These are suited to continuous control where there are high demands for speed. Cranks can be used for both fine and coarse control, depending on the degree of gearing selected.
The following recommendations apply to cranks:
1. Cranks are preferable to wheels where two or more revolutions are to be made.
2. For small cranks less than 8 cm in radius, the resistance should be at least 9 N and a maximum of 22 N when rapid movement is required.
3. Large cranks of 12.5 to 20 cm radius should have a resistance between 22 and 45 N.
4. Large cranks should be used when precision is required (accuracy between a half to one revolution), with the resistance between 10 and 35 N.
5. The handle should have a high surface friction to prevent the hand from slipping.
Wheels are used for two-handed operations. Identification of the position is very important if the wheel can be rotated through several revolutions. In addition, the following recommendations apply:
1. The turning angle should not exceed ±60 degrees from the zero position.
2. The diameter of the ring forming the outside of the wheel should be between 18 and 50 mm, and should increase as the size of the wheel increases.
3. The wheel should have a high surface friction so that the hand does not slip.
Table 5.4 shows the relative advantages of different forms of control devices for computerised process systems for four common tasks.