LECTURE №9

DRIVES AND MECHANISMS. STEPPER MOTORS AND SERVO MOTORS

9.1. STEPPER MOTOR AND SERVOMOTOR

A stepper motor is a pulse-driven motor that changes the angular position of the rotor in steps. Due to this nature of a stepper motor, it is widely used in low cost, open loop position control systems.

Servomotors are special electromechanical devices that produce precise degrees of rotation. A servo motor is a DC or AC or brushless DC motor combined with a position sensing device. Servomotors are also called control motors as they are involved in controlling a mechanical system. The servomotors are used in a closed-loop servo system as shown in Figure 9.1.1. A reference input is sent to the servo amplifier, which controls the speed of the servomotor. A feedback device is mounted on the machine, which is either an encoder or resolver. This device changes mechanical motion into electrical signals and is used as a feedback. This feedback is sent to the error detector, which compares the actual operation with that of the reference input. If there is an error, that error is fed directly to the amplifier, which will be used to make necessary corrections in control action. In many servo systems, both velocity and position are monitored. Servomotors provide accurate speed, torque, and have ability of direction control.

Fig. 9.1.1. Servo system block diagram

DC servomotors

DC operated servomotors are usually respond to error signal abruptly and accelerate the load quickly. A DC servo motor is actually an assembly of four separate components, namely:

· DC motor;

· gear assembly;

· position-sensing device;

· control circuit.

AC servo motor

In this type of motor, the magnetic force is generated by a permanent magnet and current which further produce the torque. It has no brushes so there is little noise/vibration. This motor provides high precision control with the help of high resolution encoder. The stator is composed of a core and a winding. The rotor part comprises of shaft, rotor core and a permanent magnet. Digital encoder can be of optical or magnetic type. It gives digital signals, which are in proportion of rotation of the shaft. The details about optical encoder have already discussed in Lecture 3.

Аdvantages of servo motors:

· provides high intermittent torque, high torque to inertia ratio, and high speeds;

· work well for velocity control;

· available in all sizes;

· quiet in operation;

· smoother rotation at lower speeds.

Disadvantages of servo motors:

Ø more expensive than stepper motors;

Ø require tuning of control loop parameters;

Ø not suitable for hazardous environments or in vacuum;

Ø excessive current can result in partial demagnetization of dc type servo motor.

9.2. CAMS. CLASSIFICATION OF CAMS

Cams are mechanical devices which are used to generate curvilinear or irregular motion of mechanical elements. They are used to convert rotary motion into oscillatory motion or oscillatory motion into rotary motion. There are two links namely the cam itself which acts as an input member. The other link that acts as an output member is called the follower. The cam transmits the motion to the follower by direct contact. In a cam-follower pair, the cam usually rotates while the follower translates or oscillates. Complicated output motions which are otherwise difficult to achieve can easily be produced with the help of cams. Cams are widely used in internal combustion engines, machine tools, printing control mechanisms, textile weaving industries, automated machines etc.

Necessary elements of a cam mechanism are shown in Figure 9.2.1.

· a driver member known as the cam;

· a driven member called the follower;

· a frame which supports the cam and guides the follower.

Fig. 9.2.1. Cam mechanism

Classification of cams

Wedge and Flat Cams

A wedge cam has a wedge of specified contour and has translational motion. The follower can either translate or oscillate. A spring is used to maintain the contact between the cam and the follower. Figure 9.2.2 shows the typical arrangement of Wedge cam.

Fig. 9.2.2. Wedge cam

Plate cam

In this type of cams, the follower moves in a radial direction from the centre of rotation of the cam (Figure 9.2.3). They are also known as radial or disc cam. The follower reciprocates or oscillates in a plane normal to the cam axis. Plate cams are very popular due to their simplicity and compactness.

Fig. 9.2.3. Plate cam

Cylindrical cam

Here a cylinder has a circumferential contour cut in the surface and the cam rotates about its axis (Figure 9.2.4). The follower motion is either oscillating or reciprocating type. These cams are also called drum or barrel cams.

Fig. 9.2.4. Cylindrical cam

Classification of followers

Followers can be classified based on:

· type of surface contact between cam and follower;

· type of follower motion;

· line of motion of followers.

Classification based on type of surface contact between cam and follower

Figure 9.2.5 shows the schematics of various types of followers used cam mechanisms.

Fig. 9.2.5. Types of follower based on the surface in contact

Knife edge follower

The contacting end of the follower has a sharp knife edge. A sliding motion exists between the contacting cam and follower surfaces. It is rarely used in practice because the small area of contacting surface results in excessive wear.

Roller follower

It consists of a cylindrical roller which rolls on cam surface. Because of the rolling motion between the contacting surfaces, the rate of wear is reduced in comparison with Knife edge follower. The roller followers are extensively used where more space is available such as gas and oil engines.

Flat face follower

The follower face is perfectly flat. It experiences a side thrust due to the friction between contact surfaces of follower and cam.

Spherical face follower

The contacting end of the follower is of spherical shape which overcomes the drawback of side thrust as experiences by flat face follower.

9.3. CLASSIFICATION BASED ON FOLLOWERS’ MOTION

Figure 9.3.1. shows the types of cams based followers’ motion.

Fig. 9.3.1. Classification of follower based on motion

Oscillating follower

In this configuration, the rotary motion of the cam is converted into predetermined oscillatory motion of the follower as shown in Figure 9.3.1 a).

Translating follower

These are also called as reciprocating follower. The follower reciprocates in the «guide» as the cam rotates uniformly as shown in Figure 9.3.1 b).

Applications of cams

Cams are widely used in automation of machinery, gear cutting machines, screw machines, printing press, textile industries, automobile engine valves, tool changers of machine centers, conveyors, pallet changers, sliding fork in wearhouses etc.

Fig. 9.3.2. Cam in I.C engine

Cams are also used in I.C engines to operate the inlet valves and exhaust valves.The cam shaft rotates by using prime moveres. It causes the rotation of cam. This rotation produces translatory motion of tappet against the spring. This translatory motion is used to open or close the valve. The schematic of this operation is shown in Figure 9.3.2.

Cams in automatic lathes

The cam shaft is driven by a motor. The cutting tool mounted on the transverse slide travels to desired depth and at desired feed rate by a set of plate cams mounted on the cam shaft. The bar feeding through headstock at desired feed rate is carried out by a set of plate cams mounted on the camshaft.

Automatic copying machine

The cam profile can be transferred onto the work piece by using a roller follower as shown in Figure 9.3.3. The follower can be mounted with a cutting tool. As the cam traverses, the roller follows the cam profile. The required feature can be copied onto the workpiece by the movement of follower over the cam profile.

Fig. 9.3.3. Automatic copying of cam profile