Annotated task T11 – active suspension

The initial problem specification or design brief given to the designer for this particular problem reads as follows. Suspension and shock absorption are crucial elements in the car design, particularly with regard to the passengers’ comfort. Design an active shock absorber and suggest a control strategy, so that the whole suspension mechanism would be able to adjust its properties according to the current state of the road. In addition to the absorption of shocks caused by an uneven road, we would like to adjust also the chassis clearance, so that the car has the optimal aerodynamic coefficients for a particular type of surface.

Design episode 8.2a

Initialisation of frame DC-I using similar conceptual frames

Design context: DC-I Justifying threads: J-489 – J-494

The designer received a design brief as shown above and began with an initial task analysis that was based on his knowledge of existing approaches to car suspension. First, he expressed the term ‘active suspension’ using three tenets in a language similar to that of the initial problem specification {J-490}.

Defining and clarifying the terms and customer’s requirements, he also introduced certain criteria to evaluate whether the desired objectives have been met.

In the justifying threads appears an issue of what can be considered ‘active’ when referring to car suspension. These notions were important because they were recorded allowing the follow up of any conclusions reached at the end of the initial stage {J-491 to J-494}. The initial analysis focused on two features that can be both ‘active’ and ‘passive’ – chassis clearance and shock absorption. As records numbered J-492 and J-493 show, the designer framed the problem of ‘active’ suspension in terms of a modifiable shock absorption and constant chassis clearance {J-491}. The initial frame led to the formal expression of the goals as listed in DC-II below.

Design episode 8.2b

Development of DC-I and the emergence of basic concepts

Design context: DC-I Justifying threads: J-495 – J-502

The initial framing of the design problem interpreting the requirements from the design brief led to an articulation of an ‘ideal’ solution {see notes J-495 and J-496}. The ideal solution was formulated as a suspension system whereby a car driver would not feel or observe any unevenness of the road surface.

This ideal state was further translated to the formal language of design as witnessed in J-497 to J-502.

At this stage, the designer defined basic conceptual objects of the respective design frame that he intended to use for a solution construction. For instance, a decision has been articulated that proposed using a softer suspension for uneven surfaces and a tougher one for smooth roads {J-497}. Another concept resulting from the initial framing was a monitor of the surface unevenness and a processing unit that would modify the shock absorber’s properties accordingly {J-498}. In addition to an automated response, the processing unit was required to respond to the explicit demands of the driver, hence the concept of ‘manual adjustment’ {J-499}.

Although the initial frame focused on the constant chassis clearance, the designer also articulated basic conceptual terms for adjusting the clearance. However, in comparison to the conceptual terms defined for the shock absorption, these were less precise {J-500 to J-502}.

Figure 8–2. Behaviour of a prototypic shock absorber Design episode 8.2c

Retrieval of applicable components and the move towards the first principled sketches Design context: DC-I (+ Figure 8–2) Justifying threads: J-503 – J-507

At this stage, the designer expressed the concepts identified in the initial frame in a sketch as depicted in Figure 8–2. The sketch showed how an absorber reacted to an uneven surface, and it helped to introduce more conceptual objects to the attended design frame. For example, a displacement of the wheel against the chassis was noted as a typical reaction to the car entering an uneven surface (∆^l).

This observation was also noted down as records J-503 and refined in J-504. A new concept emerging from the sketch re-defined a generic concept of ‘monitoring unevenness’ {J-498}. The new method of monitoring comprised measuring the displacement of the wheel axle against the chassis.

Another concept that emerged in the sketch was a helical spring acting as a familiar and simple shock absorber prototype. However, despite the familiarity of the spring, it was not suitable for designing a sound solution. According to J-498 and also J-504, the shock absorber’s toughness had to be modified in response to an immediate state of the road surface. A spring as a mechanical component did not exhibit the desired flexibility and simplicity for this.

Nevertheless, the spring as a prototype was instrumental in articulating what exactly had to be amended in order to change the shock absorption properties of a device. The characteristic the designer became aware of after using a spring as a prototype was ‘toughness constant K’ and its influence on the quick shock elimination (see also the next episode).

Design episode 8.2d

A new frame emerges ‘informally’

Design context: DC-II (+ Figure 8–3) Justifying threads: J-508 – J-513

As already mentioned in the episode 8.2c, a new requirement has been articulated in terms of amending the toughness constant K. Since with a spring, the constant K depended on its material (e.g. steel), the designer looked for other materials, where this constant could be more easily modified. A frame containing similar conceptual behaviours as the current one but different conceptual objects was brought forward in the form of pneumatic/hydraulic devices – pistons.

A simple pneumatic piston-based shock absorber was sketched (see Figure 8–3) to replace the old spring-based prototype (shown in Figure 8–2). The shock absorption properties of the piston depended on the pressure of the substance inside (e.g. air). Since air pressure was easily varied by pumping in more air or releasing some from the piston, the designer had a sound device for achieving the objectives recorded informally in the frame DC-I. Next, he recorded a new explicit frame DC-II.

Figure 8–3. New prototypic device (a), and the first control loop – solution (b) EXPLICIT ACHIEVEMENT 8.2/DC-II

First explicit context with a solution is formally recorded (DC-II)

Let us recapitulate what has been achieved so far. First, the designer articulated the initial specification of the problem in the following terms (see records R-1 to R6 recorded within DC-II):

– the system should be able to absorb shocks {R-1}, and it must do so automatically depending on the current surface {R-1.1}

– the system should be able to adjust the chassis clearance {R-2} but for simplicity we assume a constant clearance whilst driving {R-2.2}

– the system should be able to measure the displacement of the chassis against the road and/or wheels against the chassis {R-3, R-4}

– the system should be able to adjust dynamically the shock absorber’s properties, especially its toughness constant K {R-5}

– the system should contain a processing unit able to calculate a suitable reaction to a particular value of the wheel/chassis displacement and accordingly amend the constant K {R-6}

The concepts for constructing a solution have been discussed in the design episodes 8.2a to 8.2d so they shall not be repeated here. However, the first milestone was accomplished by articulating the solution S-1 (see also note J-514). The first solution in the form of a rule-based control loop is presented in Figure 8–3 sketch B, and its behaviour is discussed in the thread starting with J-505.

Design episode 8.2e

Proposal of an exploratory probe in a new frame – re-addressing the concept of ‘activity’

Design context: DC-II (solution S-1) Justifying threads: J-514 – J-517

From the notes following the articulation of solution S-1 {records J-515 to 517}, there was a visible emergence that the first iteration was not entirely acceptable in respect to the ‘ideal state’ {J-496 earlier}. The designer’s comment J-515 suggested that the reason for not accepting S-1 as a

fully-a

b

IF ‘∆l > 10mm’ & ‘∆vib > 5mm’

THEN ‘decrease ∆p by 0.5kPa’

IF ‘∆l > 20mm’ & ‘∆vib > 10mm’

THEN ‘decrease ∆p by 0.8kPa’

etc.

Rule-based controller

wheel deflection ∆l pressure adjust ∆p

vibration ∆vib

fledged solution, might be in its inflexibility. It took into account only two inputs (displacements of wheels and chassis), and based on these, the air pressure was reduced to achieve a softer suspension.

Also, the system was not able to perform a reversed action, i.e. returning to a higher pressure. The designer became aware of the fact that not only an uneven surface might have an adverse impact on the driver’s comfort but so would a too soft suspension; the car would start ‘oscillating’. Therefore, he proposed addressing this newly discovered requirement by amending and extending the simple control algorithm from context 8.2/DC-II {see also J-516 and J-517}.

The amended algorithm took into account the history of measurements done on the wheels and the chassis, so that when the initial shock has been eliminated, it starts increasing the pressure. The designer named this strategy as ‘try & see’ – which we believe underlines its heuristic nature. He also formulated a few new concepts that could be useful in the prediction of the type of ‘shock’ (e.g. a car entering an uneven road having a different ordering of events as a car entering a smooth road).

Design episode 8.2f

Another probe – re-addressing the chassis clearance (initial assumption of its constancy) Design context: DC-II (with solution S-1) Justifying threads: J-518 – J-527

In a similar manner to that described in design episode 8.2e, the designer was not satisfied with the restriction of constant chassis clearance. He claimed it was helpful to produce the initial solution, but it negated his understanding and definition of ‘active suspension’. Thus, he addressed this inconsistency in the threaded records starting with J-518 and ending with J-527.

The designer opted for a familiar prototype for controlling a particular variable (in this case chassis clearance). First, he established a referential value (for a stationary, empty vehicle). The displacement was measured after loading, and the individual wheels were appropriately balanced. Then a table was drawn up associating the measured shock on the chassis with the concept of ‘terrain types’. To each terrain, a recommended chassis clearance was assigned, which was to be maintained by a controller.

New requirements (or re-specified and re-addressed requirements) triggered an articulation of new conceptual objects from which sound solutions could be derived. One such new concept included a kind of ‘screw-jack’ (see sketch in Figure 8–4) that could be combined with the pneumatic piston known in the previous frame; this would implement an ‘active suspension’ {J-521}.

Design episode 8.2g

Exploratory probes accepted, move to a new context DC-III (incl. new concepts for implementation) Design context: DC-II, DC-III Justifying threads: J-528 – J-537

Before implementing the two major extensions to the context DC-II, the designer also refined his knowledge of the concepts that were already known in that context. He focused on the articulation of physical and engineering principles behind the respective functional requirements and on refining abstract concepts (see design episodes 8.2b to 8.2d). The brevity of the records and the selected concepts reflect the designer’s decision to use the existing modules for these purposes if possible.

Thus, he proposed measuring chassis vibration and its displacement against the road surface using a technique similar to that for activating airbags {J-528, J-529}. For measuring the displacement of a

piston from some initial position, he proposed a resistance-based measurement. Also, potentiometer was the selected means for measuring even diminutive changes in incremental displacement {J-530 and J-531}. Records J-532 and J-533 re-iterate the pressure modification in the pistons; the designer added an alternative of using oil instead of air (thus broadening previous frame and introducing the hydraulic components). Figure 8–4 shows the parameters measured on the system labelled as ‘IN’.

In addition to refining the ‘old’ concepts (from DC-II), the designer proposed the same level of conceptual details for the newly-emerged concept of the chassis clearance amendment. In this case, the prototypic incremental motors drove the screw-jack and move it up or down {J-536, J-537}. These motors are commonly used in controlling motion in the engineering.

Figure 8–4. Extended candidate solution with new concepts and details EXPLICIT ACHIEVEMENT 8.2/DC-III

Solution significantly extended and refined (DC-III)

As it is possible to observe in the explicit problem specification that served as a basis for this context, two statements appear refining those already formulated in context 8.2/DC-II. In addition, two new requirements were added. Below is the list of both refinements and extensions:

– the system should be able to respond to both an immediate state of the road surface and explicit demand of the driver {R-1.3} Æ refines {R-1, R-1.1}

– the system should adjust the clearance of the chassis when safety conditions allow (e.g. vehicle is stationary, steadily moving, not turning, braking, etc.) {R-2.3} Æ refines {R2, R-2.2}

– the system should contain a specific unit for controlling and adjusting the clearance, as well as appropriate ‘action body’ realising the adjustment {R-7}

– it is necessary to co-ordinate the controls coming from the suspension controller and the clearance controller to ensure smooth and safe operation {R-8}

Figure 8–5. Another solution with a co-ordinating master controller

As an embodiment of the above-presented explicit interpretation of the problem, the designer constructed a new solution that contained the informally proposed new conceptual objects in addition to those brought in earlier. The solution was referred to as S-2 in the decision sequence records, and it served as a basis for further investigation as visible from thread starting with record J-538. A sketch of the two control loops, featuring also the newly-introduced concept of a co-ordinating loop are depicted in Figure 8–5.

Design episode 8.2h

Discovery and rectification of the unacceptable co-ordination

Design context: DC-III (with solution S-2) Justifying threads: J-538 – J-541

The solution mentioned in the context 8.2/DC-III was constructed to remove the unacceptable inadequacies of the previous iteration. As the follow-up records to J-538 show, the designer did not accept S-2 as a final ‘design solution’ yet. From J-539 it seems that the designer was not satisfied with the co-ordination of the two distinct control strategies.

The candidate solution S-2 was more complex than S-1, and included several different modes of operation. The designer tackled the emerged complexity in notes J-540 and J-541, in which he made more specific commitments about the interactions between different modes, and suitable interfaces conveying the information to/from the driver. For instance, he introduced such concepts as driver overriding the automated control or ‘turning off’ the clearance adjustment under specific conditions.

These newly-articulated requirements addressed the flaws that emerged from the current solution.

Therefore, they rendered the candidate solution S-2 unacceptable and inconsistent, thus requiring further elaboration and development.

Design episode 8.2k

Consolidation of the extensions and refinements to the previous frames

Design context: DC-III Justifying threads: J-543 – J-546

The fundamental concepts differentiating the new frame from the previous ones were ‘interface’ and

‘man-system interaction’ {J-543}. The designer articulated the main functional requirement that needed to be added to the system in J-544. The functional purpose of an interface was to ‘translate’ the intuitive and qualitative values suitable for a driver to quantitative values recognisable by the suspension controllers. In the subsequent notes {J-545 and J-546}, the designer articulated the necessary concepts for implementing the new functional requirement.

First, he distinguished an ‘overall mode’ of operation, such as choosing and switching between fully automated and ‘manual’ adjustment of the chassis clearance and the shock absorbing pistons.

Another important extension was in the movement from purely quantitative controllers to fuzzy controllers. The main feature of this new type of controller was its capability to work with ranges rather than crisp values. Ranges could be associated with suitable ‘qualitative’ terms such as ‘hard’ or

‘medium’ for the suspension, and ‘low’ or ‘very low’ for the chassis clearance.

As seen in the records J-545 and J-546, this new conceptual approach benefited also the individual controllers and the control algorithms. The rules could have been written more robustly and the overall

behaviour was expected to become more flexible and less prone to harmful oscillations or destabilisation. For instance, the designer pointed out that the new approach enables better implementation of his original ‘try & see’ heuristic {J-516 and J-517}. Thus, formulating a new requirement and attending to it, helped to resolve and improve the system’s performance also in respect to another issue.