Electricity + Control July 2017

DRIVES, MOTORS + SWITCHGEAR

Matching motors and linear drive systems Previous articles have touched on this subject in an application specific way. A more systematic gener- al approach follows. Velocity, torque, power and inertia These quantities define servo size and perfor- mance. The speed reducer provides the interface to the driven load and ensures that the correct mo- tor size has been chosen. Velocity Maximum velocity of a motor is generally in the range of 2 000 – 4 000 RPM (Revolution Per Min- ute). In the case of stepper motors , maximum usa- ble velocity is around 600 RPM.This is because de- livered torque decreases with increased velocity. Torque Brushless servos have substantially constant torque throughout their velocity range. Brushed servos are not capable of simultaneous top speed and maxi- mum torque. This constraint is due to high wear of the brush gear. Both of the foregoing motors are capable of much greater peak torque for short periods. This can be an advantage when the load only requires high torque during acceleration or deceleration. Resultant heating sets a time limit. Peak torque in stepper motors should be re- stricted to 60% of available torque. This is due to the possibility of abrupt de-synchronisation of the motor and consequent loss of commanded po- sition. This is offset by the inherent much higher torque delivery at speeds below 600 RPM, com- pared to a servo motor. Power Power is the product of velocity and torque . A gearbox cannot increase motor power delivery. In contrast to conventional wisdom, power is the last parameter to be considered when sizing a system. Inertia Inertia is defined as the resistance of a body to any change in its state of motion. The force required to accelerate a body can be calculated from the body mass and the required acceleration.

Figure 6: Harmonic gearbox.

There are three parts: • A cup shaped flexible spline with external teeth which drives the auto output shaft • A ring gear • A driven wave generator As the input shaft rotates, the wave generator ec- centric action forces a portion of the spline into mesh with the ring gear. Motion is imparted be- cause the spline typically has two less teeth than the ring gear. Each turn of the wave generator moves the spline two teeth relative to the ring gear. This process is analogous to a Vernier scale where only one mark can line up at a time. Output torque is high due to the relatively large number of meshed teeth, and backlash is minimal. This all comes at the expense of relativity high friction and the need for special lubricants. Figure 7 shows a slewing ring which usually applied together with the harmonic gearbox. The slewing ring imparts high rigidity in a compact space and is seen, almost without exception, in the joints of robotic articulated arm robots.

Figure 7: Slewing ring.

12 Electricity + Control

JULY 2017