Design methods and steps for industrial robot manipulators

(4) Balance of the robot arm The advantages of balancing the gravity torque of the robot's operating arm are as follows:

If it is a painting robot, it is convenient for manual teaching.

The driver basically only needs to overcome the inertia force of the robot during movement, while ignoring the influence of gravity torque. Therefore, a driver with smaller size and lower power consumption can be selected.

Eliminates the risk of the robot arm falling under its own weight and injuring people.

In servo control, the influence of load variation is reduced, thus achieving more precise servo control.

Generally, robot manipulators do not need to be balanced because the 1-axis turret rotates. The 4th, 5th, and 6th axis arms often have very little gravity and do not need to be balanced either. Therefore, it is the gravity torque of the 2nd and 3rd axis arms that needs to be balanced.

1) Counterweight balancing mechanism: The principle of this mechanism is shown in Figure 2a. Assume that the mass of the arm is m1 and the mass of the counterweight is m2. Since the joints are on the same straight line, the unbalanced torque is

M1＝m1glcosγ

The torque generated by the counterweight is

M2＝m2gl′cosγ

The static equilibrium condition is

M1＝M2

That is, m1l = m2l'

This balancing mechanism is simple, has good balancing effect, is easy to adjust, and works reliably, but it increases the inertia of the arm and the load on the joints, and is suitable for situations where the unbalanced torque is small.

2) Spring balancing mechanism: Its principle is shown in Figure 2b. The unbalanced moment of the arm is

M1＝M11-M12＝mglcosγ-Ia

Where M11——static unbalanced moment;

M12 - moment of inertia;

I——moment of inertia of the arm about the joint axis;

a——average acceleration of arm movement.

The balancing torque produced by the spring is

Where k is the spring stiffness;

l′——the distance from the spring mounting point on the arm to the joint axis;

e——the distance from the mounting point at the other end of the spring to the joint axis;

R is the free length of spring.

The static equilibrium condition is

M2＝M11

The dynamic balance condition is

M2＝M11+M12

This balancing mechanism has a simple structure, good balancing effect, reliable operation, and is suitable for small and medium loads, but the balancing range is small.

3) Cylinder balancing mechanism: The principle of this balancing mechanism is shown in Figure 2c. The unbalanced moment of the arm is

M1＝M11+M12＝mglcosγ+Ia

The balancing torque generated by the cylinder is

Where F is the thrust of the cylinder piston;

The remaining parameters are the same as above.

The static equilibrium condition is

M2＝M11

The dynamic balance condition is

M2＝M11+M12

Cylinder balancing mechanisms are mostly used in heavy-load handling and spot welding robot manipulators. They are small in size and have strong balancing force. Pneumatic ones have good damping effects but are larger in size.

(5) Robot dynamics analysis: Since the gravity torque of each axis of the robot is basically balanced, when these axes are running, the motor mainly needs to overcome the dynamic torque caused by the rotational inertia of each axis.

Axis 1: After analysis, when the end of the robot is extended to the farthest point, the rotational inertia of axis 1 is the largest. Calculation shows that the rotational inertia of axis 1 is J1. If the starting time is T1, the dynamic torque is

M1＝J1ω1/T1

2nd axis: After analysis, when the angle between the small arm and the big arm is the largest, the moment of inertia of the 2nd axis is the largest. After calculation, it can be obtained that the moment of inertia of the 2nd axis here is J2. If the starting time is taken as T2, the dynamic torque is

M2＝J2ω2/T2

3-axis: The moment of inertia of the robot's small arm relative to the center of the upper part of the large arm is the moment of inertia of the 3-axis.

M3＝J3ω3/T3

4-axis: There is no gravity torque balancing device on the 4-axis, so the 4-axis motor must overcome both the dynamic torque at startup and the gravity torque caused by the wrist and load during operation. After calculation, the rotational inertia of the 4-axis is obtained, and then the transmission torque required by the 4-axis is calculated.

5-axis: There is no gravity torque balancing device for the 5-axis, so the 5-axis motor must overcome both the dynamic torque at startup and the gravity torque caused by the wrist and load during operation. After calculation, the rotational inertia of the 5-axis is obtained, and then the transmission torque required by the 5-axis is calculated.

6-axis: There is no gravity torque balancing device for the 6-axis, so the 6-axis motor must overcome both the dynamic torque at startup and the gravity torque caused by the wrist and load during operation. After calculation, the rotational inertia of the 6-axis is obtained, and then the transmission torque required by the 6-axis is calculated.

(6) Selection of motors Selecting a good AC servo motor is the key to manipulator design. Since robots require compact structure, light weight, and good motion characteristics, it is hoped that the motor weight should be light and small under the same power. In particular, the motor installed inside the robot's horizontal arm or vertical arm should be as light as possible and as small as possible.

According to the transmission torque required for each axis obtained by dynamic calculation, divided by the reduction ratio of the reducer, and then taking into account the efficiency of the transmission chain, such as the efficiency of the shaft, the efficiency of the bearings and the efficiency of the gears, and considering the speed required for each axis (the motion coupling factor must also be taken into account), the electric motor can be selected.

When selecting, please note that the speed of the AC servo motor is adjustable, and the torque output by the motor is constant within a fairly large speed range. Therefore, when selecting a motor, it is sufficient as long as the rated speed of the motor is greater than the maximum speed required by each axis.

At the same time, attention should be paid to the selection of the position to be configured together with the AC servo motor, and indicate whether the motor needs to be equipped with a brake, etc.

(7) Selection of reducers: Common reducers used on robots include and.

RV reducer has the advantages of long-term use without lubricant, long life, good rigidity, large reduction ratio, low vibration, high precision, convenient maintenance, etc. It is suitable for use on robots. Its transmission efficiency is 0.8, which is very high compared with the gear set with the same reduction ratio.

Its disadvantages are heavy weight and large size.

The advantages of harmonic reducers are light weight, small size, wide reduction ratio range and high precision.

In robot design, RV reducers are generally used for axes 1, 2, and 3, and harmonic reducers are commonly used for axes 4, 5, and 6.

(8) Robot arm calibration The robot arm must be checked for strength and stiffness. While meeting the strength and stiffness requirements, the arm should be made of light materials as much as possible to reduce the moment of inertia and reduce the pressure on the balancing mechanism.

(9) Calibration of mechanical parts

1) Bearing verification: All important bearings used in the design must undergo strength verification. Under the condition of meeting the size and strength requirements, domestic bearings should be selected as much as possible to reduce the cost of the robot.

2) Shaft verification: All important shafts used in the design must undergo strength verification and stiffness verification.

3) Gear selection: All gears used in the design must undergo strength verification.

4) Keys and splines: All important keys and splines used in the design must undergo strength verification.

5) Pins and screws: All important pins and screws used in the design must undergo strength check.

(10) Mechanical processing technology considerations The design of the verification robot should fully consider the convenience of processing and assembly, and should be easy to maintain and adjust.