Servos

Unless very radical systems are being used, almost all UAVs rely on flaps or articulating surfaces for pitch, roll, and yaw control. These are activated via servos that are readily available in a bewildering range of shapes, sizes, and torques. Prices vary from a few dollars to hundreds

A selection of aircraft servos from three different manufacturers

Figure 6.8 A selection of aircraft servos from three different manufacturers.

per servo: typical examples of the sort we use are shown in Figure 6.8, see also Table 6.2. We tend to always specify digital as opposed to analog servos, even though these are more expensive because of the more precise control they offer. http://www.servodatabase.com/ servos/all provides a comprehensive database of servos with information on sizes, types, torques, typical costs, and so on, currently listing nearly 2500 different options! Figure 6.9 shows how servo torque typically varies with servo weight. Regression of this data suggests that weight may be deduced from the required torque using w(oz.) = 0.0489Г(oz.in.)0 7562.

If compact, high-torque and power, metal-geared devices are used, large surfaces can be reliably controlled by each one. If digital position control feedback servos are used, they can be relied on to very accurately hold control surfaces at demanded angles provided sufficient

Table 6.2 Typical servo properties.

Type

Make

Depth

(mm)

Width

(mm)

Height

(mm)

Weight incl. horn and fixings (g)

Metal

Hitec

30

46

60

100

Metal SHT

MKS

22

40

40

79

Metal HT

Savox

22

40

40

67

Metal

Hitec

22

40

40

58

Metal

MG

22

40

40

57

Plastic

Futaba

20

40

35

45

Metal

MKS

10

30

35

34

Metal

Blue Bird

10

30

35

33

HT, high torque; SHT, super-high torque.

Variation of servo torque with weight for various manuafcturers’ servos

Figure 6.9 Variation of servo torque with weight for various manuafcturers’ servos.

electric power is available. If, however, two servos are connected to a single surface, issues can arise when they are not completely aligned to each other, leading to the servos fighting and very heavy current flows. To avoid this, it is best to ensure that some form of compliance is designed into the system, either by making the flap slightly flexible in twist or by using the flexible rubber servo mounts typically supplied with the units when purchased. In practice, we strive to avoid ever having multiple servos on a single control since this does not in any case provide redundancy. A jammed servo will prevent a surface moving even if its mate is still working reliably. Instead, if redundancy is important, we typically divide up the control surfaces, with each part having its own dedicated servo. Thus, on the SPOTTER aircraft we use four ailerons and four elevators, each with a dedicated servo, see Figure 6.10.

The servos used to control moving aerodynamic surfaces need to be solidly attached to the airframe, as very significant loads can be experienced during flight. This means bolting down the servo body itself to a suitable set of hard points, see Figure 6.11, and also attaching the (sufficiently stiff) actuating linkage securely at the other end. We typically use two-part selective laser sintering (SLS) nylon servo mounting boxes to house our servos. The base of the box is glued into the wing with epoxy, and after the servo is fitted, a cover plate is screwed over the servo just leaving the servo arm exposed. The actuating linkage joined to the servo arm is attached to what is commonly termed a servo horn - an element that sticks out from the surface of the part being moved, see Figure 6.12. The servo horn also needs to be attached to a suitable hard point. Since our control surfaces are typically made of foam, we fit load-spreader plates between the foam and the horn, also seen in Figure 6.12. It is also possible to place the servo in the moving element and attach the horn to the main airframe: we sometimes do this when we are using all moving flying surfaces, for example.

Kinematically, servos are normally configured in what are known as four-bar chains: the servo body and the structure it is bolted to forms the first bar (often called the ground link), the

SPOTTER aircraft showing multiple redundant ailerons and elevators

Figure 6.10 SPOTTER aircraft showing multiple redundant ailerons and elevators.

Servo cut-out in wing with SLS nylon reinforcement box

Figure 6.11 Servo cut-out in wing with SLS nylon reinforcement box.

rotating servo arm the second, the (adjustable-length metal) linkage the third, and the servo horn and moving surface the fourth. For such mechanisms to work well, the linkage needs ideally to form a rectangle with 90° corners in the neutral position. Moreover, it is good practice for the horn not to be mounted too far from the hinge line to avoid unnecessary compliance in the linkage, which can lead to control instabilities. The link between the two arms must also be sufficiently stiff to prevent buckling: on 20-40 kg aircraft, we use 3 mm diameter steel links, for example. Care must be taken to ensure that there are no clashes between moving elements, such as where rudder flaps and elevators are close to each other. It is also good practice to

Typical servo linkage. Note the servo arm, linkage, and servo horn (with reinforcing pad)

Figure 6.12 Typical servo linkage. Note the servo arm, linkage, and servo horn (with reinforcing pad).

avoid forcing servo arms into position by twisting them (and the internal servo mechanism) manually: rather, it is better to position them electrically. It is also critically important that before being powered up, care is taken to ensure that the servo will not try and adopt a position that cannot be reached because of the linkages attached. If this is not done, it is very easy to stall a servo, draw large currents, overheat it, and ultimately burn out the internal wiring. On high-power servos, this can happen in just a few seconds, resulting in expensive damage. We recommend that the servo arm or linkage should be fitted after the servo has been powered, placed in its neutral position, and then shut down again. Then, once the linkage has been attached, attention can be paid to trimming the servo end positions to match the mechanism design and control system settings (Ideally, the initial mechanical design of the linkage will allow the servo to move through the bulk of its range of movement while the control surface does likewise. It is poor practice for a servo to only operate over a fraction of its operational range while the controlled surface moves through its entire sweep. Clearly, the reverse situation is even worse where the linkage design does not permit full control surface movement.)

 
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