Control Dynamics
- 1. it’s not rocket science ERGONOMICS & STYLING steven casey 18 iVT International 2006 Having the right control for the job is key to getting operators to perform to the best possible standard. In extreme cases, it can even mean the difference between life and death
- 2. TECHNOLOGY 19iVT International 2006 D uring the evening of 29 June 1971, after spending 23 days aboard the world’s first space station, three Soviet cosmonauts prepared to enter Earth’s atmosphere aboard their Soyuz 11 spacecraft. The mission had been a resounding success and each man was prepared to return to a hero’s welcome on Russian soil. The re-entry operation was highly automated. Soyuz obtained the proper re-entry position over Earth and oriented herself so the cosmonauts faced backwards, away from the direction of travel. The retro rockets on the command module ignited, sending a shudder through the small spacecraft. In less than 30 minutes they would decelerate from more than 27,000km per hour to a complete stop. Minutes later the computers on board Soyuz, as programmed, shut down the retro rockets. The cabin fell silent. The instrument assembly module, the command module with the three cosmonauts and the orbital module fell freely toward the atmosphere. Everything was going as planned, and seconds later, the computers sent a signal to the explosive bolts connecting the orbital module to the command module. This separated the two modules so that the command module and three cosmonauts could return safely to Earth. Water vapour and dust particles suddenly appeared out of nowhere, suspended in the cabin, and the spacecraft began to spin unexpectedly. Flight computers, sensing that the craft was no longer properly oriented, instructed the positioning rockets to fire and keep the rear of the craft aimed toward the atmosphere below. The suddenly rotating attitude display on the console directly in front of the cosmonauts was the most obvious clue that something had gone very wrong. They quickly realised that they were depressurising and their spacecraft’s atmosphere was jetting out into space. The separation sequence had jarred open a cabin exhaust valve designed to open only after their parachute had been deployed and they were close to the ground. A manual control handle well over their heads could be turned to close the exhaust valve, but it had to be operated immediately if they were to re-establish control over their atmosphere. They had less than 60 seconds of consciousness in their rapidly evaporating atmosphere, 60 seconds in which to act. Wiggling frantically from his restrained position in his seat, one of the cosmonauts reached up over his head and turned the slow-acting valve to stop the loss of air from the cabin. The key initial failure had been mechanical in origin, but perhaps the ERGONOMICS & STYLING
- 3. most significant contributing factor would be with the design and operation of the control, its placement and the specific conditions in which it had to be operated. These human factors would determine the successful – or unsuccessful – outcome of the event. A tense landing Ground crews sat in helicopters, poised to take off to rendezvous with the spacecraft. As the Soyuz entered the uppermost layers of the atmosphere, they could see a bright yellow scratch in the dark morning sky above. The heat shield was aglow from the friction of the air. The tension grew among the helicopter crews. They made their last chute deployed, the automatic sequencer on board jettisoned the heat shield, exposing the solid fuel landing rockets. The spacecraft and its parachute were easy to see on the helicopters’ radars. Soyuz 11 floated silently toward the ground from high altitude, far above the noise and commotion. Soyuz continued her descent and the helicopters raced toward the point of landing, which was right on target. The landing rockets fired 2m above the ground, making a bright flash in the darkness and kicking up a large cloud of dust. The capsule settled to the ground, rolled onto its side and the enormous parachute collapsed onto the dirt. The helicopters landed and trained landing teams jumped out and ran toward the capsule. Designated crews took their positions near and around the capsule, while others radioed that the landing was successful. Reclining chairs were sat out next to the capsule for the three cosmonauts. They would not be expected or allowed to walk after such a long stay in zero-gravity. Ground personnel released the locking mechanism and swung open the heavy hatch. There was no movement inside the capsule. The cosmonauts were dead. Investigators would later determine that one of the cosmonauts had managed to close the valve only halfway before losing consciousness. The valve was intended to be used in just this type of emergency, yet it would have taken another minute of rapid turning of the handle for it to be Simple input dials and complex multicontrollers are not mutually exclusive A pedal worth its mettle The strain on drivers’ feet and legs has been reduced after the introduction of a double pedal in Kalmar’s hydrostatic lift-trucks. “When the right pedal is depressed, the left is raised a few millimetres above its neutral position and is locked, making a convenient support for the resting foot,” reveals design manager, Conny Christensen. “To brake, you release the active pedal upwards. The resting pedal is activated when depressed, making the opposite pedal the foot support.” The functional requirement was for the system to provide a maximum elevation of 5mm from neutral position and prevent simultaneous depression of both pedals – this was resolved with a swinging arm controlled by the active pedal locking the inactive one. Ergonomically, finding the right amount of depression force was vital. Extra springs were set in various positions, enabling the pedal force to be varied in accordance with preferences. The depression angles were selected on the basis of the required resolution of the signals from the potentiometers. The angle and position of the vertically adjustable pedal plate in relation to the pedal arm can be altered to suit, using a lever that enables the heel level to be set to various heights. ERGONOMICS & STYLING 21iVT International 2006 flight preparations and began to take off to meet the spacecraft when it landed. As it was dark, the ground crews could not see the final stages of landing, but everyone at the scene knew the sequence well. At an altitude of 9km, a single drogue chute deployed automatically, followed by the single main chute at 8km. With the main
- 4. completely closed and for the Soyuz to be airtight. The ‘two-minute valve’ could not be operated within the expected one minute of consciousness. The precise conditions under which the control would be used had not been considered during design and construction. This true story illustrates in a brutal, but memorable, way how designers’ decisions can determine the performance of users. It is often said that the practice of ergonomics – or human factors engineering – involves fitting the task to the man. A logical extension of this adage, at least in terms of designing hardware for human use, is the importance of fitting the control to the task, something that did not occur on Soyuz. Principles for dynamic controls Although many engineers and designers are familiar with traditional ergonomic concerns, such as anthropometry, hand and foot size and control actuation effort, many are not aware of the full realm of user-interface issues that must be considered when designing or selecting controls for specific tasks. With regard to dynamic hand and foot controls (as opposed to controls with simple on/off functions), there are six principles that should be considered. Firstly, the basic nature of the task should dictate the type of control. Some tasks require precision adjustment, some require rapid adjustment and some need a mixture of both. As with the spacecraft’s valve handle design and placement, careful consideration should be given to selecting the correct control for the task and the conditions in which the task will be performed. Secondly, the required precision of adjustment is critical. The greatest precision can be achieved through use of the wrist, hand and fingers. The greater the need for precision in the operator’s response, the greater the need for the hand and fingers. The forearm provides a mix of strength and precision. Feet and legs, as a rule, sacrifice some precision for strength. Where necessary, the designer should select or create a control that enables the operator to make accurate inputs. If smooth, modulated changes in a variable (such as flow) are required, then the control should allow the operator to make such inputs. Underlying physical components of controls can also be very important here. For example, consider the differences between controlling flow within a pipe when using a butterfly valve and gate valve. The butterfly valve, which is often best for rapid on/off functions, can be difficult to open ‘just a little’. Operators can find it difficult to achieve low flow rates on certain butterfly valves, especially when flow rates and pressures are high. A gated valve with proper gain between the hand control and movement of the valve enables the operator to achieve low flow rates. Conversely, a fast-acting butterfly valve may be more appropriate when the response speed is more important than the accuracy. A third consideration is that the control and display must be compatible. There are population stereotypes that govern the relationships between movements of the control and the display. For instance, most people expect engine speed to increase when a hand throttle is pushed forward. These population stereotypes can be different for cultures and product markets. The fourth principle is that the level of gain should be appropriate for the task. Gain is the relationship between the movement or force applied to a control and the corresponding 22 iVT International 2006 ERGONOMICS & STYLING Six principles should be considered when designing dynamic hand and foot controls
- 5. response of the system. Most industrial vehicle operators have driven vehicles with brakes that are either under- or oversensitive. Selecting the appropriate level of gain can shorten stopping distances and reduce the likelihood of locking the wheels. In general, the optimal amount of gain exists when the desired level of system response is achieved by quick movement of the control near the desired position, followed by a relatively quick final adjustment. Number five, the control’s level of resistance has to be suitable. Control resistance is a science in itself. Simple friction is constant resistance, which must be overcome before a control can be moved. Its primary purpose is avoiding accidental activation of the control and, sometimes, enhancing comfort. Automotive accelerator pedals, for example, have an initial friction resistance of roughly 4 lb to counter the weight of the foot resting on the floor and pedal. Pedal resistance must often be higher in industrial vehicles where the operator’s seated position is more upright, when vibration is an issue, and where footwear may be heavier. Spring resistance is often best suited for accurate positioning of a control. The further the control is moved, the greater the resistance. Subsequently, the operator is provided with constructive feedback on the position of the control. Inertial resistance is generally best suited to achieving an accurate constant input. By maintaining a constant pressure on the control, the operator can maintain a constant output, such as rate of acceleration. Viscous friction is present when the rate of movement is proportional to the force exerted by the operator. This type of friction feels like moving a paddle through water. The harder you push, the more resistance you encounter. Viscous friction is usually disadvantageous to tracking tasks or use of a joystick, but it can be used effectively to avoid large accidental movements of a control. Last, but by no means least, response lag should be minimised or eliminated. Human operators, unless they are highly experienced and skilled, have great difficulties dealing with response lags. Delays between the input to a control and the system’s response can have serious detrimental effects on operator and system performance. They can also increase the time an operator requires to learn how to operate a system. Input/ response delays of even a second or less can create control instabilities that most operators cannot correct. Operators can adapt – but only so much Humans have an unequalled ability to learn and adjust. As a consequence, we adapt to poorly designed control systems by increasing our level of skill and knowledge. As a general rule, however, if a particular machine or interface requires a great deal of time to learn to operate well, it is often an indication that the user interface is in need of improvement. Keeping in mind these human factors and characteristics when designing or selecting dynamic hand and foot controls will result in equipment that can be operated as intended and at the desired performance level. iVT References Casey, S. (1998). ‘Set Phasers on Stun’ and Other True Tales of Design, Technology, and Human Error (2nd edition). Santa Barbara, California. Aegean Publishing Company Casey, S. (2006). ‘The Atomic Chef’ and Other True Tales of Design, Technology, and Human Error. Santa Barbara, California. Aegean Publishing Company Making the skills transition simple Industrial vehicles are constantly evolving, and as operational requirements change, so do the technologies that enable them to do so. In order to cause minimum disruption (and reduce the risk of error), it is therefore imperative that operators are able to make the skills transition without extensive retraining or reorientation. Penny + Giles has been providing joysticks for OEMs of industrial vehicles for more than 20 years and believes that the key to the successful integration (and use) of its joysticks is involving the operator in the design process. In a recent project for a manufacturer of ORLs (Off-Road Loaders), the company was asked to put forward a variety of joystick options for review. Instead of lighter, smaller controllers, the professional operators – who were involved in the design process by the OEM – chose a larger joystick design that would simulate the mechanical feel associated with a hydraulic system. So although the operation was based on a new electrohydraulic system, which offers improved reliability and efficiency, the joystick remained relatively unchanged in its mechanical feel. As well as minimising the time required for the operators to familiarise themselves with the new controller, it also reduced the time from design to integration – and therefore reduced production costs for the OEM. As for the future, operator involvement will become increasingly important as generations of operators evolve. The assumption will undoubtedly be that new operators (who are likely to have had experience with games consoles, mobile phones and entertainment systems) will prefer the lighter smaller controllers. But only time will tell, and only by including them in the design process will the vehicle manufacturer – and its suppliers – be assured that the best control solution for any given application is chosen. 23iVT International 2006 ERGONOMICS & STYLING