controller that can handle a parallel actuator drive axis. For smaller gantry structures, such as the one shown above, one can have a single axis drive mechanism in the center of the workenvelope and rails at either end of the gantry to support the gantry and keep it from racking.
The parallel actuators are sometimes called a master/slave configuration. In some cases the parallel axes are joined with a mechanical linkage or gears or belts. This is usually done where the parallel actuators are close to each other. In case the parallel axes are separated by a substantial distance where the mechanical linkage might introduce backlash or racking of the 2 axes, then both axes are driven by motors that are synchronized so they both move identical distances at identical speeds simultaneously. One axis becomes the “master” and the other axis follows the master very closely and is referred to as the slave axis.
The work envelope of a Cartesian gantry robot is well defined as in the Cartesian robot situation. In both cases, creating safety enclosures becomes a relatively easy task of making an appropriate sized box.
Isel USA offers a variety of Cartesian Gantry Robots, Cartesian Robots and Belt Drive Gantry Robots.
Both Gantry Robots and Cartesian style robots can be made to have extremely large work envelopes because the base axis can be made as long as an available linear actuator. SCARA robots are limited to a few feet of reach and Articulated Arm robots are generally limited physically to about 10ft.radius.
When speed is a driving factor, SCARA and articulated arm robots have clear advantages for a given resolution. This is because the tool tip moves at a speed equal to the arm length times the angular velocity of the joint - the resulting product can be substantial. Cartesian style robots always tradeoff speed with resolution - the finer the resolution, the slower the resulting speed. The resolution is generally based on the mechanical drive mechanism - either a ball screw, rack and pinion, linear motor or belt drive.
Repeatability is very even between all the designs but significantly depends on the technology being used. In the case of Cartesian Robots, ball screws generally have higher repeatability than a rack and pinion and among ball screw driven robots, ground screws with anti-backlash ball nuts are generally more accurate and repeatable than rolled ball screws with standard ball nuts. Among SCARA and articulated arm robots, cycloidal drives and harmonic drives tend to be more precise than planetary gearboxes. Although planetary and cycloidal gearboxes tend to have higher torque capacity.
Complimentary Technologies Designed to Work TogetherArticulated arm robots and cobots are especially useful in loading/unloading applications. Because of the extension available on the arm and the rotary base, found on most COBOT style robots, they can easily load and unload CNC machines or Cartesian gantry robot style routers and glue dispensers. In those types of applications, the 2 styles are actually very complimentary. The Cartesian robots have the load capacity and rigidity to perform functions like machining or dispensing or even welding or inspection. The articulated arm cobot is especially well suited to "reaching" to load and unload parts into the Cartesian robot. Communication between collaborative machines will be discussed separately
Selecting a Cartesian RobotSelecting a Cartesian Robot first involves selecting which parameters are most important. Generally, the first consideration is work envelope. This determines the overall size of the system and is fixed by the size of the part being worked on.
The next consideration is the speed required along with the resolution of the motion. The tradeoff between speed and resolution means that the finer the resolution, the slower the resultant speed. This generally specifies the nature of the drive mechanism.
The final main point to consider is the size and weight of the payload being moved around. As noted earlier, Cartesian Robots are constructed of linear actuators. The size of the payload and the size of the travel will specify the size and capabilities of the actuator. Larger travels and larger payloads require heavier bearings. The limiting factor is generally the moment load that results - not just the weight of the payload. The offset distance of the payload is often more critical in specifying a Cartesian Robot than just the weight itself, e.g. a payload of 10 lbs is not a great deal for most linear actuators but if it is overhung from the axi by a distance of 20", a torque of 200 in.lbs is being applied to the actuator. This torque may or may not be significant, depending on the actuator specifications.