Basic design of DELTA robot

DELTA Robot Overview

I always think that DELTA is a configuration that is not appreciated. It is obviously better than SCARA in various performances, but it is far behind SCARA in sales due to price. Every two years, I always hear rumors that "DELTA robots will be popular this year", but each time it ends in failure. Although the overall sales are increasing, the sales in the entire market are only a few thousand units, which has also led many DELTAs to choose to give up the development of this model.

This also forced some manufacturers who claimed to focus on parallel robots to start developing other configurations. It is too difficult to support a company and continue to expand by selling parallel robots alone.

The main components of the more common DELTA robot include the base, active arm, driven arm, dynamic platform, and rotating axis.

Base

The base of DELTA is simply composed of a large mounting plate and three mounting planes with 120 degrees even division. Usually, a simple assembly method is adopted in the prototype stage, similar to the figure below. In terms of cost and cycle, this method is naturally very suitable in the research and development stage. However, once it enters the mass production stage, it is generally manufactured by casting. After all, the assembly method is inferior in shape and stability. On the contrary, the one-piece casting method is naturally more costly and difficult to process.

Active arm drive mode

The mainstream drive mode is a direct-connected R-type drive with a reducer and an active arm.

There are also non-mainstream methods, such as using indirect drive to convert rotation into linear motion.

For example, the pneumatic king uses a linear module instead of an active arm

Active arm

The main arm is actually not very good, and there are not many technical points to talk about. The shapes are similar. The main difference lies in the choice of materials. There are aluminum parts, carbon parts, and high-end titanium alloys.

Generally speaking, the main considerations are factors such as high strength, light weight and low cost.

To go into more detail, details such as how to bond, how to design the ball head, how to make it wear-resistant, hard limit, calibration, etc. have to be explored by the designer himself.

Active arm reducer

As mentioned before, most of the reducers used by DELTA are planetary reducers. The main reason for choosing planetary reducers is that they can achieve low reduction ratios, high transmission efficiency, and long life. However, it is relatively difficult for harmonic and RV to achieve low reduction ratios, and the transmission efficiency is slightly lower than that of planetary reducers.

However, planetary reducers also have disadvantages. The planetary reducers used by DELTA need to be below 3 arc minutes, which is less accurate than harmonics and RV. Of course, there are also planetary reducers that use 1 arc minute, but planetary reducers of this accuracy are not only difficult to find, but also very expensive.

There are not many DELTAs that use harmonics and RV, but there are some, such as QKM and Asyril. Harmonics have almost no backlash, so the grasping is smoother and more precise. This can be seen by carefully observing the details of the two robots when grasping.

Especially Asyril, because it is a desktop DELTA, the accuracy reaches the incredible 0.002mm. Please note that it is not silk level, but u level. If you have no idea, let me tell you that a hair is 60um, and the accuracy of this robot is 2um. Feel it.

Follower arm

The design of the driven arm is similar to that of the active arm. Except for some changes in shape, the solutions of various companies are similar. Most of them are aluminum parts at both ends, and carbon fiber tubes or carbon fiber rods are bonded in the middle.

The focus is still on the design of the ball joints at both ends and the production process of how to ensure that each follower arm has the same length.

Sports joints

There are different ways to connect the active arm and the passive arm.

The most common method is to use a ball joint, as shown below, which has a more flexible joint.

There are also many that use Hooke's hinges, which have the advantages of better rigidity and higher precision. For example, Asyil mentioned above has such high precision, which is also related to the use of Hooke's hinges. However, the assembly precision of this structure is required to be higher, otherwise it is easy to produce.

A few also use fish eye ball bearings.

The three solutions have their own advantages and disadvantages and are used by people. The most important thing is how to solve the wear problem.

spring

Most DELTAs use springs to connect the follower rods, one is to prevent them from falling off, and the other is to reduce noise. As for the selection of springs, they are actually the same, but the only difference is to choose the specifications with enough elasticity and wear resistance. It is easy to say, it is very easy, springs are really commonly used, but it is also difficult to say, it is not easy to choose the right spring.

The spring anti-dropout is also an interesting topic. Manufacturers using ball joints will use springs to prevent the ball head from falling out, but if the force is strong enough, the ball head will still fall out, so some manufacturers will add anti-dropout mechanisms, or use Hook joints and fisheye ball bearings to prevent it from falling out.

However, because DELTA emphasizes lightweight, for a tandem robot, the strength of the active arm and the driven arm are much smaller. Once a collision occurs, a contradiction arises. If the driven rod cannot be disengaged, it means that the collision is hard, and the driven rod may eventually be broken or bent; and if the ball head is disengaged, the driven rod will fall, which actually has certain hidden dangers. It is not easy to choose between the spear and the shield. At least for now, using only springs to prevent disengagement is still the mainstream.

The fourth axis

Regarding the fourth axis, the mainstream design is still to place the motor reducer on the base and transmit it to the moving platform through the rotating shaft. Some use spline shafts, while others use a combination of linear bearings.

There are also manufacturers who directly put the motor on the moving platform

There are also FANUCs that actually put the motor on the driven rod, which is very impressive.

There is also a genius ept that uses 4 kinematic chains to achieve 4 degrees of freedom. And it has occupied the title of the fastest DELTA for many years. Unfortunately, it is a thing of the past.

Dynamic Platform

Except for the special platform of Adept, the dynamic platforms of other configurations are similar. The main change is in the transmission realization of three-axis, four-axis, five-axis or even six-axis.

It is worth mentioning that three-axis and four-axis DELTAs are still the mainstream, while five-axis DELTAs seem to have gradually entered the market. And the six-axis DELTA, the mature products in batches are probably only FANUC, after all, the design difficulty and cost are much higher.

In fact, China began to study DELTA around 2010, and by now the configuration of traditional DELTA has been basically thoroughly studied. The powerful Chinese engineers have formed a routine for solving the problem just like cracking the college entrance examination. Some people have also reduced the cost to an extremely low level by simplifying it.

However, the ultra-low-priced DELTA has never set off the DELTA market. In the final analysis, DELTA is still a product with low entry and difficult to refine. This seems to be the case with SCARA and six-axis. Performance is always linked to cost, and cost is always linked to service and supporting technology. Reducing performance, reducing supporting technology, and lowering service levels will always have limited impact.

I hope that "DELTA will be popular this year" can get rid of the dilemma of crying wolf~~~~~